Direct observation of protein residue solvation dynamics

Dynamics of solvation of chromophores attached to a protein by amino acid residues in comparison to that by water molecules has remained a long-standing problem in the field of protein solvation dynamics. An attempt to unravel the existing controversy has been made by studying the solvation dynamics of dansyl labeled proteolytic enzyme α-chymotrypsin in both native and denatured state. In the native state the dansyl probe at the surface of the protein interacts largely with the hydration water while in the denatured state the solvation relaxation of the probe in the randomly oriented polypeptide chain is mainly governed by the polar amino acid residues of the protein. A significant structural perturbation of the protein upon denaturation due to which the probe finds itself in a non-polar environment of the peptide residues is also evident from steady-state fluorescence, circular dichroism (CD) and dynamic light scattering (DLS) experiments. High-resolution streak camera has been employed in order to study dynamic fluorescence stokes shift of the dansyl probe due to hydration water and protein residues. The time scale of solvation by polar peptide residues is found to be an order of magnitude slower than that by bulk type water molecules. In order to show the effect of environmental restriction on the solvation dynamics, the protein in both native and denatured states have been encapsulated inside reverse micelles of varying degree of hydration (w0w0). Simple theoretical models have been proposed in order to qualitatively understand the experimental findings. This study might invoke further research in the field of protein solvation.