Molecular asymmetry in extraterrestrial chemistry: Insights from a pristine meteorite

Abstract

The nonracemic amino acids of meteorites provide the only natural example of molecular asymmetry measured so far outside the biosphere. Because extant life depends on chiral homogeneity for the structure and function of biopolymers, the study of these meteoritic compounds may offer insights into the establishment of prebiotic attributes in chemical evolution as well as the origin of terrestrial homochirality. However, all efforts to understand the origin, distribution, and scope of these amino acids' enantiomeric excesses (ee) have been frustrated by the ready exposure of meteorites to terrestrial contaminants and the ubiquitous homochirality of such contamination. We have analyzed the soluble organic composition of a carbonaceous meteorite from Antarctica that was collected and stored under controlled conditions, largely escaped terrestrial contamination and offers an exceptionally pristine sample of prebiotic material. Analyses of the meteorite diastereomeric amino acids alloisoleucine and isoleucine allowed us to show that their likely precursor molecules, the aldehydes, also carried a sizable molecular asymmetry of up to 14% in the asteroidal parent body. Aldehydes are widespread and abundant interstellar molecules; that they came to be present, survived, and evolved in the solar system carrying ee gives support to the idea that biomolecular traits such as chiral asymmetry could have been seeded in abiotic chemistry ahead of life.

Scheme 1.

Fig. 1.

Chiral gas chromatographic separation of the CR2 95229 alloisolleucine/leucine diastereomers (mass spectrometric detection, single ion traces).

Scheme 2.

Scheme 3.

Fig. 2.

Gas chromatography carbon isotope ratio mass spectroscopy of a GRA 95229 unhydrolyzed water extract showing the elution of isoleucine and alloisoleucine enantiomers (a), an alloisoleucine/isoleucine standard (b), and their δ¹³C values. m/z traces 44, 45, and 46, on a Cirasil-Dex column. Prior GC-MS analyses of the sample showed that elutions of the four meteoritic diasteromers were free of interference. A more extended chromatogram is shown in SI Fig. 4.