Extra-Terrestrial Gas-Phase Stereoinversion in Amino Acid Leucine: Thermal and Photochemical Channels

Enantiomeric-excess of the left-handed enantiomer of proteinogenic amino acids has been reported in the meteoritic samples of carbonaceous chondrites. Such chiral-bias is recently being believed due to the removal of right-handed enantiomer through its selective destruction by circularly polarized ultraviolet (UV) radiations in interstellar space. The present work through quantum mechanical computations explores various thermal and photochemical channels for stereoinversion in proteinogenic amino acid Leucine, under the gas-phase conditions akin to different temperature zones of interstellar medium (ISM). A thermochemical and kinetic analysis for the feasibility of the proposed channels is also carried out in different regions of ISM. The initiation of stereoinversion along all the ground-state thermal channels is found to be restricted by a very high energy barrier which, however, is proposed to be substantially reduced if the initial step proceeds via an excited-state. This work reveals that the stereoinversion in Leucine is quite feasible in ISM, provided it is initiated photochemically by UV radiations, which are abundant in ISM.