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 eg g 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 it-as observed, which was caused by lyso zyme molecules in the solution interacting with each other with strong repu lsive electrostatic force. At 45 and 63% (v/v) ethanol, the slow relaxation mode was also observed, which showed translational diffusive nature, simil ar to that observed in salt free polyelcetrolyte 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 tho se from the fast mode at 0-63% (v/v). The reported alcohol-induced conforma tional transformation of lysozyme molecules at > 60% (v/v) ethanol from the ir native structure to an alpha -helix-rich structure might cause such dras tic decrease in the mutual diffusion coefficients. At the highest ethanol c oncentration of 90% (v/v), the slow mode reappeared, and its relaxation rat e 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 closel y relates to the diffusion of molecules and their aggregation. (C) 2001 Joh n Wiley & Sons, Inc.