There is a 7.5‰ difference in δ 13C values between PAHs isomers containing a five-carbon ring (e.g., fluoranthene) and those without (e.g., pyrene), which has been interpreted as evidence of two possible pathways for the formation of PAHs ( Gilmour and Pillinger, 1994 Naraoka et al., 2000).
The isotopic heterogeneity displayed by aromatic compounds in Murchison and Asuka-881458 may also contain evidence for different synthetic pathways. This has led to the suggestion that during the synthetic processes that led to bond formation, isotopic fractionation was at its most extreme, implying that synthesis took place in a low-temperature environment such as interstellar space ( Sephton and Gilmour, 2000). Compounds, with relatively high molecular weights, but which differ by only one or two carbon atoms also display significant differences in their δ 13C values.
The δ 13C values obtained for extractable aromatic hydrocarbons in CM2 meteorites display a significant amount of isotopic heterogeneity with a range in δ 13C values of over 20‰. The predominant aromatic trend is consistent that observed for C 1–C 5 compounds from Murchison and indicates an origin by a synthetic process progressively adding 12C to the carbon skeleton with kinetic isotopic fractionation determining the distribution of carbon isotopes between compounds rather than thermodynamic equilibrium ( Gilmour and Pillinger, 1994 Naraoka et al., 2000). The δ 13C values of many of the aromatic compounds extracted (e.g., Asuka-881458 fluoranthene: –8.3‰) are markedly more 13C-enriched than typical terrestrial PAHs confirming the indigenous nature of these compounds. Although the abundance of the alkyl amides in Murchison have not been reported, the carboxy lactams, lactams, hydantoins, and N-acetyl amino acids are all converted to amino acids by hydrolysis as acid-labile amino acid precursors and may account for the roughly twofold increase in total amino acids after acid hydrolysis of hot water extracts ( Glavin et al., 2010).Įxtractable C 6–C 22 aromatic compounds from the Murchison (CM2) and Asuka-881458 (CM2) meteorites also appear to follow a systematic trend when their δ 13C values are plotted against carbon number ( Figure 3), although benzene and toluene from Murchison and naphthalene and biphenyl from Asuka-881458 are clear outliers. It is possible that some amides formed by partial hydrolysis of interstellar nitriles such as acetonitrile and propionitrile or from a reaction involving the addition of another known interstellar molecule (cyanate) to amino acids or their derivatives ( Cooper and Cronin, 1995). There are likely many more alkyl amides present in Murchison with isomers and homologs up to eight carbons. These include monocarboxylic and dicarboxylic acid amides, cyclic imides, carboxy lactams, lactams, amino acid hydantoins, N-acetyl amines acids, and hydroxy acid amines. Twenty seven different alkyl amide compounds have been identified in water extracts of the Murchison meteorite ( Cooper and Cronin, 1995). Hikaru Yabuta, in Primitive Meteorites and Asteroids, 2018 3.3.2.2.2 Alkyl Amides