Structure and dynamics of lysozyme in DMSO-water binary mixture: fluorescence correlation spectroscopy

Proteins' structural stability and activity in aqueous-organic solvent mixtures is a fascinating topic of research in biochemistry. Here, the effect of dimethyl sulfoxide (DMSO) on the structure and conformational dynamics of lysozyme (labeled with alexa 488) is studied by fluorescence correlation spectroscopy (FCS). Circular dichroism and tryptophan emission indicate that DMSO molecules are accumulated around the tryptophan residues of lysozyme. According to the FCS data, the hydrodynamic radius (r(H)) of the protein increases steeply from 18 angstrom at 0 mol% DMSO to 33 angstrom at 5 mol% DMSO. This suggests unfolding of the protein on addition of DMSO. On addition of DMSO beyond 5 mol%, size of lysozyme gradually decreases until 30 mol% of DMSO, and increases thereafter. The rate constants (k(+) and k(-)) of fast folding and unfolding dynamics (contact between alexa 488 and amino containing residues e.g. tryptophan) show a chevron like plot on increasing DMSO concentration which indicates a two state pathway of folding dynamics.