Molecular and isotopic analyses of the hydroxy acids, dicarboxylic acids, and hydroxicarboxylic acids of the Murchison meteorite

The hydroxymonocarboxylic acids, dicarboxylic acids, and hydroxydicarboxylic acids of the Murchison meteorite were analyzed as their tert-butyldimethylsilyl derivatives using combined gas chromatography-mass spectrometry. The hydroxydicarboxylic acids have not been found previously in meteorites. Each class of compounds is numerous with carbon chains up to C8 or C9 and many, if not all, chain and substitution position isomers represented at each carbon number. The alpha-hydroxycarboxylic acids and alpha-hydroxydicarboxylic acids correspond structurally to many of the known meteoritic alpha-aminocarboxylic acids and alpha-aminodicarboxylic acids, a fact that supports the proposal that a Strecker synthesis was involved in the formation of both classes of compounds. Isotopic analyses show these acids to be D-rich relative to terrestrial organic compounds as expected; however, the hydroxy acids appear to be isotopically lighter than the amino acids with respect to both carbon and hydrogen. The latter finding would not be expected if both classes of compounds came exclusively from common precursors as would have been the case for a Strecker synthesis.     

Linear and cyclic aliphatic carboxamides of the Murchison meteorite: hydrolyzable derivatives of amino acids and other carboxylic acids

Analyses of fractionated aqueous extracts of the Murchison meteorite by gas chromatography-mass spectrometry after silylation with N-methyl-N (tert-butyldimethylsilyl) trifluoroacetamide have revealed an extensive series of linear and cyclic aliphatic amides. These include monocarboxylic acid amides, dicarboxylic acid monoamides, hydroxy acid amides, lactams, carboxy lactams, lactims, N-acetyl amino acids, and substituted hydantoins. Numerous isomers and homologues through at least C8 were observed in all cases, except for the N-acetyl amino acids and hydantoins. Carboxy lactams, lactams, hydantoins, and N-acetyl amino acids are converted to amino acids by acid hydrolysis, thus, these compounds qualitatively account for the earlier observation of acid-labile amino acid precursors in meteoritic extracts. Laboratory studies of the spontaneous decomposition of N-carbamyl-alpha-amino acids and their dehydration products, the 5-substituted hydantoins, have led to the recognition of a series of aqueous phase reactions by which amino acids and cyanic acid/cyanate ion in the primitive parent body might have given rise to several of the observed classes of amides, as well as to monocarboxylic acids, dicarboxylic acids, and hydroxy acids. A previously undescribed reaction of 5-substituted hydantoins with cyanic acid/cyanate ion to give carboxamides of the 5-substituent groups was observed in the course of these studies. The presence of an extensive suite of amides in a CM chondrite appears to be consistent with the interstellar-parent body formation hypothesis for the organic compounds of these meteorites. The presence of carboxy lactams and lactams along with free amino acids suggests the possibility of further chemical evolution of meteorite amino acids by thermal polymerization. The cyclic amides, given their potential for hydrogen-bonded pair formation, might be considered candidate bases for a primitive sequence coding system.     

Stable isotope mass fragmentography: identification and hydrogen-deuterium exchange studies of eight Murchison meteorite amino acids

The concentration of eight protein amino acids found in extracts of the Murchison carbonaceous chondrite has been measured by quadrupole mass fragmentography. This result was obtained by using deuterated amino acids as internal standards. In addition, hydrogendeuterium exchange in amino acids was studied by two methods. First, nondeuterated amino acids were added to the meteorite and the amount of deuterium incorporated after extraction with deuterium oxide was determined. Second, deuterated amino acids were added to the dry meteorite and the loss of deuterium after extraction with H₂O was measured. It was observed that the degree of hydrogen-deuterium exchange increased with increasing severity of extraction conditions. This exchange resulted in some racemization, presumably catalyzed by constituents of the meteorite. The degree of racemization for each amino acid was determined by gas chromatography of the corresponding N-trifluoroacetyl-O-( + )-2-butyl esters.     

Iminodicarboxylic acids in the Murchison meteorite: Evidence of Strecker reactions

α-Amino acids and α-hydroxy acids are well known constituents of several carbonaceous meteorites. One proposed mechanism of their formation is the reactions of CN⁻, NH₃, aldehydes and ketones in aqueous solution, a Strecker-like synthesis. Iminodicarboxylic acids, relatively unusual in molecular structure, are significant by-products of laboratory Strecker syntheses of α-amino acids. It is therefore notable that an analogous suite of imino acids has not been reported in CM2 chondrites where amino and hydroxy acids are abundant. In this work, aqueous extracts of the Murchison meteorite were examined for the presence of imino acids; GC-MS and HPLC molecular analyses revealed a complex suite of such acids. With the exception of one of the seven-carbon members, all of the C4 through C7 imino acids were observed in Murchison. These observations suggest that the Strecker synthesis made, at least, some contribution to the formation of extraterrestrial amino acids.     

Summary and implications of reported amino acid concentrations in the Murchison meteorite

A study of literature reports of the concentrations of amino acids in extracts from the Murchison meteorite shows that many of the concentration ratios are constant. There are two possible interpretations of these ratios. One is that they are controlled by the pathways through which the amino acids formed, from which it follows that the amino acids are distributed in the same proportions throughout the meteorite. The other interpretation is that the ratios result from the analytical procedures used to extract the amino acids from the meteorite. These methods rely heavily on high-temperature (100 degrees C) aqueous extraction and subsequent high-temperature acid hydrolysis. A correlation was observed in the present study between the relative concentrations of several amino acids in the meteorite extracts and their relative aqueous solubilities at 100 degrees C (alanine, valine, leucine, isoleucine, norleucine, aspartic acid, glutamic acid and glycine). The extract solutions are dilute, and far from the saturation limits, but these correlations suggest that the sampling procedure affects directly the reported concentrations for these amino acids. Ratios of the concentration of serine to those of glycine are also constant but cannot be accounted for solely by relative solubilities, and, as suggested elsewhere, serine as well as phenylalanine and methionine may be terrestrial contaminants. Data for beta-alanine, alpha-aminobutyric acid, proline, sarcosine, alloisoleucine, beta-aminoisobutyric acid, beta-aminobutyric acid, and threonine also show constant abundances relative to glycine, but lack of solubility data at extraction conditions prohibits evaluating the extent of possible sampling bias for these amino acids. If the extraction process does not bias the results, and all extractable amino acids are removed from meteorite samples, then the properties of amino acids which control both their solubilities and their concentrations in the meteorite need to be established. The possibility of sampling bias needs to be tested experimentally before concluding that extraction is complete, and that the constant relative abundances indicate that the relative concentrations of amino acids are homogeneous in the meteorite.     

Non-racemic amino acids in the Murray and Murchison meteorites.

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.     

Amino acids of the Murchison meteorite: II. Five carbon acyclic primary β-, γ-, and δ-amino alkanoic acids

All ten of the possible five-carbon acyclic primary β-, γ-, and δ-amino alkanoic acids (amino position isomers of the valines) have been positively identified in hot-water extracts of the Murchison meteorite using combined gas chromatography-mass spectrometry (GC-MS) and ion exchange chromatography. With the exception of δ-aminovaleric acid, none of these amino acids has been previously reported to occur in meteorites or in any other natural material. The γ-amino acids (4-aminopentanoic acid, 4-aminc-2-meth-ylbutanoic acid, and 4-amino-3-methylbutanoic acid) are present at higher concentrations (about 5 nmol g^?1) than are the β-amino isomers (3-aminopentanoic acid, 3-amino-2-methylbutanoic acid, allo-3-amino-2-methylbutanoic acid, 3-amino-3-methylbutanoic acid, 3-amino-2-ethylpropanoic acid, and 3-amino-2,2-dimethylpropanoic acid) which are present at concentrations of 1–2 nmol g^?1. These amino acids are less abundant in the meteorite than either the corresponding α-amino acids or the four-carbon homologues. Thirty-six amino acids have now been positively identified in the Murchison meteorite, 17 of which are apparently unique to carbonaceous chondrites. The fact that the meteorite contains all possible five-carbon acyclic primary α-, β-, γ-, and δ-amino alkanoic acids is consistent with a synthetic process involving random combination of single-carbon precursors.     

Nonracemic isovaline in the Murchison meteorite: chiral distribution and mineral association

The enantiomeric and carbon-isotopic composition of the amino acid isovaline have been analyzed in several samples of the Murchison meteorite and one sample of the Murray meteorite. l-Enantiomeric excesses of the amino acid were found to range from 0 to 15.2%, varying significantly both between meteorite stones and at short distances within a single stone. The upper limit of this range is the largest enantiomeric excess measured to date for a biologically rare meteoritic amino acid and raises doubts that circularly polarized light irradiation could have been the sole cause of amino acids chiral asymmetry in meteorites. Individual d- and l-isovaline δ¹³C values ware found to be about +18‰, with no significant differences between the two enantiomers to suggest terrestrial contamination. The amino acid relative abundance also varied between samples, with isovaline/alanine ratios of 0.5 to 6.5. X-ray diffraction analyses of contiguous meteorite fragments suggest a possible correlation between isovaline and hydrous silicates abundances.     

Molybdenum Isotopic Composition of Individual Presolar Silicon Carbide Grains from the Murchison Meteorite

We report the isotopic composition of molybdenum in twenty-three presolar SiC grains from the Murchison meteorite which have been measured by resonant ionization mass spectrometry (RIMS). Relative to terrestrial abundance (and normalized to s-process-only ⁹⁶Mo), the majority of the analyzed grains show strong depletions in the p-process isotopes ⁹²Mo and ⁹⁴Mo and the r-process isotope ¹⁰⁰Mo. Sixteen of these grains have δ-values <−600% for these three isotopes. The observed isotopic patterns of Mo from mainstream SiC grains clearly reveal the signature of s-process nucleosynthesis. Three-isotope plots of all grain data (δⁱMo vs. δ⁹²Mo) show strong linear correlations with characteristic slopes. This finding suggests mixing of solar-like material and pure s-process material in the parent stars. Comparison with evolutionary calculations of nucleosynthesis and mixing in red giants suggests that low-mass thermally-pulsed symptotic giant branch (TP-AGB) stars are the most likely site for the observed s-process nucleosynthesis.     

Gradual and stepwise pyrolyses of insoluble organic matter from the Murchison meteorite revealing chemical structure and isotopic distribution

To study the detailed structural and isotopic heterogeneity of the insoluble organic matter (IOM) of the Murchison meteorite, we performed two types of pyrolytic experiments: gradual pyrolysis and stepwise pyrolysis. The pyrolysates from the IOM contained 5 specific organic groups: aliphatic hydrocarbons, aromatic hydrocarbons, sulfur-bearing compounds, nitrogen-bearing compounds, and oxygen-bearing compounds. The release temperatures and the compositions of these pyrolysates demonstrated that the IOM is composed of a thermally unstable part and a thermally stable part. The thermally unstable part mainly served as the linkage and substituent portion that bound the thermally stable part, which was dispersed throughout the IOM. The linkage and substituent portion consisted of aliphatic hydrocarbons from C₄ to C₈, aromatic hydrocarbons with up to 6 rings, sulfo and thiol groups (the main reservoirs of sulfur in the IOM), and carboxyl and hydroxyl groups (the main reservoirs of oxygen). However, the thermally stable part was composed of polycyclic aromatic hydrocarbons (PAHs) containing nitrogen heterocycles in the IOM. Isotopic data showed that the aliphatic and aromatic hydrocarbons in the linkage and substituent portion were rich in D and ¹³C, while the thermally stable part was deficient in D and ¹³C. The structural and isotopic features suggested that the IOM was formed by mixing sulfur- and oxygen-bearing compounds rich in D and ¹³C (e.g., polar compounds in the interstellar medium (ISM)) and nitrogen-bearing PAHs deficient in D and ¹³C (e.g., polymerized compounds in the ISM).     

Small SiC grains and a nitride grain of circumstellar origin from the Murchison meteorite: implications for stellar evolution and nucleosynthesis

We report the results of SIMS isotopic analyses of carbon, nitrogen, oxygen, and silicon made on 849 small (approximately 1 micrometer) individual silicon carbide grains from the Murchison meteorite. The isotopic compositions of the major elements carbon and silicon of most grains (mainstream) are similar to those observed in larger grain studies suggesting an AGB star origin of these grains. In contrast, the trace element nitrogen shows a clear dependency on grain size. 14N/15N ratios increase with decreasing grain size, suggesting different stellar sources for grains of different size. Typically observed 14N/15N ratios in the small grains of this study are approximately 2700, clearly larger than the values expected from model calculations of AGB stars. In addition to the three dredge-up episodes characteristic for the evolution of AGB stars, extra-mixing of CNO-processed matter in low mass AGB stars appears to be a promising possibility in order to explain the high 14N/15N ratios of the small circumstellar SiC grains. A small fraction of grains shows a silicon isotopic signature not observed in larger circumstellar SiC grains from Murchison. Their stellar origin is still uncertain. The minor type A, B, Y, and X grains were found to be present at a level of a percent, which is similar to their abundance in the larger-grain SiC separates from Murchison. Oxygen isotopic compositions are normal within the experimental uncertainties of several 10%, indicating that oxygen of stellar origin is rare or even absent in the SiC grains. We conclude that most of the oxygen is a contaminant which was introduced into the SiC grains after their formation, e.g., during sample processing in the laboratory. We identified a nitride grain, most likely Si3N4 with little carbon, with highly anomalous isotopic compositions (12C/13C = 157 +/- 33, 14N/15N = 18 +/- 1, delta 29 Si = -43 +/- 56%, delta 30 Si = -271 +/- 50%). The isotopic patterns of carbon, nitrogen, and silicon resemble those of the rare SiC X grains suggesting that these two rare constituents of circumstellar matter formed in the same type of stellar source, namely, Type II supernovae.     

A comparative study of the hydroxy acids from the Murchison, GRA 95229 and LAP 02342 meteorites

The hydroxy acid suites extracted from the Murchison (MN), GRA 95229 (GRA) and LAP 02342 (LAP) meteorites have been investigated for their molecular, chiral and isotopic composition. Substantial amounts of the compounds have been detected in all three meteorites, with a total abundance that is lower than that of the amino acids in the same stones. Overall, their molecular distributions mirror closely that of the corresponding amino acids and most evidently so for the LAP meteorite. A surprising l-lactic acid enantiomeric excess was found present in all three stones, which cannot be easily accounted by terrestrial contamination; all other compounds of the three hydroxy acid suites were found racemic. The branched-chain five carbon and the diastereomer six-carbon hydroxy acids were also studied vis-a-vis the corresponding amino acids and calculated ab initio thermodynamic data, with the comparison allowing the suggestion that meteoritic hydroxyacid at these chain lengths formed under thermodynamic control and, possibly, at a later stage than the corresponding amino acids. ¹³C and D isotopic enrichments were detected for many of the meteoritic hydroxy acids and found to vary between molecular species with trends that also appear to correlate to those of amino acids; the highest δD value (+3450‰) was displayed by GRA 2-OH-2-methylbutyric acid. The data suggest that, while the amino- and hydroxy acids likely relate to common presolar precursor, their final distribution in meteorites was determined to large extent by the overall composition of the environments that saw their formation, with ammonia being the determining factor in their final abundance ratios.     

Solid-state (H and C) nuclear magnetic resonance spectroscopy of insoluble organic residue in the Murchison meteorite: a self-consistent quantitative analysis

Complementary, double- and single-resonance solid-state (¹H and ¹³C) nuclear magnetic resonance (NMR) experiments were performed on a solvent extracted and demineralized sample of Murchison meteorite organic macromolecule. These NMR data provide a consistent picture of a complex organic solid composed of a wide range of organic (aromatic and aliphatic) functional groups, including numerous oxygen-containing functional groups. The fraction of aromatic carbon within the Murchison organic residue (constrained by three independent experiments) lies between 0.61 and 0.66. The close similarity in cross-polarized and single-pulse spectra suggests that both methods detect the same distribution of carbon. With the exception of interstellar diamond (readily detected in slow magic angle spinning single-pulse NMR experiments), there is no evidence in the solid-state NMR data for a significant abundance of large laterally condensed aromatic molecules in the Murchison organic insoluble residue. Given the most optimistic estimation, such carbon would not exceed 10% and more likely is a fraction of this maximum estimate. The fraction of aromatic carbon directly bonded to hydrogen is low (∼30%), indicating that the aromatic molecules in the Murchison organic residue are highly substituted. The bulk hydrogen content, H/C, derived from NMR data, ranges from a low of 0.53 ± 0.06 and a high of 0.63 ± 0.06. The hydrogen content (H/C) determined via elemental analysis is 0.53. The range of oxygen-containing organic functionality in the Murchison is substantial. Depending on whether various oxygen-containing organic functional groups exist as free acids and hydroxyls or are linked as esters and ethers results in a wide range in O/C (0.22 to 0.37). The lowest values are more consistent with elemental analyses, requiring that oxygen-containing functional groups in the Murchison macromolecule are highly linked. The combined ¹H and ¹³C NMR data reveal a high proportion of methine carbon, which requires that carbon chains within the Murchison organic macromolecule are highly branched.     

The lipid chemistry of an interfacial sediment from the Peru Continental Shelf: Fatty acids,alcohols, aliphatic ketones and hydrocarbons

A sample of the sediment-water column interface which lies on the continental shelf under the Peru upwelling regime, has been examined for fatty acids, fatty alcohols, ketones and hydrocarbons. Fatty acids were the most abundant compound class, ranging from C₁₂-C₂₄, with 16:0 as the major component (765.5 μg/g dry sediment). The alcohols were dominated by 3,7,11,15-tetramethylhexadeca-2-en-ol (phytol), with even-chain n-alcohols in the range C₁₄-C₂₀. The ketones consisted of C₃₇-C₃₉ di- and tri-unsaturated alken-2-ones and alken-3-ones. Both alkanes and alkenes were present in the hydrocarbon fraction; the alkanes ranging from C₁₃ — C₂₀ and comprising both straight chain and isoprenoid compounds; the alkenes consisting of isomeric pairs of C₂₅ branched trienes and tetraenes. The data indicate that the organic content has been contributed very largely from marine sources (probably mainly from phytoplankton and bacteria), showing little terrigenous influence. The presence of labile compounds such as polyunsaturated fatty acids (with two to six double bonds), implies that the sediment has undergone very little diagenetic alteration, and the lipids are probably largely unchanged from the state in which they actually reached the sediment. They may therefore serve as a useful baseline in assessing diagenesis in older sediments, where diagenetic transformations are more advanced.     

Hydropyrolysis of insoluble carbonaceous matter in the Murchison meteorite: new insights into its macromolecular structure

The major organic component of carbonaceous chondrites is a solvent-insoluble, high molecular weight macromolecular material that constitutes at least 70% of the total organic content in these meteorites. Analytical pyrolysis is often used to thermally decompose macromolecular organic matter in an inert atmosphere into lower molecular weight fragments that are more amenable to conventional organic analytical techniques. Hydropyrolysis refers to pyrolysis assisted by high hydrogen gas pressures and a dispersed catalytically-active molybdenum sulfide phase. Hydropyrolysis of meteorites has not been attempted previously although it is ideally suited to such studies due to its relatively high yields. Hydropyrolysis of the Murchison macromolecular material successfully releases significant amounts of high molecular weight PAH including phenanthrene, carbazole, fluoranthene, pyrene, chrysene, perylene, benzoperylene and coronene units with varying degrees of alklyation. Analysis of both the products and residue from hydropyrolysis reveals that the meteoritic organic network contains both labile (pyrolysable) and refractory (nonpyrolysable) fractions. Comparisons of hydropyrolysis yields of Murchison macromolecular materials with those from terrestrial coals indicate that the refractory component probably consists of a network dominated by at least five- or six-ring PAH units cross-linked together.