We have studied the femtosecond hydration dynamics of Monellin, a protein with a single tryptophan residue at its surface. Tryptophan was selectively used as a probe of the dynamics, and through monitoring of its fluorescence Stokes shift with time we obtained the hydration correlation function, which decays due to rotational and translational motions of water at the protein surface and in bulk. The decay exhibits a “bimodal” behavior with time constants of 1.3 and 16 ps, mirroring relaxation of the free/quasifree water molecules and surface-bound water layer (minimum binding energy of 1–2 kcal/mol). The observed slow decay of 16 ps for tryptophan in the native protein differs by more than an order of magnitude from that of bulk water because of the dynamical exchange in the layer. To examine the effect of unfolding, we also studied hydration dynamics when Monellin was denatured in a 6 M guanidine hydrochloride solution and obtained a totally different behavior: 3.5 and 56 ps. Comparing with the results of experiments on free tryptophan in the same concentration of the denaturing solution, we conclude that the fast component of 3.5 ps comes from bulk-type solvation in the 6 M guanidine hydrochloride. However, the absence of the 16-ps decay and appearance of the 56-ps component reflects a more “rigid solvation,” which is likely to involve the motions of the protein backbone in the random-coiled state. With the help of polymer theory, this time scale is reproduced in agreement with experimental observations
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