Mechanism and kinetics of astrophysically relevant gas-phase stereoinversion in glutamic acid: A computational study

Abstract

Enantiomeric excess of amino acids observed in the meteoritic samples of carbonaceous chondrites has incited many researchers to search for an extra-terrestrial origin of life on prebiotic Earth. However, in a non-catalytic environment, only racemic amino acids are synthesized. This computational quantum-mechanical study explores non-catalytic mechanistic pathways for stereoinversion in proteinogenic L-glutamic acid, which may be observable under gas-phase conditions of interstellar medium (ISM). The multi-step stereoinversion pathways proposed in this study are traced through a global reaction route mapping (GRRM) strategy utilizing density-functional and coupled-cluster theories. Notably, a few of the pathways are observed to proceed through simultaneous intramolecular hydrogen atom and proton transfer as well as through a proton-coupled electron transfer mechanism. The intermediates explored along the stereoinversion pathways resemble ammonium ylide and imine, the key ingredients in Strecker synthesis of amino acids. The thermodynamic and kinetic analysis of the stereoinversion pathways in different temperature regions of ISM are also carried out, predicting the streoinversion to proceed over any dissociation of intermediates and conformers of glutamic acid along the pathways. However, initial step of the pathways involves an unsurmountable energy barrier though the key step responsible for stereoinversion has a very low energy barrier and is predicted to proceeds with significant rates. The work suggests the possibility of observing stereoinversion of glutamic acid in the warmer regions of ISM.