STRUCTURAL-CHANGES OF T4 LYSOZYME UPON ADSORPTION TO SILICA NANOPARTICLES MEASURED BY CIRCULAR-DICHROISM

The change in the secondary structure of T4 lysozyme upon adsorption t o silica particles was studied with circular dichroism, Two different mutants of the protein along with the wild type were investigated, The mutants differ from wild type by substitution of isoleucine for cyste ine or tryptophan at position 3 and were chosen to represent a range o f stability as quantified by their energies of thermal unfolding, The mutants differ in Delta G, at 65 degrees C: and pH 6.5, compared to th e wild-type enzyme with -2.8 and 1.2 kcal/mol for the tryptophan and c ysteine mutants, respectively. After adsorption to 9-nm silica nanopar ticles for 90 min, a large change in the spectrum was observed for the less stable tryptophan mutant, while the changes were smaller for the wild type and the cysteine mutant, The spectral changes before and af ter adsorption corresponded to a calculated loss of alpha-helix of 12% for the wild type, 9% for the cysteine mutant, and 29% for the trypto phan mutant, Structural changes during adsorption of the proteins were also followed kinetically at 222 nm, The rate of conformational chang e differed among the three proteins and was fastest for the tryptophan mutant, In the case of the tryptophan mutant the time required for ha lf of the measured change to occur was approximately 5 min, while for the cysteine mutant and the wild-type T4 lysozyme more than 10 min was required. (C) 1995 Academic Press, Inc.