Isomers are compound with the same molecular formula but with a different structural arrangement of atoms, leading to different properties. There are several types of isomers, including structural isomers, stereoisomers, and geometric isomers. Enantiomers are a type of isomers that are non-superimposable mirror images of each other, often involving a chiral center (an atom, usually carbon, with four different substituents). By convention, one enantiomer is labeled left-handed (L) and the other is labeled right-handed (R). Earth life uses L-amino acids to form proteins. As a result, mixtures of those biological amino acids are said to have high enantiomeric excess, and sometimes, but not always, they display perfect enantiomeric excess (ee = 100%). In contrast, enantiomeric ratios in abiotic amino acid mixtures tend to be racemic (ee = 0%) or have low enantiomeric excess. This reflects abiotic reaction kinetics and thermodynamics. This distinction can serve as a signature of life.
Abundance in the prebiotic environment: Amino acids are abundant in a diversity of prebiotically-relevant environments such as primitive oceans, meteorites, and hydrothemal systems. This increases the likelihood that they will be incorporated into incipient biochemistries. [General]
EnvironmentsEvolutionary forces: Enzymes catalyze reactions most effectively when interacting with homochiral substrates. Since evolutionary processes will favor biochemical economy and efficiency life will always tend to form homochiral polymers. In the case of peptides and proteins, this will lead to amino acid mixtues with enantiomeric excess. [General]
EnvironmentsProtein functionality: Functional proteins and peptides must be homochiral to ensure proper folding, stability, and biological activity. Homochirality allows consistent interactions between amino acids, enabling the formation of specific 3D structures essential for the protein's function. This results in biotic amino acid mixtures with enantiomeric excess. [General]
EnvironmentsProtein functionality: There are known cases of functional proteins and peptides that are not homochiral. This suggests that life forms could exists, perhaps shortly after the origin of life, that generate amino acid mixtures with low enantiomeric excess. [General]
EnvironmentsEnzymatic specificity: Protein making machinery is optimized to work with homochiral amino acids, that is, with amino acids of the same handedness, resulting in mixtures that have enantiomeric excesses. [General]
EnvironmentsEnzymatic specificity: It is possible for the ribosome to use amino acids of the opposite handedness during protein synthesis, thereby lowering the enantiomeric excess of the resulting amino acid mixtures. [General]
EnvironmentsUse of amino acids of opposite handedness in specialized roles: Not all mixtures of terrestrial biological amino acids are homochiral because some D-amino acids are crucial in specific roles such as structural support of cell walls. Thus, the existence of D-amino acids and the enzymatic processes creating them in Earth biochemistry demonstrate that biological systems can function with non-homochiral amino acid mixtures. [General]
EnvironmentsBiologial activity: If extant life is present, it will enzymatically maintain an enantiomeric excess in the environment by the recycling of free amino acids that have racemized. [Production, Survivability]
EnvironmentsBiological activity: If extant life is present, it may decrease the enantiomeric excess in the environment through its capability to enzymatically racemize free amino acids. [Production, Survivability]
EnvironmentsPreservation under cold and dry conditions: Extremely cold and dry conditions on the surface of ocean worlds will help preserve the enantiomeric ratios of amino acids for geologic timescales. [Survivability]
EnvironmentsPreservation under cold and dry conditions: Amino acids near the surface of Mars (top 2 meters) will degrade relatively fast from exposure to ionizing radiation, which may limit their detectability. [Survivability]
EnvironmentsPreservation in liquid water: The enantiomeric excess of amino acids within fluids will be lost over time. [Survivability]
EnvironmentsProduction of minor enantiomers: There can be substantial production of free amino acids of the minor enantiomer (on Earth D-amino acids) for non-enzymatic roles which can lower the overall enantiomeric ratios in biological amino acid mixtures. [Production, Survivability]
EnvironmentsEffects of ionizing radiation: Enantiomeric excess will be preserved under ionizing radiation over geologic timescales, even if overall amino acid levels are reduced by radiolysis [Survivability]
EnvironmentsAbundance in the prebiotic environment: Amino acids are abundant in a diversity of prebiotically-relevant environments such as primitive oceans, meteorites, and hydrothemal systems. This increases the likelyhood that they will be incorporated into incipient biochemistries. [General]
EnvironmentsAbiotic chirality amplification: There are abiotic chemical reactions that can preferentially amplify one chiral form of amino acids. The resulting abiotic amino acid mixtures can have a relatively large enantiomeric excesses. [General]
EnvironmentsEnantiomeric excess in meteorites: Some abiotic amino acids found in carbonaceous chondrite meteorites have a preferential abundance of one chiral form, which can lead to amino acid mixtures with enantiomeric excess. [General]
EnvironmentsEnantiomeric excess in meteorites: Most abiotic amino acids found in carbonaceous chondrites are racemic, and the resulting amino acid mixtures have relatively low enantiomeric excess. [General]
EnvironmentsPreservation under cold and dry conditions: Extremely cold and dry conditions on the surface of ocean worlds will help preserve the enantiomeric ratios of amino acids for geologic timescales. [Survivability]
EnvironmentsPreservation in liquid water: Enantiomeric enrichments in abiotic alpha-amino acids will not be preserved for geologic timescales in an environment under wet conditions [Survivability]
EnvironmentsEffects of hydrolysis during sample preparation: Hydrolysis can promote racemization of amino acids and large enantiomeric excesses can be converted into a racemic mixture of equal amounts [Survivability]
EnvironmentsEffects of ionizing radiation: Enantiomeric enrichments in abiotic, chiral alpha-amino acids will be preserved for geologic timescales even after exposure to ionizing radiation [Survivability]
EnvironmentsEffects of ionizing radiation: Enantiomeric enrichments in some abiotic chiral alpha-amino acids will decay over geologic timescales due to exposure to ionizing radiation (i.e., radioracemization) [Survivability]
Environments