Mechanism and Kinetics of the Gas-Phase Stereoinversion in Proteinogenic l-Threonine and Its Astrophysical Relevance.

Abstract

Quantum-mechanical computations are performed to trace the mechanistic pathways for the gas-phase stereoinversion in proteinogenic l-threonine, an amino acid with two stereocenters. The pathways are explored employing density functional and coupled cluster theories along with a global reaction route mapping strategy to locate various intermediates and transition states along the stereoinversion pathways on the complex potential energy surface of l-threonine. A simultaneous intramolecular proton and hydrogen atom transfer is observed to drive the stereoinversion in threonine. The kinetics analysis of the stereoinversion pathways is also carried out using transition state theory while accounting for the quantum mechanical tunnelling under conditions akin to various temperature regions of interstellar medium (ISM). The key step leading to stereoinversion through an achiral intermediate or transition state is predicted to involve a low energy barrier with high stereoinversion rates. The temperature region of 500-1000 K corresponding to protoplanetary disks was found to be an optimum region for stereoinversion to occur in l-threonine with quite significant reaction rates. However, in the cold molecular clouds of ISM the stereoinversion is predicted to be a less likely event despite involving significant proton tunnelling. The stereoinversion pathways proposed in this work pay gainful insights, particularly, to the researchers looking for the complex organic molecules in outer space.