Evidence

Amino acids are likely to be available in prebiotic environments such as the primeval oceans of Earth

In the opinion of the authors, the basic reason that amino acids are used in terrestrial biology is their abundance in the primitive ocean.

Evidence is Sourced from:

Title

Reasons for the occurrence of the twenty coded protein amino acids

Authors

Weber, Arthur L and Miller, Stanley L

Abstract

Factors involved in the selection of the 20 protein L-a-amino acids during chemical evolution and the early stages of Darwinian evolution are discussed. The selection is considered on the basis of the availability in the primitive ocean, function in proteins, the stability of the amino acid and its peptides, stability to racemization, and stability on the transfer RNA. We conclude that aspartic acid, glutamic acid, arginine, lysine, serine and possibly threonine are the best choices for acidic, basic and hydroxy amino acids. The hydrophobic amino acids are reasonable choices, except for the puzzling absences of a-amino-n-butyric acid, norvaline and norleucine. The choices of the sulfur and aromatic amino acids seem reasonable, but are not compelling. Asparagine and glutamine are apparently not primitive. If life were to arise on another planet, we would expect that the catalysts would be poly-a-amino acids and that about 75% of the amino acids would be the same as on the earth.

Citation

Download BibTeX   |   Download Endnote

Evidence

Amino acids are commonly found in carbonaceous chondries 

The prevalence of amino acids in carbonaceous chondriets means these compounds are likely delivered to planetary bodies during accretion, and can be available for the origin of life

Evidence is Sourced from:

Title

Amino acids in meteorites.

Authors

Cronin JR,Pizzarello S

Abstract

Carbonaceous chondrites carry a record of chemical evolution that is unparalleled among presently accessible natural materials. Within the complex suite of organic compounds that characterize these meteorites, amino acids occur at a total concentration that may reach 0.6 micromole g-1 meteorite (approximately 60 ppm). Both free amino acids and acid-labile amino acid derivatives have been found in hot-water extracts of a CI1 and seven CM2 chondrites. Although the amino acid composition of all CM2 chondrites is not the same, differences may be largely explicable on the basis of spontaneous and biologically-caused decomposition occurring during their terrestrial residence. The amino acids of the Murchison meteorite (CM2) have been extensively analyzed and 52 amino acids have been positively identified. Thirty three of these amino acids are unknown in natural materials other than carbonaceous chondrites. Thus the Murchison meteorite has recently been the major source of new naturally-occurring amino acids. The Murchison amino acids comprise a mixture of C2 through C8 cyclic and acyclic monoamino alkanoic and alkandioic acids of nearly complete structural diversity. Within the acyclic monoamino alkanoic acid series, primary alpha-amino alpha-branched amino acids are predominant. The concentrations of individual amino acids decline exponentially with increasing carbon number within homologous series. Amino acid enantiomers are found in approximately equal amounts. Eight of the terrestrial protein amino acids have been found.

Citation

Download BibTeX   |   Download Endnote

Evidence

Alkaline hydrothermal vents may be energetically a suitable environment for the synthesis and polymerization of amino acids.

Experimental high-temperature volume and heat capacity data reported in the literature for several amino acids, together with correlation algorithms and equations of state were used to evaluate the synthesis of the 210 protein amino acids under alkaline hydrotherml vent conditions.

Evidence is Sourced from:

Title

Energetics of amino acid synthesis in alkaline hydrothermal environments

Authors

Kitadai, Norio

Abstract

Alkaline hydrothermal systems have received considerable attention as candidates for the origin and evolution of life on the primitive Earth. Nevertheless, sufficient information has not yet been obtained for the thermodynamic properties of amino acids, which are necessary components for life, at high temperatures and alkaline pH. These properties were estimated using experimental high-temperature volume and heat capacity data reported in the literature for several amino acids, together with correlation algorithms and the revised Helgeson–Kirkham–Flowers (HKF) equations of state. This approach enabled determination of a complete set of the standard molal thermodynamic data and the revised HKF parameters for the 20 protein amino acids in their zwitterionic and ionization states. The obtained dataset was then used to evaluate the energetics of amino acid syntheses from simple inorganic precursors (CO2, H2, NH3 and H2S) in a simulated alkaline hydrothermal system on the Hadean Earth. Results show that mixing between CO2-rich seawater and the H2-rich hydrothermal fluid can produce energetically favorable conditions for amino acid syntheses, particularly in the lower-temperature region of such systems. Together with data related to the pH and temperature dependences of the energetics of amino acid polymerizations presented in earlier reports, these results suggest the following. Hadean alkaline hydrothermal settings, where steep pH and temperature gradients may have existed between cool, slightly acidic Hadean ocean water and hot, alkaline hydrothermal fluids at the vent–ocean interface, may be energetically the most suitable environment for the synthesis and polymerization of amino acids.

Citation

Download BibTeX   |   Download Endnote

Evidence

Evolutionary pressures inevitably lead to homochiral biochemistries

Primordial life could have evolved in both homochiral forms, but evolutionary pressures toward a more efficient biochemistry led to the survival of only one of these chiral forms

Evidence is Sourced from:

Title

Homochirality in life: Two equal runners, one tripped

Authors

Green, Mark M and Jain, Vipul

Abstract

Strong arguments can be found in the literature addressed to the question of the origin of homochirality in life, supporting the hypothesis that primordial life could have evolved in both homochiral forms and that early on when life was still rarely found, random events led to the survival of only one of these living mirror images. This proposal is an alternative to the generally accepted view that small enantiomeric excesses of biologically important molecules were amplified to homochirality prior to life’s origin. Acceptance of the possibility of “two equal runners” leads to the importance of research investigations on routes to formation of ensembles of racemic mixtures of isotactic biologically interesting polymers, supramolecular entities and aggregates.

Citation

Download BibTeX   |   Download Endnote

Evidence

Stereospecificity is inherent in the catalytic action of enzymes

This paper argues that It is impossible for a chiral catalyst to maximally speed the rate of a chemical reaction without distinguishing enantiomers or enantiotopic moieties

Evidence is Sourced from:

Title

Enzymes and stereochemistry

Authors

J.W. Cornforth

Abstract

No abstract provided

Citation

Download BibTeX   |   Download Endnote

Evidence

The longest protein that contains D-amino acids only has 42 residues, which suggests that larger, functional proteins must be built with amino acids of the same chiral form.

A survey of a protein data bank for proteins and peptides containing D-amino acids. They claim that the longest D-amino acid containing protein was 42 residues. This points to the fact that larger, functional proteins require homochirality in their structure.

Evidence is Sourced from:

Title

D‐amino acid residues in peptides and proteins

Authors

John B.O. Mitchell, James Smith

Abstract

We have investigated the D‐amino acid residues present in Protein Data Bank (PDB) entries, categorizing them into “real” D‐residues and artifacts. In polypeptide chains of more than 20 residues, only a single instance of a “real” D‐residue, other than those deliberately designed or engineered, was found. This example was the result of a slow chemical epimerization process. Another 12 designed D‐residues were found in these longer polypeptide chains. Smaller peptides of 20 or fewer residues contained 479 “real” D‐residues, the majority in various gramicidin, actinomycin, or cyclosporin structures. We found 148 PDB entries with “real” D‐residues and a further 186, in which all apparent D‐residues are artifacts. Investigating the (ϕ, ψ) preferences of the “real” D‐residues, we found that the region around (−60°, −45°) was almost completely unoccupied, even though it is not formally disallowed. We link the low propensity to occupy this region with the α‐helix destabilizing properties of D‐residues. Proteins 2003;50:563–571. © 2003 Wiley‐Liss, Inc.

Citation

Download BibTeX   |   Download Endnote

Evidence

The presence of D-amino acids destabilizes the helical secondary structure of proteins

Examples of how D-amino acids destabilize secondary structures in proteins. Proteins need to be homochiral for proper structure and function.

Evidence is Sourced from:

Title

The Helix-Destabilizing Propensity Scale of d-Amino Acids:  The Influence of Side Chain Steric Effects

Authors

Eberhard Krause, Michael Bienert, Peter Schmieder, and Holger Wenschuh

Abstract

Although d-amino acids are found in various naturally occurring peptides and frequently used for structure−activity studies, not much is known about their impact on the helical secondary structures formed by l-amino acids. Although several previous accounts reported on the α-helical propensity of l-amino acids, the present contribution addresses this subject for the first time for the corresponding d-enantiomers of all of the proteinogenic amino acids. Thus, the helix-destabilizing abilities of the 19 d-amino acids in the host sequence acetyl-KLALKLALxxLKLALKLA-amide (x9,10-KLA) were evaluated by means of circular dichroism (CD) spectroscopy, nuclear magnetic resonance, and reversed-phase HPLC. CD and HPLC data enabled calculation of differences in the free energy of helix formation for d-amino acid x relative to glycine (ΔΔGt) or to the corresponding l-amino acid (ΔΔGd-l). The data show that the helix-destabilizing propensity is highly dependent on the amino acid side chain and not related to the structure propensity of the corresponding l-amino acid. In consequence, the d-amino acids can be grouped into (i) weak helix destabilizers (d-His, d-Asp, d-Glu, d-Cys, d-Gln, d-Asn, d-Ser), (ii) medium helix destabilizers (d-Leu, d-Arg, d-Met, d-Lys, d-Trp, d-Ala), and (iii) strong helix destabilizers (d-Thr, d-Phe, d-Val, d-Ile, d-Tyr, d-Pro). Accordingly, the d-isomers of bulky and β-branched amino acids are the most effective in destabilizing the amphipathic KLA-helix by induction of turn-like structures. Such d-isomers disrupt the secondary structure in a manner similar to that of l-proline.

Citation

Download BibTeX   |   Download Endnote

Evidence

The beta-sheet structure of proteins has more stability when made from a single enantiomer

In the lab beta-sheets were synthesized with different ratios of L and D amino acids and then acid hydrolyzed. Regions enriched in L-AA were more stable than the random coil portions.

Evidence is Sourced from:

Title

Beta-Structures of Polypeptides with L- and D-Residues

Authors

Andre Brack and Gerard Spach

Abstract

It was previously shown that nuclei of beta-sheets surrounded by unordered segments are formed in polypeptide chains built up with alternating hydrophobic and hydrophilic residues and containing both L- and D-enantiomers. It was also established that segments of residues having the same configuration tend to segregate in these nuclei when the starting composition of stereomonomers departs from the racemic mixture. Soft acidic hydrolysis of these polymers has been studied. Kinetic measurements show two pseudo first order rate constants, in agreement with the existence of two conformational species. The unordered part of the chains in hydrolyzed more rapidly, allowing the isolation of the a beta-fraction enriched in one enantiomer. Thus, a plausible process of enrichment in enantiomer during prebiotic evolution has been described, which however does not explain the preference of one enantiomer over the other one.

Citation

Download BibTeX   |   Download Endnote

Evidence

Ribosomes discriminate against incorporation of D-amino acids into proteins 

During protein synthesis, ribosomes discriminate chirality of amino acids and prevent incorporation of D-amino acids into nascent proteins by slowing down the rate of peptide bond formation. Poor D-amino acid reactivity in the ribosomal A site stems from the suboptimal positioning of the reactive α-amino group, which hinders the nucleophilic attack and blocks the proton shuttle during the peptide bond formation.

Evidence is Sourced from:

Title

{Mechanistic insights into the slow peptide bond formation with D-amino acids in the ribosomal active site}

Authors

Melnikov, Sergey V and Khabibullina, Nelli F and Mairhofer, Elisabeth and Vargas-Rodriguez, Oscar and Reynolds, Noah M and Micura, Ronald and Söll, Dieter and Polikanov, Yury S

Abstract

{During protein synthesis, ribosomes discriminate chirality of amino acids and prevent incorporation of D-amino acids into nascent proteins by slowing down the rate of peptide bond formation. Despite this phenomenon being known for nearly forty years, no structures have ever been reported that would explain the poor reactivity of D-amino acids. Here we report a 3.7Å-resolution crystal structure of a bacterial ribosome in complex with a D-aminoacyl-tRNA analog bound to the A site. Although at this resolution we could not observe individual chemical groups, we could unambiguously define the positions of the D-amino acid side chain and the amino group based on chemical restraints. The structure reveals that similarly to L-amino acids, the D-amino acid binds the ribosome by inserting its side chain into the ribosomal A-site cleft. This binding mode does not allow optimal nucleophilic attack of the peptidyl-tRNA by the reactive α-amino group of a D-amino acid. Also, our structure suggests that the D-amino acid cannot participate in hydrogen-bonding with the P-site tRNA that is required for the efficient proton transfer during peptide bond formation. Overall, our work provides the first mechanistic insight into the ancient mechanism that helps living cells ensure the stereochemistry of protein synthesis.}

Citation

Download BibTeX   |   Download Endnote

Evidence

Incorporation of homochiral amino acids may be favored by reaction rates

In a statistical analysis in which the values for L and D reactions rates are permitted to randomly fluctuate about a given mean, the authors found that for differing L and D reaction rates, a substantial amount of chiral excess may be reached at steady-state.

Evidence is Sourced from:

Title

Chiral polymerization in open systems from chiral-selective reaction rates

Authors

Marcelo Gleiser, Bradley J Nelson, Sara Imari Walker

Abstract

We investigate the possibility that prebiotic homochirality can be achieved exclusively through chiral-selective reaction rate parameters without any other explicit mechanism for chiral bias. Specifically, we examine an open network of polymerization reactions, where the reaction rates can have chiral-selective values. The reactions are neither autocatalytic nor do they contain explicit enantiomeric cross-inhibition terms. We are thus investigating how rare a set of chiral-selective reaction rates needs to be in order to generate a reasonable amount of chiral bias. We quantify our results adopting a statistical approach: varying both the mean value and the rms dispersion of the relevant reaction rates, we show that moderate to high levels of chiral excess can be achieved with fairly small chiral bias, below 10%. Considering the various unknowns related to prebiotic chemical networks in early Earth and the dependence of reaction rates to environmental properties such as temperature and pressure variations, we argue that homochirality could have been achieved from moderate amounts of chiral selectivity in the reaction rates.

Citation

Download BibTeX   |   Download Endnote

Evidence

Peptidyl transferase specificity prevents incorporation of D-amino acids during protein synthesis

The specificity of the acceptor site of peptidyl transferase for substrates with an L-configuration may prevent D-amino acids from being incorporated during peptide elongation.

Evidence is Sourced from:

Title

Substrate Specificity of Ribosomal Peptidyl Transferase

Authors

I Rychlík, J Cerná, S Chládek, P Pulkrábek, J Zemlicka

Abstract

The effect of the nature of the amino acid residue on the acceptor activity of substrates of ribosomal peptidyl transferase was investigated. We tested 2′(3′)‐O‐aminoacyladenosines containing the following amino acid residues: l‐alanine, l‐glutamine, glycine, l‐3‐(1‐benzyl‐4‐imidazolyl)‐alanine, l‐leucine, l‐lysine, l‐methionine, l‐methionine S‐oxide, l‐phenylalanine, d‐phenylalanine, l‐proline, l‐serine and l‐valine as acceptors of the acylaminoacyl residue transferred from peptidyl‐tRNA. The acceptor activity of these compounds was dependent on the nature of the side chain of the amino acid residues bound to adenosine. The acceptor activity was also affected by the nature of the donor tRNA derivative. With l‐acetylphenylalanyl‐tRNA or l‐acetylleucine npetanucleotide fragments derived from l‐acetylleucyl‐tRNA donating the acetylphenylalanyl and the acetylleucyl residue, respectively, a high acceptor activity was shown by the basic 2′(3′)‐O‐l‐lysyladenosine or 2′(3′)‐O‐l‐3‐(1‐benzyl‐4‐imidazolyl)‐alanyladenosine which acted only as weak acceptors of lysine peptides from (Lys)n‐tRNA. The transfer reaction catalyzed by peptidyl transferase was stereospecific with respect to acceptor substrates. The acylaminoacyl residue was transferred from tRNA to 2′(3′)‐O‐l‐phenylalanyladenosine, whereas 2′(3′)‐O‐d‐phenylalanyladenosine, containing a d‐phenylalanine residue, was completely inactive as acceptor.

Citation

Download BibTeX   |   Download Endnote

Evidence

Life uses racemases to convert the non-preferred to the preferred enantiomer

Cultures from extreme environments show that bacteria will consume both the preferred (L) and non-preferred (D) amino acids when they are provided as nutrients, but the D consumption only occurs after a lag phase. The D consumption coincides with the onset of racemase enzyme activity demonstrating the functionality of this enzyme for allowing conversion and then use. This suggests that the enzyme is primarily used by biology to maintain the preferred enatiomeric configuration and hence the enantiomeric excess.

Evidence is Sourced from:

Title

Racemization in reverse: evidence that D-amino acid toxicity on Earth is controlled by bacteria with racemases.

Authors

Zhang G,Sun HJ

Abstract

D-amino acids are toxic for life on Earth. Yet, they form constantly due to geochemical racemization and bacterial growth (the cell walls of which contain D-amino acids), raising the fundamental question of how they ultimately are recycled. This study provides evidence that bacteria use D-amino acids as a source of nitrogen by running enzymatic racemization in reverse. Consequently, when soils are inundated with racemic amino acids, resident bacteria consume D- as well as L-enantiomers, either simultaneously or sequentially depending on the level of their racemase activity. Bacteria thus protect life on Earth by keeping environments D-amino acid free.

Citation

Download BibTeX   |   Download Endnote

Evidence

Cold and dry conditions enhance the preservation of amino acids over geologic timescales 

This evidence demonstrates that amino acids will not racemize on timescales equal to the age of the solar system when preserved cold and dry at temperatures <250K.

Evidence is Sourced from:

Title

Amino acid racemization on Mars: implications for the preservation of biomolecules from an extinct martian biota.

Authors

Bada JL,McDonald GD

Abstract

Using kinetic data, we have estimated the racemization half-lives and times for total racemization of amino acids under conditions relevant to the surface of Mars. Amino acids from an extinct martian biota maintained in a dry, cold (<250 K) environment would not have racemized significantly over the lifetime of the planet. Racemization would have taken place in environments where liquid water was present even for time periods of only a few million years following biotic extinction. The best preservation of both amino acid homochirality and nucleic acid genetic information associated with extinct martian life would be in the polar regions.

Citation

Download BibTeX   |   Download Endnote

Evidence

Enantiomeric excess in aqueous solutions is likely to be left behind and seep into the soil preserving it in high salt concentrations (e. g. evaporites).

Amino acids iso-leucine and phenylalanine are widely used by organisms to build proteins. They were found with enantiomeric excess L-isomer in salt and sediment samples from the Atacama desert, a hyperarid Mars analog environment. These results support the idea that enantiomeric excess in biogenic amino acids tends to be preserved under hyperarid conditions.

Evidence is Sourced from:

Title

{Biosignature Analysis of Mars Soil Analogs from the Atacama Desert: Challenges and Implications for Future Missions to Mars}

Authors

Aerts, Joost W. and Riedo, Andreas and Melton, Daniel J. and Martini, Simone and Flahaut, Jessica and Meierhenrich, Uwe J. and Meinert, Cornelia and Myrgorodska, Iuliia and Lindner, Robert and Ehrenfreund, Pascale

Abstract

The detection of biosignatures on Mars is of outstanding interest in current Astrobiology and drives various fields of research, ranging from new sample collection strategies to the development of more sensitive detection techniques. Detailed analysis of the organic content in Mars analogue materials collected from extreme environments on Earth improves the current understanding of biosignature preservation and detection under conditions similar to Mars. In this paper we examined the biological fingerprint of several locations in the Atacama desert (Chile) which include different wet and dry, and intermediate to high elevation salt flats (also named salars). Liquid Chromatography and Multidimensional Gas Chromatography Mass Spectrometry measurement techniques were used for the detection and analysis of amino acids extracted from the salt crusts and sediments using sophisticated extraction procedures. Illumina 16S amplicon sequencing was used for the identification of microbial communities associated with the different sample locations. Although amino acid load and organic carbon and nitrogen quantities were generally low, it was found that most of the samples harbored complex and versatile microbial communities, which were dominated by (extremely) halophilic microorganisms (most notably by species of the Archaeal family Halobacteriaceae). The dominance of salts (i.e. halites and sulphates) in the investigated samples leaves its mark on the composition of the microbial communities but does not appear to hinder the potential of life to flourish since it can clearly adapt to the higher concentrations. Although the Atacama desert is one of the driest and harshest environments on Earth, it is shown that there are still sub-locations where life is able to maintain a foothold, and as such, salt flats could be considered as interesting targets for future life exploration missions on Mars.

Citation

Download BibTeX   |   Download Endnote

Evidence

Amino acids can be preserved for geologically long periods in sulfate mineral matrices

Sulfate minerals show increased capability to preserve enantiomeric excess of amino acids over billions of years. The Martian soil was found to be rich in iron-oxide which provides a barrier against radiation, preserving organic material like amino acids and its enantiomeric excess, and this protection might be increased in deeper regolith. Further, the presence of sulfate minerals together with iron-oxide may produce an even greater shielding against radiation, and thus radiolysis. UV radiation is a large concern to the loss of enantiomeric excess, but results shows that a few millimeters of impure gypsum crust is opaque to radiation below 400nm, shielding the organic material (amino acids) against UV metal-catalyzed oxidation.

Evidence is Sourced from:

Title

Sulfate minerals and organic compounds on Mars

Authors

Aubrey, Andrew; Cleaves, H. James ; Chalmers, John H. ; Skelley, Alison M. ; Mathies, Richard A. ; Grunthaner, Frank J. ; Ehrenfreund, Pascale ; Bada, Jeffrey L.

Abstract

Strong evidence for evaporitic sulfate minerals such as gypsum and jarosite has recently been found on Mars. Although organic molecules are often codeposited with terrestrial evaporitic minerals, there have been no systematic investigations of organic components in sulfate minerals. We report here the detection of organic material, including amino acids and their amine degradation products, in ancient terrestrial sulfate minerals. Amino acids and amines appear to be preserved for geologically long periods in sulfate mineral matrices. This suggests that sulfate minerals should be prime targets in the search for organic compounds, including those of biological origin, on Mars.

Citation

Download BibTeX   |   Download Endnote

Evidence

Enantiomeric excess of amino acids in comets can be preserved for geologic timescales

Enantiomeric excess is able to be preserved under gamma radiation conditions for the lifespan of the solar system if samples are shielded appropriately from cosmic radiation

Evidence is Sourced from:

Title

Amino acids in comets and meteorites: stability under gamma radiation and preservation of the enantiomeric excess

Authors

Susana Iglesias-Groth, Franco Cataldo, Ornella Ursini, and Arturo Manchado

Abstract

Amino acids in Solar system bodies may have played a key role in the chemistry that led to the origin of life on Earth. We present laboratory studies testing the stability of amino acids against high energy radiation. All the 20 chiral amino acids in the levo form found in the proteins of the current terrestrial biochemistry were irradiated in the solid state with γ -radiation to a dose of 3.2 MGy, which is the dose equivalent to that produced by radionuclide decay in comets and asteroids in 1.05 × 109 yr. For each amino acid the radiolysis degree and the radioracemization degree were measured by differential scanning calorimetry and by optical rotatory dispersion spectroscopy. From these measurements, a radiolysis rate constant kdsc and a radioracemization rate constant krac were determined for each amino acid and extrapolated to a dose of 14 MGy, which corresponds to the expected total dose delivered by natural radionuclide decay to all the organic molecules present in comets and asteroids in 4.6 × 109 yr, the age of the Solar system. It is shown that all the amino acids studied can survive a radiation dose of 14 MGy, although certain fractions of them are lost as a result of radiolytic processes. Similarly, the radio- racemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. Knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentrations of the amino acids at the time of the formation of the Solar system, starting from the concentration found today in carbonaceous chondrites. For some amino acids the concentration in the pre-solar nebula could have been up to 6 times higher than that currently observed in meteorites. The preservation of an original enatiomeric excess is also expected. This study adds experimental support to the suggestion that amino acids were formed in the interstellar medium and in chiral excess and then were incorporated into comets and asteroids at the epoch of Solar system formation.

Citation

Download BibTeX   |   Download Endnote

Evidence

Amino acids are likely to be available in prebiotic environments such as the primeval oceans of Earth 

In the opinion of the authors, the basic reason that amino acids are used in terrestrial biology is their abundance in the primitive ocean.

Evidence is Sourced from:

Title

Reasons for the occurrence of the twenty coded protein amino acids.

Authors

Weber AL,Miller SL

Abstract

Factors involved in the selection of the 20 protein L-alpha-amino acids during chemical evolution and the early stages of Darwinian evolution are discussed. The selection is considered on the basis of the availability in the primitive ocean, function in proteins, the stability of the amino acid and its peptides, stability to racemization, and stability on the transfer RNA. We conclude that aspartic acid, glutamic acid, arginine, lysine, serine and possibly threonine are the best choices for acidic, basic and hydroxy amino acids. The hydrophobic amino acids are reasonable choices, except for the puzzling absences of alpha-amino-n-butyric acid, norvaline and norleucine. The choices of the sulfur and aromatic amino acids seem reasonable, but are not compelling. Asparagine and glutamine are apparently not primitive. If life were to arise on another planet, we would expect that the catalysts would be poly-alpha-amino acids and that about 75% of the amino acids would be the same as on the earth.

Citation

Download BibTeX   |   Download Endnote

Evidence

Amino acids are commonly found in carbonaceous chondries  

The prevalence of amino acids in carbonaceous chondriets means these compounds are likely delivered to planetary bodies during accretion, and can be available for the origin of life

Evidence is Sourced from:

Title

Amino acids in meteorites.

Authors

Cronin JR,Pizzarello S

Abstract

Carbonaceous chondrites carry a record of chemical evolution that is unparalleled among presently accessible natural materials. Within the complex suite of organic compounds that characterize these meteorites, amino acids occur at a total concentration that may reach 0.6 micromole g-1 meteorite (approximately 60 ppm). Both free amino acids and acid-labile amino acid derivatives have been found in hot-water extracts of a CI1 and seven CM2 chondrites. Although the amino acid composition of all CM2 chondrites is not the same, differences may be largely explicable on the basis of spontaneous and biologically-caused decomposition occurring during their terrestrial residence. The amino acids of the Murchison meteorite (CM2) have been extensively analyzed and 52 amino acids have been positively identified. Thirty three of these amino acids are unknown in natural materials other than carbonaceous chondrites. Thus the Murchison meteorite has recently been the major source of new naturally-occurring amino acids. The Murchison amino acids comprise a mixture of C2 through C8 cyclic and acyclic monoamino alkanoic and alkandioic acids of nearly complete structural diversity. Within the acyclic monoamino alkanoic acid series, primary alpha-amino alpha-branched amino acids are predominant. The concentrations of individual amino acids decline exponentially with increasing carbon number within homologous series. Amino acid enantiomers are found in approximately equal amounts. Eight of the terrestrial protein amino acids have been found.

Citation

Download BibTeX   |   Download Endnote

Evidence

Alkaline hydrothermal vents may be energetically a suitable environment for the synthesis and polymerization of amino acids. 

Experimental high-temperature volume and heat capacity data reported in the literature for several amino acids, together with correlation algorithms and equations of state were used to evaluate the synthesis of the 210 protein amino acids under alkaline hydrotherml vent conditions.

Evidence is Sourced from:

Title

Energetics of amino acid synthesis in alkaline hydrothermal environments

Authors

Kitadai, Norio

Abstract

Alkaline hydrothermal systems have received considerable attention as candidates for the origin and evolution of life on the primitive Earth. Nevertheless, sufficient information has not yet been obtained for the thermodynamic properties of amino acids, which are necessary components for life, at high temperatures and alkaline pH. These properties were estimated using experimental high-temperature volume and heat capacity data reported in the literature for several amino acids, together with correlation algorithms and the revised Helgeson–Kirkham–Flowers (HKF) equations of state. This approach enabled determination of a complete set of the standard molal thermodynamic data and the revised HKF parameters for the 20 protein amino acids in their zwitterionic and ionization states. The obtained dataset was then used to evaluate the energetics of amino acid syntheses from simple inorganic precursors (CO2, H2, NH3 and H2S) in a simulated alkaline hydrothermal system on the Hadean Earth. Results show that mixing between CO2-rich seawater and the H2-rich hydrothermal fluid can produce energetically favorable conditions for amino acid syntheses, particularly in the lower-temperature region of such systems. Together with data related to the pH and temperature dependences of the energetics of amino acid polymerizations presented in earlier reports, these results suggest the following. Hadean alkaline hydrothermal settings, where steep pH and temperature gradients may have existed between cool, slightly acidic Hadean ocean water and hot, alkaline hydrothermal fluids at the vent–ocean interface, may be energetically the most suitable environment for the synthesis and polymerization of amino acids.

Citation

Download BibTeX   |   Download Endnote

Evidence

Enantiomeric enrichment of racemic alpha-amino acids occurs during sublimation in the presence of another enantiomerically enriched alpha-amino acid. 

Electric discharge experiments of Miller and others showed that amino acids can be produced in racemic mixtures from simple chemical reactions. In an effort to see how enantiomeric pure amino acids might be obtained from such a mixture, the authors extended the high temperature sublimation experiments of Viedma et al. The results with mixtures of an enantiopure alpha-amino acid plus lower amounts of racemic alpha-amino acids were that the most abundant amino acid imposes the sense of its handedness on the other alpha-amino acids undergoing sublimation. A greater number of racemic amino acids in the starting mixture gave a greater enantio-enrichment of each amino acid after sublimation. It was also found that the enantiotopic excesses and yields are strongly directly dependent on the temperature of sublimation but relatively independent of the atmosphere present. In experiments with 6% enantioenriched L-valine and 0.25 equivalents of racemic leucine, the enrichment of L-valine and L-leucine was low (to 7.6% and 1.3% respectively). However, the presented data strongly suggest that several cycles of sublimation would eventually yield very high levels of enantio-excess for both amino acids. As high temperature sublimations are compatible with the drastic conditions of syntheses occurring four billions years ago on the warm surface of the young Earth, these conditions could plausibly yield high enantiotopic purity of many, if not all, alpha-amino acids.

Evidence is Sourced from:

Title

High temperature sublimation of a-amino acids: a realistic prebiotic process leading to large enantiomeric excess

Authors

Tarasevych, A.V., Sorochinsky, A.E., Kukharb, V.P., and Guillemin, J-C.

Abstract

The reiterative high temperature co-sublimation of an enantiopure or an enantioenriched alpha-amino acid mixed with racemic alpha-amino acids leads to deracemization of the latter. A synergistic effect is observed for complex mixtures, and the sense of the handedness is, for all compounds, identical to that of the enantioenriched starting material.

Citation

Download BibTeX   |   Download Endnote

Evidence

Crystallization of complex organic mixtures containing a plethora of amino acids generated under prebiotic conditions can generate crystals with an excess of L or D amino acids, depending upon the conditions of crystallization. 

Electric discharge experiments of Miller and others showed that amino acids can be produced in racemic mixtures from simple chemical reactions. The experiments by Jiang et al. examined the possibility of spontaneous asymmetric resolution of alpha-amino acids obtained from electric discharge experiment through self-crystallization. The authors show data supporting the conclusion that an enrichment of L or D isomers of some amino acids can be achieved, depending upon the precise conditions of recrystallization. Under one set of conditions, recrystallization of seven pure racemic amino acids afforded enantiomeric excesses of the D-enantiomer by 41±3% for aspartic acid, 36±4% for serine, 30±3% for alanine and 42±1% for asparagine (n = 3). A different set of conditions, mimicking possible conditions of early earth, with a “prebiotic” mixture gave crystals with excesses of L-enantiomers of 89:11 and 58:42 for serine and alanine respectively. These data support the authors’ conclusion that complex organic mixtures containing a plethora of amino acids generated under prebiotic conditions can undergo spontaneous generation of asymmetry. This conclusion, in turn, suggests that multiple cycles of such recrystallizations could eventually give, under abiotic conditions, a pool of essentially 100% L-enantiomers of each of the 21 alpha-amino acids seen in proteins.

Evidence is Sourced from:

Title

Abiotic synthesis of amino acids and self-crystallization under prebiotic conditions

Authors

Jiang, L., Dziedzic, P., Spacil, Z., Zhao, G-L., Nilsson, L., Ilag, L.L., and Córdova, A.

Abstract

Building on previous research on the origin and homochirality of life, this study focuses on analyses profiling important building blocks of life: the natural amino acids. The spark discharge variation of the iconic Miller experiment was performed with a reducing gas mixture of ammonia, methane, water and hydrogen. Amino acid analysis using liquid chromatography coupled with tandem mass spectrometry after pre-column derivatization revealed the generation of several amino acids including those essential for life. Re-crystallization of the synthetic products and enantiomeric ratio analysis were subsequently performed. Results from liquid chromatography coupled with either fluorescent detector or tandem mass spectrometry after pre-column derivatization with chiral reagent revealed spontaneous and effective asymmetric resolution of serine and alanine. This work describes a useful analytical platform for investigation of hypotheses regarding the origin and homochirality of amino acids under prebiotic conditions. The formation of numerous amino acids in the electric discharge experiment and the occurrence of high enantiomeric ratios of amino acids in re-crystallization experiment give valuable implications for future studies in unraveling fundamental questions regarding origins and evolution of life.

Citation

Download BibTeX   |   Download Endnote

Evidence

Enantiomeric selection of amino acids can be achieved in amounts of up to 100% simply by recrystallizing a mixture of 12 racemic amino acids plus a four-fold excess (over the combined 12 amino acids) of racemic asparagine. 

Electric discharge experiments of Miller and others showed that amino acids can be produced in racemic mixtures from simple chemical reactions. In an effort to see how enantiomeric pure amino acids might be obtained from such a mixture, the authors conducted recrystallization experiments with mixture of 12 racemic amino acids plus a four-fold excess (over the combined 12 amino acids) of racemic asparagine (Asn). In 12 separate trials, the enantiomeric excess (ee) for Asn crystallized from a racemic solution varied from -100 to +100%. In each trial the ee of co-crystallized amino acids was highly correlated with the value measured for Asn. Interestingly, these ee values were often much higher than those observed in the recrystallization of each amino acid alone with racemic Asn. These results demonstrate that racemic Asn can cause an enantio-selective crystallization of a co-existing racemic amino acids. It was incidental whether the enrichment took place with L- or D-Asn. However, once the selection was made, the co-existing amino acid with the same configuration at the alpha-carbon was preferentially involved. The authors propose that enantio-selective crystallization of racemic amino acids induced by spontaneous resolution of a co-existing racemic molecule such as racemic Asn, contributed to the selection of L-amino acids in the biosphere and that the resulting ee is sufficiently high to account for the predominance of L-amino acids on the earth.

Evidence is Sourced from:

Title

Racemic D,L-asparagine causes enantiomeric excess of other coexisting racemic D,L-amino acids during recrystallization: a hypothesis accounting for the origin of L-amino acids in the biosphere.

Authors

Kojo, S., Uchino H., Yoshimura, M., and Tanaka, K.

Abstract

When recrystallizations were performed using a mixture of 12 D,L-amino acids (alanine, aspartic acid, arginine, glutamic acid, glutamine, histidine, leucine, methionine, serine, valine, phenylalanine, and tyrosine) with excess D,L-asparagine, all amino acids with the same configuration as asparagine were preferentially co-crystallized, indicating that it is the nature of a mixture of racemic amino acids to produce a spontaneous high enantiomeric excess.

Citation

Download BibTeX   |   Download Endnote

Evidence

Crystals of high enantiomeric purity can be obtained via enantiomer-specific oriented attachment during which a racemic mixture of nano- or microcrystals undergo oriented attachment to form single crystal aggregates. 

Electric discharge experiments of Miller and others showed that amino acids can be produced in racemic mixtures from simple chemical reactions. An unresolved question is how enantiomerically pure material can evolve from such racemic mixtures. The present study looks at the ability of a racemic mixture of nano- or microcrystals to undergo enantiomer-specific oriented attachment (ESOA) to form single crystal aggregates of high enantiomeric purity. Most of the study is with crystals of NaClO3 but similar results were obtained with the amino acid threonine.

Evidence is Sourced from:

Title

Enantiomer-specific oriented attachment: formation of macroscopic homochiral crystal aggregates from a racemic system.

Authors

Viedma, C., McBride, J. M., Kahr, B., and Cintas, P.

Abstract

No abstract given

Citation

Download BibTeX   |   Download Endnote

Evidence

This review article covers recent results suggesting various chemical models of catalysis-based mechanisms for amplifying chirality under prebiotic conditions on Earth. A theoretical example is presented for alpha-amino acids but no experimental data.

Electric discharge experiments of Miller and others showed that amino acids can be produced in racemic mixtures from simple chemical reactions. An unresolved question is how enantiomerically pure material can evolve from such racemic mixtures. Several examples of mechanisms by which achiral solutions can yield an excess of chiral products are given. The focus is on catalysis-based models relying on far-from-equilibrium chemical mechanisms of enantiomeric enrichment (e.g., asymmetric autocatalytic reactions, reactions with asymmetric autoamplification, asymmetric catalytic reactions with “hyperpositive” nonlinear effects, and asymmetric catalytic reactions without nonlinear effects). The authors prefer the models of chirality amplification based on dynamic nonlinear effects (DNLEs), i.e., on asymmetric autocatalysis and asymmetric autoamplification/dynamic chirality control because of their similarity with the expected conditions for mimicking natural selection in chemical systems but cannot rule out other candidate mechanisms.

Evidence is Sourced from:

Title

Chemical mechanisms of prebiotic chirality amplification.

Authors

Bryliakov, K. P.

Abstract

This review article surveys the recent experimental findings that suggest alternative chemical models of directed chirality amplification at the early, prebiotic Earth. It is believed that the chirality amplification step followed the initial emergence of small enantiomeric imbalance and preceded (as a necessary condition) the occurrence of homochiral biopolymers, assembled from enantiomerically pure building blocks. This work focuses on the chemical nature of possible mechanisms of primordial chirality enhancement, without going into detail of the preceding and subsequent phases of origination of biological homochirality and life on Earth. These mechanisms are discussed through the prism of integrity of biological natural selection and chemical kinetic selection.

Citation

Download BibTeX   |   Download Endnote

Evidence

Small L-enantiomeric excesses ranging from 1 to 9% were found in six abiotic amino acids in Murchison and Murray meteorites

Small L-enantiomeric excesses ranging from 1 to 9% were found in six alpha-methyl amino acids in Murchison and Murray. L-nantiomeric excesses were not observed in the alpha hydrogen amino acids in these meteorites or were attributed to terrestrial contamination.

Evidence is Sourced from:

Title

Non-racemic amino acids in the Murray and Murchison meteorites

Authors

Pizzarello, S and Cronin, JR

Abstract

Small (1.0-9.2%) L-enantiomer excesses were found in six alpha-methyl-alpha-amino alkanoic acids from the Murchison (2.8-9.2%) and Murray (1.0-6.0%) carbonaceous chondrites by gas chromatography-mass spectroscopy of their N-trifluoroacetyl or N-pentafluoropropyl isopropyl esters. These amino acids [2-amino-2,3-dimethylpentanoic acid (both diastereomers), isovaline, alpha-methyl norvaline, alpha-methyl valine, and alpha-methyl norleucine] are either unknown or rare in the terrestrial biosphere. Enantiomeric excesses were either not observed in the four alpha-H-alpha-amino alkanoic acids analyzed (alpha-amino-n-butyric acid, norvaline, alanine, and valine) or were attributed to terrestrial contamination. The substantial excess of L-alanine reported by others was not found in the alanine in fractionated extracts of either meteorite. The enantiomeric excesses reported for the alpha-methyl amino acids may be the result of partial photoresolution of racemic mixtures caused by ultraviolet circularly polarized light in the presolar cloud. The alpha-methyl-alpha-amino alkanoic acids could have been significant in the origin of terrestrial homochirality given their resistance to racemization and the possibility for amplification of their enantiomeric excesses suggested by the strong tendency of their polymers to form chiral secondary structure.

Citation

Download BibTeX   |   Download Endnote

Evidence

L-isovaline excesses ranging from 15.2 - 18.5% were identified in the aqueously altered Murchison and Orgueil meteorites.

L-isovaline excesses ranging from 15.2 - 18.5% were identified in the aqueously altered CM meteorite Murchison and the CI Orgueil. No L-isovaline enrichment was found int he most primitive, unaltered Antarctic CR meteorites. These results are inconsistent with UV circularly polarized light as the primary mechanism for L-isovaline enrichment and indicate that amplification of a small initial isovaline asymmetry in Murchison and Orgueil occurred during an extended aqueous alteration phase on the meteorite parent bodies.

Evidence is Sourced from:

Title

Enrichment of the amino acid L-isovaline by aqueous alteration on CI and CM meteorite parent bodies.

Authors

Glavin DP, Dworkin JP

Abstract

The distribution and enantiomeric composition of the 5-carbon (C(5)) amino acids found in CI-, CM-, and CR-type carbonaceous meteorites were investigated by using liquid chromatography fluorescence detection/TOF-MS coupled with o-phthaldialdehyde/N-acetyl-L-cysteine derivatization. A large L-enantiomeric excess (ee) of the alpha-methyl amino acid isovaline was found in the CM meteorite Murchison (L(ee) = 18.5 +/- 2.6%) and the CI meteorite Orgueil (L(ee) = 15.2 +/- 4.0%). The measured value for Murchison is the largest enantiomeric excess in any meteorite reported to date, and the Orgueil measurement of an isovaline excess has not been reported previously for this or any CI meteorite. The L-isovaline enrichments in these two carbonaceous meteorites cannot be the result of interference from other C(5) amino acid isomers present in the samples, analytical biases, or terrestrial amino acid contamination. We observed no L-isovaline enrichment for the most primitive unaltered Antarctic CR meteorites EET 92042 and QUE 99177. These results are inconsistent with UV circularly polarized light as the primary mechanism for L-isovaline enrichment and indicate that amplification of a small initial isovaline asymmetry in Murchison and Orgueil occurred during an extended aqueous alteration phase on the meteorite parent bodies. The large asymmetry in isovaline and other alpha-dialkyl amino acids found in altered CI and CM meteorites suggests that amino acids delivered by asteroids, comets, and their fragments would have biased the Earth's prebiotic organic inventory with left-handed molecules before the origin of life.

Citation

Download BibTeX   |   Download Endnote

Evidence

Large L-excesses of aspartic acid and glutamic acid up to ~60% were measured in the Tagish Lake meteorite.

Large L-excesses of aspartic acid and glutamic acid up to ~60% were measured in the Tagish Lake meteorite. Carbon isotope measurements of these amino acids confirm that the L-excesses are extraterrestrial in origin. Another common protein amino acid in Tagish Lake, alanine, was found to be nearly racemic. These results can be explained by differences in the solid–solution phase behavior of aspartic acid and glutamic acid, which can form conglomerate enantiopure solids during crystallization, and alanine, which can only form racemic crystals. Amplification of a small initial L-enantiomer excess during aqueous alteration on the meteorite parent body could have led to the large L-enrichments observed for aspartic acid and other conglomerate amino acids in Tagish Lake.

Evidence is Sourced from:

Title

Unusual nonterrestrial l-proteinogenic amino acid excesses in the Tagish Lake meteorite

Authors

GLAVIN, Daniel P. and ELSILA, Jamie E. and BURTON, Aaron S. and CALLAHAN, Michael P. and DWORKIN, Jason P. and HILTS, Robert W. and HERD, Christopher D. K.

Abstract

Abstract– The distribution and isotopic and enantiomeric compositions of amino acids found in three distinct fragments of the Tagish Lake C2-type carbonaceous chondrite were investigated via liquid chromatography with fluorescence detection and time-of-flight mass spectrometry and gas chromatography isotope ratio mass spectrometry. Large l-enantiomeric excesses (lee ∼ 43–59\%) of the α-hydrogen aspartic and glutamic amino acids were measured in Tagish Lake, whereas alanine, another α-hydrogen protein amino acid, was found to be nearly racemic (d ≈ l) using both techniques. Carbon isotope measurements of d- and l-aspartic acid and d- and l-alanine in Tagish Lake fall well outside of the terrestrial range and indicate that the measured aspartic acid enantioenrichment is indigenous to the meteorite. Alternate explanations for the l-excesses of aspartic acid such as interference from other compounds present in the sample, analytical biases, or terrestrial amino acid contamination were investigated and rejected. These results can be explained by differences in the solid–solution phase behavior of aspartic acid, which can form conglomerate enantiopure solids during crystallization, and alanine, which can only form racemic crystals. Amplification of a small initial l-enantiomer excess during aqueous alteration on the meteorite parent body could have led to the large l-enrichments observed for aspartic acid and other conglomerate amino acids in Tagish Lake. The detection of nonterrestrial l-proteinogenic amino acid excesses in the Tagish Lake meteorite provides support for the hypothesis that significant enantiomeric enrichments for some amino acids could form by abiotic processes prior to the emergence of life.

Citation

Download BibTeX   |   Download Endnote

Evidence

Enantiomeric excess in aqueous solutions is likely to be left behind and seep into the soil preserving it in high salt concentrations (e. g. evaporites). 

Amino acids iso-leucine and phenylalanine are widely used by organisms to build proteins. They were found with enantiomeric excess L-isomer in salt and sediment samples from the Atacama desert, a hyperarid Mars analog environment. These results support the idea that enantiomeric excess in biogenic amino acids tends to be preserved under hyperarid conditions.

Evidence is Sourced from:

Title

{Biosignature Analysis of Mars Soil Analogs from the Atacama Desert: Challenges and Implications for Future Missions to Mars}

Authors

Aerts, Joost W. and Riedo, Andreas and Melton, Daniel J. and Martini, Simone and Flahaut, Jessica and Meierhenrich, Uwe J. and Meinert, Cornelia and Myrgorodska, Iuliia and Lindner, Robert and Ehrenfreund, Pascale

Abstract

The detection of biosignatures on Mars is of outstanding interest in current Astrobiology and drives various fields of research, ranging from new sample collection strategies to the development of more sensitive detection techniques. Detailed analysis of the organic content in Mars analogue materials collected from extreme environments on Earth improves the current understanding of biosignature preservation and detection under conditions similar to Mars. In this paper we examined the biological fingerprint of several locations in the Atacama desert (Chile) which include different wet and dry, and intermediate to high elevation salt flats (also named salars). Liquid Chromatography and Multidimensional Gas Chromatography Mass Spectrometry measurement techniques were used for the detection and analysis of amino acids extracted from the salt crusts and sediments using sophisticated extraction procedures. Illumina 16S amplicon sequencing was used for the identification of microbial communities associated with the different sample locations. Although amino acid load and organic carbon and nitrogen quantities were generally low, it was found that most of the samples harbored complex and versatile microbial communities, which were dominated by (extremely) halophilic microorganisms (most notably by species of the Archaeal family Halobacteriaceae). The dominance of salts (i.e. halites and sulphates) in the investigated samples leaves its mark on the composition of the microbial communities but does not appear to hinder the potential of life to flourish since it can clearly adapt to the higher concentrations. Although the Atacama desert is one of the driest and harshest environments on Earth, it is shown that there are still sub-locations where life is able to maintain a foothold, and as such, salt flats could be considered as interesting targets for future life exploration missions on Mars.

Citation

Download BibTeX   |   Download Endnote

Evidence

Amino acids can be preserved for geologically long periods in sulfate mineral matrices 

Sulfate minerals show increased capability to preserve enantiomeric excess of amino acids over billions of years. The Martian soil was found to be rich in iron-oxide which provides a barrier against radiation, preserving organic material like amino acids and its enantiomeric excess, and this protection might be increased in deeper regolith. Further, the presence of sulfate minerals together with iron-oxide may produce an even greater shielding against radiation, and thus radiolysis. UV radiation is a large concern to the loss of enantiomeric excess, but results shows that a few millimeters of impure gypsum crust is opaque to radiation below 400nm, shielding the organic material (amino acids) against UV metal-catalyzed oxidation.

Evidence is Sourced from:

Title

Sulfate minerals and organic compounds on Mars

Authors

Aubrey, Andrew; Cleaves, H. James ; Chalmers, John H. ; Skelley, Alison M. ; Mathies, Richard A. ; Grunthaner, Frank J. ; Ehrenfreund, Pascale ; Bada, Jeffrey L.

Abstract

Strong evidence for evaporitic sulfate minerals such as gypsum and jarosite has recently been found on Mars. Although organic molecules are often codeposited with terrestrial evaporitic minerals, there have been no systematic investigations of organic components in sulfate minerals. We report here the detection of organic material, including amino acids and their amine degradation products, in ancient terrestrial sulfate minerals. Amino acids and amines appear to be preserved for geologically long periods in sulfate mineral matrices. This suggests that sulfate minerals should be prime targets in the search for organic compounds, including those of biological origin, on Mars.

Citation

Download BibTeX   |   Download Endnote

Evidence

Amino acids will not racemize on geologic timescales under dry conditions at temperatures <250K.

This evidence demonstrates that amino acids will not racemize on timescales equal to the age of the solar system when preserved cold and dry at temperatures <250K.

Evidence is Sourced from:

Title

Amino acid racemization on Mars: implications for the preservation of biomolecules from an extinct martian biota.

Authors

Bada JL,McDonald GD

Abstract

Using kinetic data, we have estimated the racemization half-lives and times for total racemization of amino acids under conditions relevant to the surface of Mars. Amino acids from an extinct martian biota maintained in a dry, cold (<250 K) environment would not have racemized significantly over the lifetime of the planet. Racemization would have taken place in environments where liquid water was present even for time periods of only a few million years following biotic extinction. The best preservation of both amino acid homochirality and nucleic acid genetic information associated with extinct martian life would be in the polar regions.

Citation

Download BibTeX   |   Download Endnote

Evidence

Twenty proteinaceous amino acids can survive in relatively large amounts a gamma-radiation dose of 14 MGy

All studied amino acids can survive a radiation dose of 14MGy, although certain fractions of them are lost as a result of radiolytic processes. Similarly, the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment.

Evidence is Sourced from:

Title

Amino acids in comets and meteorites: stability under gamma radiation and preservation of the enantiomeric excess

Authors

S. Iglesias-Groth, F. Cataldo, O. Ursini, and A. Manchado

Abstract

Amino acids in Solar system bodies may have played a key role in the chemistry that led to the origin of life on Earth. We present laboratory studies testing the stability of amino acids against high energy radiation. All the 20 chiral amino acids in the levo form found in the proteins of the current terrestrial biochemistry were irradiated in the solid state with γ-radiation to a dose of 3.2 MGy, which is the dose equivalent to that produced by radionuclide decay in comets and asteroids in 1.05 × 109 yr. For each amino acid the radiolysis degree and the radioracemization degree were measured by differential scanning calorimetry and by optical rotatory dispersion spectroscopy. From these measurements, a radiolysis rate constant kdsc and a radioracemization rate constant krac were determined for each amino acid and extrapolated to a dose of 14 MGy, which corresponds to the expected total dose delivered by natural radionuclide decay to all the organic molecules present in comets and asteroids in 4.6 × 109 yr, the age of the Solar system. It is shown that all the amino acids studied can survive a radiation dose of 14 MGy, although certain fractions of them are lost as a result of radiolytic processes. Similarly, the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. Knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentrations of the amino acids at the time of the formation of the Solar system, starting from the concentration found today in carbonaceous chondrites. For some amino acids the concentration in the pre-solar nebula could have been up to 6 times higher than that currently observed in meteorites. The preservation of an original enatiomeric excess is also expected. This study adds experimental support to the suggestion that amino acids were formed in the interstellar medium and in chiral excess and then were incorporated into comets and asteroids at the epoch of Solar system formation.

Citation

Download BibTeX   |   Download Endnote

Evidence

A fraction of the 20 proteinaceous amino acids can survive a massive radiation dose preserving also an enantiomeric enrichment.

The 20 amino acids studied survived a radiation dose of 3.2 MGy, although part of them is lost in radiolytic processes. Similarly also the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. The amino acids that showed the best level of radiolysis and radioracemization resistance are those commonly found in enantiomeric excess in meteorites.

Evidence is Sourced from:

Title

Radiolysis and radioracemization of 20 amino acids from the beginning of the Solar System

Authors

Franco Cataldo, Ornella Ursini, Giancarlo Angelini, Susana Iglesias-Groth, Arturo Manchado

Abstract

A series of chiral amino acids in the levo form used in the current terrestrial biochemistry were irradiated in the solid and dry state with γ-radiation to a dose of 3.2 MGy which is the dose equivalent to that derived by radionuclide decay in comets and asteroids in 1.05 × 109 years at a depth >20 m. For each amino acids, the radiolysis degree and the radioracemization degree was measured, respectively by differential scanning calorimetry (DSC) and by optical rotatory dispersion (ORD) spectroscopy. From these measurements, a radiolysis rate constant k dsc and a radioracemization rate constant k rac were determined for each amino acid and extrapolated to a dose of 14 MGy which corresponds to the expected total dose delivered by the natural radionuclides decay to all the organic molecules present in comets and asteroids in 4.6 × 109 years, the age of the Solar System. It is shown that all the amino acids studied survive a radiation dose of 3.2 MGy although part of them is lost in radiolytic processes. Similarly also the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. Even the extrapolation to 14 MGy corresponding to 4.6 × 109 years shows the partial survival of all amino acids studied and their enantiomeric enrichment. The knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentration of the amino acids at the times of the formation of the Solar System starting from the concentration found today in carbonaceous chondrites. Based on these results, it is not at all a surprise that amino acids have been found in meteorites and in measureable chiral excess. Furthermore, the amino acids showing the best level of radiolysis and radioracemization resistance are just those commonly found in enantiomeric enrichment in meteorites.

Citation

Download BibTeX   |   Download Endnote