PHOTOCATALYTIC RACEMIZATION OF AMINO-ACIDS IN AQUEOUS POLYCRYSTALLINECADMIUM(II) SULFIDE DISPERSIONS

L-Lysine in an aqueous solution is partly racemized by photoirradiatio n (at wavelengths > 300 nm) in the presence of suspended cadmium(II) s ulfide particles under a de-aerated atmosphere at room temperature. Lo ading of a small amount of platinum or its oxide inhibited the racemiz ation, but enhanced N-cyclization of L-lysine into DL-pipecolinic acid . H-2 and H-1 NMR measurements of the racemate from photoirradiated re action mixtures in deuterium oxide solutions revealed the substitution of the alpha-hydrogen of lysine with deuterium alongside the racemiza tion. These facts suggest a mechanism for photocatalytic racemization that includes both reduction and oxidation by photoexcited electrons a nd positive holes, respectively. Similarly, the other amino acids, leu cine and phenylalanine, undergo photocatalytic racemization under reac tion conditions where the alpha-amino group is deprotonated, although glutamic acid gives no racemate during photocatalytic decomposition. A mong the CdS photocatalysts used in this study, well grown cubic cryst allites with a specific surface area of ca. 8 m2 g-1 showed the highes t selectivity for the racemization of L-lysine.