Solvent-exposed tryptophans probe the dynamics at protein surfaces

The dynamics of single tryptophan (W) side chain of protease subtilisin Car lsberg (SC) and myelin basic protein (MBP) were used for probing the surfac e of these proteins. The W side chains are exposed to the solvent, as shown by the extent of quenching of their fluorescence by KI. Time-resolved fluo rescence anisotropy measurements showed that the rotational motion of W is completely unhindered in the case of SC and partially hindered in the case of MBP. The rotational correlation time (phi) associated with the fast loca l motion of W did not scale linearly with the bulk solvent viscosity (eta) in glycerol-water mixtures. In contrast, phi values of either W side chains in the denatured proteins or the free W scaled almost linearly with eta, a s expected by the Stokes-Einstein relationship. These results were interpre ted as indicating specific partitioning of water at the surface of the prot eins in glycerol-water mixtures.