Interactions between thioglycolic acid capped CdSe/ZnS nanoparticles and papain

The paper presents the interactions among carboxyl surfaced Cadmium selenide/Zinc sulphide (CdSe/ZnS) core/shell nanoparticles (NPs) with papain, studied by various spectrometric methods. CdSe/ZnS NPs strongly quench the intrinsic fluorescence of papain and show contribution of both static and dynamic mechanisms. The binding constants for papain-CdSe/ZnS NPs conjugates were determined by studying the fluorescence quenching of papain at different temperatures. Using Vant-Hoff analysis, the change in enthalpy, entropy and Gibbs free energy of papain-CdSe/ZnS NPs interaction have been calculated. The results infer that electrostatic interactions exist in carboxyl surfaced NPs and papain provides two independent binding sites for the NPs. The distance between donor and acceptor for papain-CdSe/ZnS NPs conjugates is calculated to be 3.02 nm from Forster resonance energy transfer. The intrinsic fluorescence of CdSe/ZnS NPs has also been enhanced by the papain but only at low concentrations 1-20 mu l. All the observations evidence the formation of papain-CdSe/ZnS NPs conjugates, where papain retains the enzymatic activity dependent on temperature and pH. The optimum range for activity of papain-CdSe/ZnS NPs conjugates is pH 6-7.6, which is same as that of free papain.