Proton-Coupled Electron Transfer in Electrochemical Alanine Formation from Pyruvic Acid: Mechanism of Catalytic Reaction at the Interface between TiO₂(101) and Water

We investigate the feasibility for conversion of biomass-derived raw materials into useful substances, which is important from the perspective of sustainable resource utilization. Our focus is on the electrochemical alanine formation from pyruvic acid. The reaction pathway was surveyed using first principles methods to propose a plausible mechanism for amino acid production from biomass-derived pyruvic acid at the cathode of TiO2. Our calculations show that amino acid formation from pyruvic acid involves seven elementary reactions mainly described by proton transfer. Our detailed spin density analysis for N-H bond formation revealed that the reaction is mediated by a proton-coupled electron transfer reaction, which sufficiently lowers the activation energy compared to that in simple hydrogen transfer. All proton transfer reactions occur via water bridges between the TiO2 surface oxygens and the reactant, suggesting that water is essential in the reaction mechanism.