Quantum Tunneling Governs Dual-Pathway Amino Acid Racemization via Water Bridges

The intrinsic contradiction between homochirality and the phenomenon of racemization poses both challenges and opportunities for the exploration of life's origin, urgently necessitating a deep understanding of the racemization mechanism of essential small life molecules, particularly amino acids. Herein, we demonstrate that the racemization of amino acid molecules in aqueous environments can occur by a dual-path coexistence (DPC) mechanism via the carboxyl (COOH) and amino (NH₂) groups simultaneously, which provides a new interpretation of previous experimental results obtained in neutral to weakly acidic environments. Results show that the quantum mechanical tunneling (QMT) effect plays the pivotal role in this DPC mechanism, as evidenced by the tunneling hindrance of the NH₂ reaction and the tunneling enhancement of the COOH reaction. Further, the disparity in the QMT effect contributes to a crossover between COOH and NH₂ reactions at ∼250 K, such that NH₂ reactions dominate at high temperatures (>300 K), whereas COOH reactions dominate at low temperatures (<200 K). This study proposes a DPC mechanism for amino acid racemization, mediated by the QMT effect, that fills a gap in the understanding of amino acid racemization mechanisms and offers valuable insights into the origin of homochirality in the extreme environments of the early Earth.