Carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter from the Murchison meteorite

Abstract

Abstract-Low molecular weight monocarboxylic acids, including acetic acid, are some of the most abundant organic compounds in carbonaceous chondrites. So far, the 13 C-and D-enriched signature of water-extractable carboxylic acids has implied an interstellar contribution to their origin. However, it also has been proposed that monocarboxylic acids could be formed by aqueous reaction on the meteorite parent body. In this study, we conducted hydrous pyrolysis of macromolecular organic matter purified from the Murchison meteorite (CM2) to examine the generation of monocarboxylic acids with their stable carbon isotope measurement. During hydrous pyrolysis of macromolecular organic matter at 270-330 °C, monocarboxylic acids with carbon numbers ranging from 2 (C 2 ) to 5 (C 5 ) were detected, acetic acid (CH 3 COOH; C 2 ) being the most abundant. The concentration of the generated acetic acid increased with increasing reaction temperature; up to 0.48 mmol acetic acid/g macromolecular organic matter at 330 °C. This result indicates that the Murchison macromolecule has a potential to generate at least ~0.4 mg acetic acid/g meteorite, which is about four times higher than the amount of water-extractable acetic acid reported from Murchison. The carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter is ~−27‰ (versus PDB), which is much more depleted in 13 C than the water-extractable acetic acid reported from Murchison. Intramolecular carbon isotope distribution shows that methyl (CH 3 -)-C is more enriched in 13 C relative to carboxyl (-COOH)-C, indicating a kinetic process for this formation. Although the experimental condition of this study (i.e., 270-330 °C for 72 h) may not simulate a reaction condition on parent bodies of carbonaceous chondrite, it may be possible to generate monocarboxylic acids at lower temperatures for a longer period of time.

Two homologous series of alkylpyridines in the Murchison meteorite

“…Therefore, the alkylpyridines should be a candidate for more primitive organic compounds in meteorites. In comparison, chemical oxidation process could be effected by OH radicals or aqueous alteration on the parent bodies, in which the progressive oxidation of methylene carbon yields alcohol to ketone followed by carboxylic acid (Cody and Alexander, 2005;Oba and Naraoka, 2006). The carbon double bond in unsaturated alkylpyridines would easily be oxidized to yield alkylpyridine carboxylic acids and dicarboxylic acids, being consistent with the reports that various alkylpyridine carboxylic acids are present in CCs (Stoks and Schwartz, 1982; Pizzarello et al, 2001;Smith et al, 2014).…”

“…Since all CCs had been subjected to some extent of aqueous alteration, the nature and composition of the meteoritic organic matter could have been altered by chemical oxidation associated with the aqueous activity. This oxidation could induce incorporation of oxygen into the original organic matter, resulting in the production of O-rich and H 2 O-soluble compounds such as carboxylic acids (Oba and Naraoka, 2006). The large abundance of carboxylic acids in CCs (Naraoka et al, 1999) may be due to such oxidation processes.…”

Origin and Evolution of Prebiotic Organic Matter As Inferred from the Tagish Lake Meteorite

“…Some of these compounds may be the result of hydrothermal alteration oflOM in the meteorite parent bodies (10)(11)(12), but which formed in this manner is an open question.…”

La Histidina Como Un Posible Precursor en El Origen De La Vida

“…Por otro lado, la presencia del grupo imidazol se ha confirmado en distintas meteoritas carbonosas (Ehrenfreund, y Charnley, 2000) como el Murchison (Oba, y Naraoka, 2006). Esto nos hace suponer que la síntesis de este compuesto pudo darse durante su viaje espacial.…”