Dual-Path Mechanism of Amino Acid Racemization Mediated by Quantum Mechanical Tunneling

TL;DR

This study reveals that amino acid racemization occurs via two quantum tunneling-mediated pathways involving the carboxyl and amino groups, with temperature-dependent dominance, providing insights into early Earth's homochirality origins.

Contribution

It introduces a dual-path mechanism mediated by quantum tunneling, clarifying the atomic-level racemization process previously misunderstood for over a century.

Findings

Quantum tunneling significantly influences amino acid racemization.

A crossover between pathways occurs around 200-257 K.

NH2 pathway dominates at higher temperatures, COOH at lower temperatures.

Abstract

The racemization of amino acids constitutes one of the most elemental and critical reactions, holding primitive significance for understanding the life's origin and maintenance. Nevertheless, its mechanism at the atomic level has been persistently misunderstood for more than a century. In this work, we demonstrate that the racemization of amino acid molecules in aqueous environments can occur simultaneously by two pathways via the carboxyl (COOH) and amino (NH2) groups. Behind this result, the quantum mechanical tunneling (QMT) effect plays a pivotal role, as evidenced by the tunneling hindrance of the NH2 reaction and the tunneling enhancement of the COOH reaction. Notably, the disparity in the QMT effect leads to a crossover between the COOH and NH2 reactions within 200-257 K, such that NH2 reactions dominate at high temperatures and COOH reactions dominate at low temperatures. Our work emphasizes the significance of QMT effect in the racemization of amino acids and therefore introduces a dual-path coexistence mechanism, offering valuable insights into the origin of homochirality in extreme environments of the early Earth.