Selective decomposition of proteins by photocatalytic Ti(IV)-doped calcium hydroxyapatite particles from mixed-protein systems

The decomposition of protein molecules from a mixed-protein solution on the surface of calcium hydroxyapatite (CaHap) and Ti(IV)-doped CaHap (TiHap) particles with a Ti/(Ca + Ti) atomic ratio (X Ti) of 0.10 and 0.20 under UV irradiation of 365 nm in wavelength was investigated. Acidic bovine serum albumin (BSA) and basic lysozyme (LSZ) were employed as a model of pathogenic proteins. The photocatalytic activities of TiHap particles were estimated from the decomposition of BSA and LSZ from the BSA (2.5 mg/cm3)–LSZ(1.0 mg/cm3) mixture under 1 mW/cm2 UV irradiation dispersed in a 10-mL quartz tube. No change in BSA concentration by UV irradiation was observed for all the unheated original CaHap and TiHap particles without and with low photocatalytic activities, respectively. Similar results were observed for the systems that employed heat-treated particles endowed a high photocatalytic activity by heat treatment at 650 °C for 1 h. On the other hand, a selective photocatalytic decomposition was observed for the LSZ, i.e., only LSZ molecules were decomposed completely from the BSA (2.5 mg/cm3)–LSZ(1.0 mg/cm3) mixture by using heat-treated TiHap particles with X Ti = 0.10 and 0.20. This selective decomposition by TiHap particles was interpreted by higher adsorption affinity of positively charged LSZ to highly negatively charged TiHap together with low molecular weight and rigid structure of LSZ molecules.