Extraterrestrial hydroxy amino acids in CM and CR carbonaceous chondrites

Abstract

The abundances, distributions, and enantiomeric ratios of a family of three‐ and four‐carbon hydroxy amino acids (HAAs) were investigated in extracts of five CM and four CR carbonaceous chondrites by gas chromatography‐mass spectrometry analyses. HAAs were detected in both the acid hydrolysates of the hot water extracts and the 6 M HCl extracts of all the CM and CR chondrites analyzed here with total hot water and HCl extractable HAA concentrations ranging from 6.94 to 315 nmol g−1. The HAA analyses performed in this study revealed: (1) the combined (hot water + HCl) extracts of CR2 chondrites contained greater abundances of α‐HAAs than that of CM2 chondrites and (2) the combined extracts of CM and CR chondrites contained roughly similar abundances of β‐ and γ‐HAAs. Application of the new GC‐MS method developed here resulted in the first successful chromatographic resolution of the enantiomers of an α‐dialkyl HAA, d,l‐α‐methylserine, in carbonaceous chondrite extracts. Meteoritic α‐methylserine was found to be mostly racemic within error and did not show l‐enantiomeric excesses correlating with the degree of aqueous alteration, a phenomenon observed in meteoritic isovaline, another α‐dialkyl amino acid. The HAAs identified in CM and CR chondrite extracts could have been produced during parent body alteration from the Strecker cyanohydrin reaction (for α‐HAAs) and an ammonia‐involved formose‐like reaction (for β‐, and γ‐HAAs).

Synthesis of Amino Acids from Aldehydes and Ammonia: Implications for Organic Reactions in Carbonaceous Chondrite Parent Bodies

“…Various amino acids were identified, including glycine, alanine, β-alanine, and C 3 and C 4 hydroxy amino acids (HAAs) [hydroxyl (−OH) group-bearing amino acids] with α-, β-, and γ-structural isomers. Although the distribution of HAAs in carbonaceous chondrites has received little attention except for proteinogenic amino acids serine and threonine, we previously detected a family of α-, β-, and γ-HAAs in various CM and CR chondrites. , The ubiquity of α-, β-, and γ-HAAs in carbonaceous chondrites suggests that a similar mechanism as in our experiments using aldehydes and ammonia could have proceeded in the meteorite parent bodies.…”

“…In the same manner, isoserine (β-HAA) or 4-A-2-HBA (γ-HAA) may also form from the reaction of ammonia with 2-hydroxy-3-oxopropanoic acid (β-oxoacid) or 2-hydroxy-4-oxobutanoic acid (γ-oxoacid), respectively (Figure ). Therefore, the reaction between oxoacids and ammonia can be hypothesized to produce various amino acids, including the α-, β-, and γ-HAAs found in CM and CR carbonaceous chondrites. , Note that the hydrolysis experiment of N -oxalylglycine demonstrated that the precursor compound gradually hydrolyzes at 60 °C. In the case of organic reactions in meteorite parent bodies, N -oxalylglycine and similar precursor compounds (Figure ) are likely to hydrolyze completely to form amino acids after prolonged aqueous alteration (e.g., 5–20 Myr) in meteorite parent bodies .…”

“…The hydroxy radicals produced by photolysis and radiolysis of H 2 O are a candidate oxidizing reagent . The CM chondrite parent bodies appear to be more conducive to oxidation than CR chondrites based on their elemental distributions , and mineralogy. , The distribution of HAA structural isomers in carbonaceous chondrites may also imply that a formose-like reaction was more prominent in the parent bodies of CM chondrites than in CR chondrites . Further investigation is needed to reveal the relationship between the predominance of the ammonia-involved formose-like reaction and the oxidizing conditions promoted by molecular oxygen and/or other oxidizing reagents in the meteorite parent body.…”

“…If this hypothesis is true, it would be possible to describe the formation pathways of α-, β-, and γ-amino acids from the corresponding α-, β-, and γ-oxoacids and ammonia. Therefore, an ammonia-involved formose-like reaction can be considered a plausible formation mechanism that comprehensively explains the distribution of meteoritic amino acids, including HAAs found in CM and CR chondrites . In future investigations, the proposed formation pathways for meteoritic amino acids need to be demonstrated via the detection of precursor compounds, including N -oxalylglycine, in the least aqueously altered carbonaceous chondrites.…”

“…The delivery of meteoritic amino acids to early Earth may have played an important role in prebiotic chemistry before the origin of life . In the past 50 years, advances in chromatography and mass spectroscopy have facilitated the evaluation of the distributions of structural isomers and enantiomers of amino acids in various types of carbonaceous chondrites. − In particular, excess abiotic L -enantiomers of amino acids have been found only in carbonaceous chondrites, which implies a potential relationship with the origin of l -amino acid homochirality in extant life. Hence, understanding the formation of meteoritic amino acids would provide valuable insights into the prebiotic chemistry that allowed life to emerge on early Earth.…”