Towards a better understanding of the initial steps in the photocatalyzed mineralization of amino acids at the titania/water interface. An experimental and theoretical examination of L-alanine, L-serine and L-phenylalanine

The pathway to the photomineralization of the three amino acids L-serine (L-Ser), L-phenylalanine (L-Phe) and L-a-alanine (L-Ala) is described experimentally on the basis of CO2 evolution and conversion of the amino group to NH4+ and NO3- ions, and theoretically on the basis of molecular orbital calculations to define frontier electron densities and point charges on all the individual atoms. The relatively high negative point charges on the carboxylate oxygens are consistent with adsorption of the amino acids to the TiO2 particle surface through the carboxylate function. Mineralization to carbon dioxide is complete for L-Ser (similar to 98%) and nearly so for L-Ala (similar to 90%), whereas for L-Phe the extent of mineralization is 59% corresponding to the total photooxidation of the phenyl ring carbons; for the amine function the extent of conversion is 87% for L-Ser, 97% for L-Ala and 91% for L-Phe. Relative formation yields of NH4+ and NO3- ions depend on the structural fragment R attached to the a-amino carboxylic acid functions, R-CH(NH2)COOH. Primary attack of the amino acids by the (OH)-O-. radical is correlated with the frontier electron densities. Ammonia is formed through a photoreductive step by electron attachment onto the zwitterionic form of the amino acids, whereas nitrate is produced through a photooxidative step implicating a very tortuous series of events. (C) 1998 Elsevier Science S.A. All rights reserved.