Nitrogen-containing compounds in two CR2 meteorites: ¹⁵N composition, molecular distribution and precursor molecules

Abstract

Amino acids, amines and aldehydes were obtained from the water extracts of two CR2 carbonaceous chondrites from Antarctica and analyzed for their molecular and ¹⁵N isotopic content. These compounds were found to differ significantly from those of CM chondrites in both overall abundances and molecular distribution. The amino acids suites comprise a preponderant abundance of linear, 2-H amino acids, show rapid non-linear decrease with the compounds’ increasing chain length and include protein amino acids never identified in meteorites before, such as threonine, tyrosine and phenylalanine. The presence of tertiary amines as well as a diverse, large abundance of aldehydes and ketones also distinguishes both CR2 organic suites. The δ¹⁵N values determined for CR2 amino acids have a distribution between molecular subgroups that is opposite to the one of their δD values, with 2-H amino acids having higher δ¹⁵N and lower δD values than 2-methyl amino acids, while the opposite is true for 2-methyl amino acids. Based on theoretical data, these isotopic findings would place the formation of the two amino acid groups or their direct precursors at different ISM stages of star formation.

Introduction

Soluble organic compounds are known to be abundant in CM carbonaceous chondrites (CC). They comprise a complex and diverse suite of both water-, and solvent-extractable molecular species that are distinguished by large, sometime complete, isomerism. Their broad ranges of δ¹³C and δD values also indicate a varied formation that possibly involved several solar as well as presolar synthetic regimes (see Pizzarello et al., 2006, and references therein for a review). The study of the isotopic composition of Murray and Murchison amino acids was particularly interesting in this respect, because it determined isotopic differences between the isomers of various structural subgroups of the compounds. For example, δD values of amino acids with branched alkyl chains are much higher than for linear amino acids (Pizzarello and Huang, 2005) and the ¹³C content within amino acid subgroups varies with carbon number for some but not all, depending on the amino position along the alkyl chain (Pizzarello et al., 2004). These data point to a diversity of synthetic pathways even for meteoritic molecules of closely related composition.

The organic composition of CR (Renazzo type) chondrites is known in far less detail, as these meteorites make up a more recent classification of several “finds” from Antarctica and the Sahara desert that have petrology closely similar to that of the Renazzo meteorite. Cody and Alexander (2005) showed by nuclear magnetic resonance (NMR) spectroscopy that insoluble organic material (IOM) of the CR2 EET92042 Antarctic meteorite contains the largest alkyl component of all IOMs analyzed so far. Given that Tagish Lake (ungrouped CC) ranked at the other extreme of the measurements with the lowest alkyl portion in its IOM and that this trait could indicate extensive reprocessing as well as possible oxidation of the material, the NMR data appeared to indicate that the EET92042 parent body processes had not substantially altered its insoluble organics. The comparative elemental and isotopic analyses of four other CR2 meteorites by Alexander et al. (2007), have supported this assumption thus far.

As for the soluble organic composition of CR2s, we had previously analyzed the amino acid content of small amounts (∼100 mg) of Renazzo powders drilled from two pieces of the meteorite, obtained at the Arizona State University Center for Meteorite Studies and the University of Bologna Museum. We found that these samples contained amino acids considered to be indigenous in CM meteorites, such as α-amino isobutyric acid (aiba) and isovaline (ival), but they were also accompanied by larger amount of proteinogenic amino acids, e.g., glycine, alanine, threonine and serine. At the time, employing liquid chromatography with fluorescence detection and without the benefit of chiral analyses, we concluded that the Renazzo fragments had likely been exposed to bacterial contamination during the long terrestrial residence of the meteorite (Renazzo fell in 1824). More recent repeat analysis of the ASU specimen by chromatography–mass spectroscopy (GC–MS) found that the amounts of aiba had in fact decreased, as it would be expected in the case of an ongoing bacterial contamination (Oró and Tornabene, 1965).

In 2007, after a considerable hiatus since this meteorite was collected in Antarctica, a first communication (Pizzarello and Garvie, 2007) reported on the soluble molecular composition of the GRA95229 CR2 meteorite as well as on the deuterium enrichment of some of its amino acids. Two subsequent manuscripts described at more length the organic composition of CR2 meteorites. Martins et al. (2007) analyzed the amino acid content of three CR2 Antarctic meteorites while Pizzarello et al. (2008) studied both the water and solvent soluble organic compounds of the GRA95229. This latter study has offered a yet unknown view of the synthetic capabilities of extraterrestrial environments by revealing a soluble organic suite that, unlike those of CMs, is made up of mainly water-soluble compounds, with predominant N-containing species and where some of the amino acids display the highest deuterium enrichment ever measured for extraterrestrial molecules by direct analyses. Intriguingly, the study of this meteorite’s diastereomer amino acids also suggested that their precursor aldehydes carried enantiomeric excesses (ee) during the aqueous phase reactions that took place in the meteorites’ asteroidal parent bodies.

The new findings have posed anew the questions of what syntheses and cosmic locals were responsible for the formation of abiotic organic compounds that have identical counterparts in terrestrial biomolecules and of whether, amongst their varied pathways of formation, any of the resulting organic suites might have been more advantageous to prebiotic molecular evolution. We report here on the molecular and isotopic analyses of the amino acids, amines and ammonia in the LAP02342 CR2 meteorite as well as on the molecular distribution of the Murchison, GRA95229 and LAP02342 aldehydes. The study was undertaken with the intent of addressing the above questions and offers new insights on CR meteorites’ organic composition, the conditions of their asteroidal locals and the likely cosmic provenance of their N-containing water soluble molecules.