Adsorption and decomposition of a dipeptide (Ala-Trp) in TiO2 photocatalytic systems

The adsorption and photodecomposition of a dipeptide, alanyl-tryptophan (Ala-Trp) on a TiO2 surface were investigated by zeta potential measurements and 1H NMR spectroscopy in TiO2 aqueous suspension systems. The photodecomposition rate of Ala-Trp was lower than that of Trp but higher than that of Ala (Ala < Ala-Trp < Trp), reflecting the adsorbability and the photocatalytic reactivity of the component amino acids. The most efficient photocatalysis of Ala-Trp takes place by the adsorption of the heterocyclic ring (3-indolyl group) on the basic terminal hydroxyl sites of TiO2. The adsorptions of the molecule on the basic terminal OH sites through the carboxyl group or the peptide bond, or on the acidic bridged OH sites through NH2 group were also suggested. However, the photocatalytic reactivities for such adsorptions are considered to be lower. A plausible adsorption scheme is presented with the decomposition efficiency.