Insights into Abiotically-Generated Amino Acid Enantiomeric Excesses Found in Meteorites

Abstract

Biology exhibits homochirality, in that only one of two possible molecular configurations (called enantiomers) is used in both proteins and nucleic acids. The origin of this phenomenon is currently unknown, as nearly all known abiotic mechanisms for generating these compounds result in equal (racemic) mixtures of both enantiomers. However, analyses of primitive meteorites have revealed that a number of amino acids of extraterrestrial origin are present in enantiomeric excess, suggesting that there was an abiotic route to synthesize amino acids in a non-racemic manner. Here we review the amino acid contents of a range of meteorites, describe mechanisms for amino acid formation and their potential to produce amino acid enantiomeric excesses, and identify processes that could have amplified enantiomeric excesses.

Possible Roles of Amphiphilic Molecules in the Origin of Biological Homochirality

“…Amino acid and sugar are thought to be important chiral organic molecules, since they are chiral building blocks of proteins and nucleic acids. It is interesting that many of the amino acids and sugars found in meteorites have excess L and D configurations, respectively [101,102]. The chirality coincides with the handedness of the building blocks of biopolymers of present life and the fact supports that the source of the organic molecules are extraterrestrial.…”

Evaluating Biosignatures for Life Detection

“…Significant excess of one enantiomer was interpreted as evidence of biological activity. The discovery of amino acids of abiotic origin in meteorites markedly biased toward one optical isomer (Pizzarello et al, 2012 ) is assigned high value as evidence contradicting biological origins of homochirality in general, even though the underlying mechanism might be highly specific to a narrow range of meteorites (Burton and Berger, 2018 ). A similar bias is also observed in evaluating probabilities.…”

Monosaccharides and Their Derivatives in Carbonaceous Meteorites: A Scenario for Their Synthesis and Onset of Enantiomeric Excesses

“…A couple of factors may point to an early indigenous racemic nature of the deoxy sugar acids: (i) although, from a glance at the structure of 2,3-erythro- and 2,3-threo-dihydroxybutyric acids, one could envision a mechanism whereby early ee could have been lost through racemization, e.g., via isomerization between adjacent (and polar) ‒C‒OH groups, this would still leave the question of why the same type of mechanism did not racemize the 4C erythronic and threonic acids ( Figure 5 ); and (ii) another deoxy sugar acid, 2-methylglyceric acid ( Figure 3 ), has a methyl group on the second carbon and would therefore have been very difficult to racemize if it ever possessed an ee , yet it is racemic while structurally comparable amino acids (i.e., 2-methyl-2-amino) show at least some ee in the vast majority of CCs [ 26 , 76 ].…”

“…As a proxy to possible reactions of polyols on meteorite parent bodies, we must use the much wider knowledge gained from the studies of meteoritic amino acids, in particular their abundance and ee distribution [ 26 ]. It appears that aqueous processes on meteorite parent bodies increased the l -enantiomeric excess of the rare amino acid isovaline [ 95 ], CH 3 ‒CH 2 ‒C(NH 2 )(CH 3 )‒CO 2 H. This amino acid, as with the diastereomeric polyols, is important in the study of meteoritic ee because it is very difficult to racemize, i.e., it has a methyl group instead of an easily exchangeable H bonded to the carbon (the “alpha”, α, carbon) next to the carboxyl group.…”

“…We will address this general question in the discussion of possible locales and origins of polyol ee ( Section 5 ). While amino acids have been the subject of a long history of analysis [ 9 , 26 ], there are few reports of polyol analysis in meteorites. In the following section we will focus on these publications, which include molecular identifications and the recent analysis of enantiomer ratios of individual polyols.…”