Gas-Phase Stereoinversion in Aspartic Acid: Reaction Pathways, Computational Spectroscopic Analysis, and Its Astrophysical Relevance

Abstract

Noncatalytic reaction pathways for the gas-phase stereoinversion in aspartic acid are mapped employing a global reaction route mapping strategy using quantum mechanical computations. The species including the transition states (TSs) traced along the stereoinversion pathways are characterized using rotational and vibrational computational spectroscopic analysis while accounting for the vibrational corrections to rotational constants and anharmonic effects. Notably, the TS structures traced along the stereochemical pathways resemble the achiral ammonium ylide and imine intermediates as observed in the Strecker synthesis of chiral amino acids. A few of the probable stereoinversion pathways proposed proceed through the proton or hydrogen atom transfer. The feasibility of the pathways under conditions akin to interstellar medium (ISM) is further discussed in terms of natural bond orbital analysis. The stereoinversion pathways proposed in this work may proceed via photoirradiation in the ISM, which though can be revealed by exploring the excited-state potential energy surface. In this context, the spectroscopic data generated in this work can provide valuable assistance toward the astrophysical detection of chiral molecules in outer space.

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

“…The intermediate ( S )‐EQ2 R3 then transforms to its non‐superimposable mirror image ( R )‐EQ2 R3 via an achiral transition state TS R3 . Similar stereoinversion route reported for other amino acids has also been observed to involve a very high energy barrier of same range . Although, note that the present case involves participation of a non‐polar side chain along this route.…”

“…The energy requirement for the first step of this stereoinversion route is similar to that along a pathway reported for Alanine, whereas the barrier for amino acids with polar side chain (namely, in Serine, Aspartic acid and Glutamic acid) is reported to be ca. 218 kJ/mol . Therefore, there is no significant difference in the barrier for this type of stereoinversion pathway traced for different amino acids, except, for an additional step in the present case due to a longer side chain in Leucine.…”

“…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.…”

“…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.…”