User:SpaceStones/Carbonaceous chondrite

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Organic matter[edit]

Murchison meteorite

Most of the organic carbon in CI and CM carbonaceous chondrites is an insoluble complex material. That is similar to the description for kerogen. A kerogen-like material is also in the ALH84001 Martian meteorite (an achondrite).

The CM meteorite Murchison has over 96 extraterrestrial amino acids and other compounds including carboxylic acids, hydroxy carboxylic acids, sulphonic and phosphonic acids, aliphatic, aromatic and polar hydrocarbons, fullerenes, heterocycles, carbonyl compounds, alcohols, amines and amides.

Extraterrestrial Amino Acids[edit]

Amino acids in carbonaceous chondrites have important implications for theories describing the delivery of organic compounds to the early Earth and the subsequent development of life. Shortly after its fall and recovery in Australia in 1969, the Murchison meteorite was found to host five protein amino acids (glycine, alanine, valine, proline, and glutamic acid) in addition to 12 non-proteinogenic amino acids including α-aminoisobutyric acid and isovaline which are rare on Earth.[1] Since then, the number of characterized amino acids in the Murchison meteorite has risen to 96, including 12 of the 20 common biological amino acids, along with hundreds more that have been detected, but remain uncharacterized.[2] While the abundance of amino acids present in terrestrial soils presents a potential source of contamination, most of the amino acids characterized in Murchison are terrestrially rare or absent.[3]

Amino acids may be structurally chiral, meaning that they have two possible non-superimposable mirror image structures, termed enantiomers. Conventionally, these are referred to as left-handed (L) and right-handed (D) by analogy with glyceraldehyde. Living beings use L-amino acids, although there is no apparent reason why one enantiomer is favoured over the other as they behave equivalently in biological systems.[4] In contrast with terrestrial biology, early laboratory studies, including the famous Miller-Urey Experiment, have shown that amino acids may form under a range of possible abiotic conditions with equal (racemic) mixtures of D- and L-enantiomers.[5] Thus, the ratios between enantiomers for a given amino acid may discriminate between biotic and abiotic formation mechanisms. In the first characterization of amino acids in Murchison, all chiral examples were present in racemic mixtures indicating an abiotic origin.[1] This is consistent with proposed sythetic pathways, as the formation of isovaline and other α-dialkyl amino acids in CM chondrites has been attributed to the Strecker synthesis which produces racemic mixtures of enantiomers.[6]

The Strecker synthesis of alpha amino acids from carbonyl compounds in the presence of ammonia and cyanide.

Ehrenfreund et al. (2001)[7] found that amino acids in CI chondrites Ivuna and Orgueil were present at much lower concentrations than in CM chondrites (~30%), and that they had a distinct composition high in β-alanine, glycine, γ-ABA, and β-ABA but low in α-aminoisobutyric acid (AIB) and isovaline. This implies that they had formed by a different synthetic pathway, and on a different parent body from the CM chondrites.

Enantiomeric excesses observed in extraterrestrial amino acids[edit]

More recently, identification of amino acids from several meteorites with significant L-enantiomeric excesses has shifted our current understanding of prebiotic chemistry. L-excesses from 3 – 15% in several non-protein α-dialkyl amino acids have been found in the Murchison and Murray meteorites.[8] Their extraterrestrial origin is indicated by their absence in biological systems and significant heavy isotope enrichments in 13C and deuterium compared to terrestrial values.[9] Further characterization of L-isovaline excesses up to 20.5% in a range of carbonaceous chondrite groups have supported a hypothesis that increasing hydrothermal alteration of the host meteorite correlates with increasing observed L-enantiomeric excess.[10] Large L-excesses for α-H amino acids have also been reported, but these are more problematic due to the potential for terrestrial contamination.[11] The ungrouped C2 chondrite Tagish Lake has L-aspartic acid excesses up to ~60%, with carbon isotope measurements indicating an extraterrestrial origin due to significant enrichments in 13C.[12] In Tagish Lake, proteinogenic amino acids show both significant L-excesses, and racemic mixtures: glutamic acid, serine, and threonine were found to have ~50 – 99% L-excesses, while alanine was racemic.[12]

It has been proposed that extraterrestrial amino acid L-excesses observed in carbonaceous chondrites are a result of differences in the crystallization behaviour of the enantiomers.[13] Circularly polarized ultraviolet light has been shown to generate L-excesses in crystallizing amino acids for experimental conditions mimicking alteration on asteroids, and this is thought to be the dominant extraterrestrial source of chiral symmetry breaking (i.e., the favouring of one enantiomer over another).[14] It is notable that only excesses of the L-enantiomer have been observed in extraterrestrial amino acids, suggesting that the abiotic process responsible for enantiomeric enrichments may be the original source of the L-amino acid selectivity currently observed in terrestrial life.

Implications for extraterrestrial biosignatures[edit]

NASA have proposed a “Ladder of Life Detection” threshold of >20% enantiomeric excess in amino acids to distinguish extraterrestrial biosignatures. But, as previously mentioned, recent studies of carbonaceous chondrites and complementary experimental investigations have demonstrated that even larger enantiomeric excesses may be produced by abiotic pathways. To identify chiral asymmetry (enantiomeric excess) of biological origin, Glavin et al. (2020)[13] emphasize three criteria that must be met: chiral asymmetry, light 13C isotopic composition, and simplified distribution of structural isomers. If a distribution of amino acids in an extraterrestrial sample is found to be chirally asymmetric, display structural isomeric preference, and carry 13C, 15N, and D depletions relative to associated inorganic material, a compelling case may be made for its biological origin. With the current interest in sample return missions from carbonaceous asteroids (e.g., OSIRIS-REx) and Mars headed by NASA and other space agencies , the subsequent analysis of returned samples devoid of terrestrial contamination will provide the best opportunity to discover potential biosignatures in our Solar System.

References[edit]

  1. ^ a b Kvenvolden, Keith; Lawless, James; Pering, Katherine; Peterson, Etta; Flores, Jose; Ponnamperuma, Cyril; Kaplan, I. R.; Moore, Carleton (1970). "Evidence for Extraterrestrial Amino-acids and Hydrocarbons in the Murchison Meteorite". Nature. 228 (5275): 923–926. doi:10.1038/228923a0. ISSN 1476-4687.
  2. ^ Glavin, Daniel P.; Alexander, Conel M. O'D.; Aponte, José C.; Dworkin, Jason P.; Elsila, Jamie E.; Yabuta, Hikaru (2018-01-01), Abreu, Neyda (ed.), "Chapter 3 - The Origin and Evolution of Organic Matter in Carbonaceous Chondrites and Links to Their Parent Bodies", Primitive Meteorites and Asteroids, Elsevier, pp. 205–271, doi:10.1016/b978-0-12-813325-5.00003-3, ISBN 978-0-12-813325-5, retrieved 2023-05-01
  3. ^ Cronin, John R.; Chang, Sherwood (1993), Greenberg, J. M.; Mendoza-Gómez, C. X.; Pirronello, V. (eds.), "Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite", The Chemistry of Life’s Origins, Dordrecht: Springer Netherlands, pp. 209–258, doi:10.1007/978-94-011-1936-8_9, ISBN 978-94-011-1936-8, retrieved 2023-05-01
  4. ^ Milton, R. C. deL.; Milton, S. C. F.; Kent, S. B. H. (1992-06-05). "Total Chemical Synthesis of a D-Enzyme: The Enantiomers of HIV-1 Protease Show Reciprocal Chiral Substrate Specificity". Science. 256 (5062): 1445–1448. doi:10.1126/science.1604320. ISSN 0036-8075.
  5. ^ Miller, Stanley L. (1953-05-15). "A Production of Amino Acids Under Possible Primitive Earth Conditions". Science. 117 (3046): 528–529. doi:10.1126/science.117.3046.528. ISSN 0036-8075.
  6. ^ Wolman, Yecheskel; Haverland, William J.; Miller, Stanley L. (1972). "Nonprotein Amino Acids from Spark Discharges and Their Comparison with the Murchison Meteorite Amino Acids". Proceedings of the National Academy of Sciences. 69 (4): 809–811. doi:10.1073/pnas.69.4.809. ISSN 0027-8424. PMC 426569. PMID 16591973.{{cite journal}}: CS1 maint: PMC format (link)
  7. ^ Ehrenfreund, Pascale; Daniel P. Glavin; Oliver Botta; George Cooper; Jeffrey L. Bada (2001). "Extraterrestrial amino acids in Orgueil and Ivuna: Tracing the parent body of CI type carbonaceous chondrites". Proceedings of the National Academy of Sciences. 98 (5): 2138–2141. Bibcode:2001PNAS...98.2138E. doi:10.1073/pnas.051502898. PMC 30105. PMID 11226205.
  8. ^ Cronin, John R.; Pizzarello, Sandra (1997-02-14). "Enantiomeric Excesses in Meteoritic Amino Acids". Science. 275 (5302): 951–955. doi:10.1126/science.275.5302.951. ISSN 0036-8075.
  9. ^ Elsila, Jamie E.; Callahan, Michael P.; Glavin, Daniel P.; Dworkin, Jason P.; Brückner, Hans (2011). "Distribution and Stable Isotopic Composition of Amino Acids from Fungal Peptaibiotics: Assessing the Potential for Meteoritic Contamination". Astrobiology. 11 (2): 123–133. doi:10.1089/ast.2010.0505. ISSN 1531-1074.
  10. ^ Glavin, Daniel P.; Callahan, Michael P.; Dworkin, Jason P.; Elsila, Jamie E. (2010). "The effects of parent body processes on amino acids in carbonaceous chondrites: Amino acids in carbonaceous chondrites". Meteoritics & Planetary Science. 45 (12): 1948–1972. doi:10.1111/j.1945-5100.2010.01132.x.
  11. ^ Glavin, Daniel P.; Elsila, Jamie E.; McLain, Hannah L.; Aponte, José C.; Parker, Eric T.; Dworkin, Jason P.; Hill, Dolores H.; Connolly, Harold C.; Lauretta, Dante S. (2021). "Extraterrestrial amino acids and L‐enantiomeric excesses in the CM 2 carbonaceous chondrites Aguas Zarcas and Murchison". Meteoritics & Planetary Science. 56 (1): 148–173. doi:10.1111/maps.13451. ISSN 1086-9379.
  12. ^ a b Glavin, Daniel P.; Elsila, Jamie E.; Burton, Aaron S.; Callahan, Michael P.; Dworkin, Jason P.; Hilts, Robert W.; Herd, Christopher D. K. (2012). "Unusual nonterrestrial l-proteinogenic amino acid excesses in the Tagish Lake meteorite: l-amino acid excesses in the Tagish Lake meteorite". Meteoritics & Planetary Science. 47 (8): 1347–1364. doi:10.1111/j.1945-5100.2012.01400.x.
  13. ^ a b Glavin, Daniel P.; Burton, Aaron S.; Elsila, Jamie E.; Aponte, José C.; Dworkin, Jason P. (2020-06-10). "The Search for Chiral Asymmetry as a Potential Biosignature in our Solar System". Chemical Reviews. 120 (11): 4660–4689. doi:10.1021/acs.chemrev.9b00474. ISSN 0009-2665.
  14. ^ Garcia, Adrien D.; Meinert, Cornelia; Sugahara, Haruna; Jones, Nykola C.; Hoffmann, Søren V.; Meierhenrich, Uwe J. (2019-03-16). "The Astrophysical Formation of Asymmetric Molecules and the Emergence of a Chiral Bias". Life. 9 (1): 29. doi:10.3390/life9010029. ISSN 2075-1729. PMC 6463258. PMID 30884807.{{cite journal}}: CS1 maint: unflagged free DOI (link)
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