Meteoritic amino acids as chemical tracers of parent-body chemistries

Abstract

ABSTRACT The analysis of the organic matter of meteorites made it possible to identify over 70 amino acids (AA), including 8 of those found in living organisms. However, their relative abundances vary drastically with the type of the carbonaceous chondrite, even for isomers of same chemical formula. In this report, we address the question whether this difference may have its origin in the relative stability of these isomers according to the conditions they experienced when they were formed and after. To this end, we rely on the fact that for most of the species observed so far in the interstellar medium (ISM), the most abundant isomer of a given generic chemical formula is the most stable one (minimum energy principle, MEP). Using quantum density functional theory (DFT) simulations, we investigate the relative stability of the lowest energy isomers of alanine (Ala) and amino butyric acid (ABA) in the neutral, protonated, and zwitterionic structures together with corresponding nitrile precursors. It is shown that β-alanine and γ-ABA are the most stable in a protonated form, whereas α-AA are the most stable in the zwitterionic and nitrile structures. The different composition of the carbonaceous chondrites CIs and CMs could be linked to the chemical context of the aqueous alterations of the parent bodies.