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.

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

“…Besides this, the HOMO of corresponding transition state involved indicates a PCET mechanism during this migration because there is a simultaneous shift of electron density from asymmetric carbon to carbonyl oxygen via carbonyl carbon as evident in SI Figure S3. Such type of pathway has been reported previously only for amino acid Threonine . However, the formation of an axially chiral enol type intermediate EQ1 R2b along this route involves a high activation barrier of ca.…”

“…Further, this type of multi‐step stereoinversion channel has previously been traced in other amino acids with single asymmetric carbon . For all the cases, such a pathway also involved a high initial energy barrier of ca.…”

“…Besides this, the conventional transition state theory (CTST) was employed to compute the rate coefficients along each elementary step of the proposed stereoinversion pathways. The details on the calculation of CTST rate coefficients are provided in our previous work . A few of the pathways explored proceed via migration of −H as an atom or proton (see later in the next section), therefore, the effect of quantum tunnelling has also been incorporated during kinetic analysis while employing an asymmetric Eckart potential .…”

“…The racemization of chiral molecular species in outer space can be hypothesised to occur either during their synthesis or through some chemical and physical processes subsequent to their formation. Within the latter framework, our research group has been elucidating the mechanism and kinetics of gas‐phase stereoinversion in amino acids, through quantum‐mechanical computations . The molecular species being investigated in this study is an essential amino acid Leucine.…”

“…Leucine, 2‐amino‐4‐methylpentanoic acid, is a non‐polar aliphatic alpha amino acid due to the presence of isobutyl group as a side chain. Except for alanine, the amino acids that have been investigated so far for tracing their stereoinversion routes, all exhibits a polar side chain. Therefore, this study to the best of our knowledge, is the first such study on an amino acid with a long non‐polar side chain.…”