Comprehensive Search of Stable Isomers of Alanine and Alanine Precursors in Prebiotic Syntheses

Enantiomeric excesses of l-amino acids have been detected in meteorites; however, their molecular mechanism and prebiotic syntheses are still a matter of debate. To elucidate the origin of homochirality, alanine and the chiral precursors formed in prebiotic processes were investigated with regard to their stabilities among their isomers by employing the minimum energy principle, namely, the abundancy of a molecule in the interstellar medium is directly correlated to the stability among isomers. To facilitate the search for possible isomers, we developed a new isomer search algorithm, the random connection method, and performed a thorough search for all the stable isomers within a given chemical formula. We found that alanine and most of its precursors are located at higher energy by more than 5.7 kcal mol^-1, with respect to the most stable isomer that consists of a linear-chain structure, whereas only the 2-aminopropanenitrile is the most stable isomer among all others possible. The inherent stability of the α-amino nitrile suggests that the 2-aminopropanenitrile is the dominant contribution in the formation of the common enantiomeric excess over α-amino acids.