Gas-phase stereoinversion in proteinogenic amino acid isoleucine: its astrophysical importance

With outer space being speculated as the source of handedness in molecules of biological importance, there is a great need to explore the gas-phase stereoinversion in molecules under the conditions akin to interstellar medium. In this work, quantum-mechanical computations are performed to trace the pathways for gas-phase stereoinversion in isoleucine, an amino acid with two chiral centres. Contrary to the previous such work on a two-chiral centre proteinogenic amino acid, threonine, the present work could not trace a complete stereoinversion pathway on the potential energy surface of isoleucine. The failure in case of isoleucine is attributed to its branched aliphatic side chain which hindered a complete inversion in isoleucine. However, the diastereomeric conversion pathways explored in this work for isoleucine support the distribution of different stereoisomers reported in the meteoritic composition. Notably, this study also revealed that the chirality of the aldehydic precursors may hold the key to enantio-biasing observed for isoleucine in the meteoritic samples.