Photocatalytic racemization of amino acids in aqueous polycrystalline cadmium(II) sulfide dispersions

Abstract

L-Lysine in an aqueous solution is partly racemized by photoirradiation (at wavelengths > 300 nm) in the presence of suspended cadmium(II) sulfide particles under a de-aerated atmosphere at room temperature. Loading of a small amount of platinum or its oxide inhibited the racemization, but enhanced N-cyclization of L-lysine into DL-pipecolinic acid. ²H and ¹H NMR measurements of the racemate from photoirradiated reaction mixtures in deuterium oxide solutions revealed the substitution of the α-hydrogen of lysine with deuterium alongside the racemization. These facts suggest a mechanism for photocatalytic racemization that includes both reduction and oxidation by photoexcited electrons and positive holes, respectively. Similarly, the other amino acids, leucine and phenylalanine, undergo photocatalytic racemization under reaction conditions where the α-amino group is deprotonated, although glutamic acid gives no racemate during photocatalytic decomposition. Among the CdS photocatalysts used in this study, well grown cubic crystallites with a specific surface area of ca. 8 m² g^–1 showed the highest selectivity for the racemization of L-lysine.