Effect of surface concentration on secondary and tertiary conformational changes of lysozyme adsorbed on silica nanoparticles

Kinetics of tertiary conformation of lysozyme adsorbed on 90 nm silica nanoparticles was inferred using tryptophan fluorescence for different surface concentrations (0.24 to 0.92 mg/m2), pH (4, 7 and 9), ionic strength (10 and 100 mM), 2,2,2-trifluoroethanol (TFE) (5, 15 and 30%) and Dithiothreitol (DTT) (0.5 mg/ml) concentrations. A rapid initial unfolding, followed by a much slower refolding and subsequent unfolding, were observed with the extent of unfolding being higher at lower surface concentration, higher ionic strengths, higher TFE and DTT concentrations and at pH 9. The rate of unfolding was found to be higher at lower surface concentrations, pH 4, higher ionic strengths, higher TFE and DTT concentrations. In contrast, earlier results showed that β lactoglobulin unfolded slower and exhibited only an initial rapid and a subsequent slow unfolding phase. Circular Dichroism spectra showed that α helix content was lower for adsorbed lysozyme compared to bulk with a corresponding increase in β sheet and random coil. This decrease in α helix was found to be more pronounced at lower surface concentrations. DTT decreased α helix with a corresponding increase in random coil while TFE was found to have negligible effect on secondary structure.