Denaturation and aggregation of hen egg lysozyme in aqueous ethanol solution studied by dynamic light scattering

We applied dynamic light scattering technique on the model system of hen egg lysozyme in salt-free aqueous ethanol solution to study the mechanism of denaturation and aggregation of protein. At low ethanol concentration [0-63% (v/v)], the fast relaxation mode was observed, which was caused by lysozyme molecules in the solution interacting with each other with strong repulsive electrostatic force. At 45 and 63% (v/v) ethanol, the slow relaxation mode was also observed, which showed translational diffusive nature, similar to that observed in salt-free polyelectrolyte solution. At 72 or 81% (v/v) ethanol, the slow mode disappeared, leaving only the fast mode. However, the mutual diffusion coefficients obtained from the fast mode at 72 and 81% (v/v) ethanol decreased by about one order of magnitude compared with those from the fast mode at 0-63% (v/v). The reported alcohol-induced conformational transformation of lysozyme molecules at >60% (v/v) ethanol from their native structure to an alpha-helix-rich structure might cause such drastic decrease in the mutual diffusion coefficients. At the highest ethanol concentration of 90% (v/v), the slow mode reappeared, and its relaxation rate was decreasing with elapsed time, which is possibly due to the growth of aggregates of lysozyme molecules. X-ray diffraction results suggested that the intermolecular beta-sheet formation caused the aggregation. Thus, our results indicated that the change in molecular structure of lysozyme closely relates to the diffusion of molecules and their aggregation.