Hydration at the surface of the protein Monellin: Dynamics with femtosecond resolution

Abstract

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 free͞quasifree water molecules and surface-bound water layer (minimum binding energy of 1-2 kcal͞mol). 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. W ater at the surface of a protein defines a molecular layer that has been termed ''biological water'' and exhibits unique characteristics. The structure and dynamics of such layers, which are determined by the hydrophobic and hydrophilic interactions with the residues exposed to the water in the folded state, are important for the stability of proteins as well as for the mechanisms of protein-protein and protein-ligand association (1). For example, the energetics and dynamics of water desolvation have been suggested to be a determining factor in the process of protein-ligand recognition (2). In molecular dynamics simulations, water molecules have been found to mediate or bridge the interaction between DNA and proteins through their hydrogen bonding (3).In a recent communication (4), we described how the ultrafast Stokes shift of the fluorescence from a single tryptophan amino acid (Trp) at the surface of a protein can be used to probe the dynamics of the biological water layer. Briefly, we use a femtosecond UV pulse to selectively excite the indole chromophore of the Trp residue situated at the protein surface. Although the initial excitation might involve two overlapping transitions for the indole chromophore ( 1 L a and 1 L b ), it has been demonstrated that any electronic mixing that leads to the 1 L a fluorescing state occurs in under 100 fs (5, 6); the 1 L a state of Trp has a significantly larger dipole moment than the ground state. Subsequent to the formation of this dipole by femtosecond excitatio...

Protein hydration dynamics in solution: a critical survey

“…Figure 7 shows DSS curves for tryptophan in water and in the protein subtilisin Carlsberg (Pal et al 2002b). Similar results have been reported for other proteins (Pal et al 2002c;Peon et al 2002). For tryptophan in water, the DSS curve exhibits a sub-picosecond inertial decay, associated with water librations ( Jimenez et al 1994), followed by a diffusive decay with a time constant DSS bulk Ϸ 1 ps (Shen & Knutson 2001;Pal et al 2002b).…”

“…In the protein monellin, where DSS solv = 16 ps was reported (Peon et al 2002), one face of the indole ring in the examined Trp-3 side chain is solvent exposed, but there are six charged groups within 8 Å of this residue (in the crystal structure 4MON). A recent 2 ns molecular dynamics simulation of monellin in water, using an excited state charge distribution corresponding to a dipole moment of 5.7 D for the Trp-3 indole, shows that the energy correlation function C(t) is dominated by intraprotein interactions, which decay on the time-scale of the experimental DSS solv , whereas the smaller water contribution decays on a much shorter time-scale (L. Nilsson, unpublished results), as expected from the MRD results.…”

Hydrogen Bond Network of Water around Protein Investigated with Terahertz and Infrared Spectroscopy

“…Molecular dynamics (MD) simulations (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) and other experimental approaches (23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37) have revealed the protein is surrounded by dynamically retarded hydration water, with the innermost shell having a density higher than that of bulk water. However, even today, experimentally characterizing the dynamics and the structure of the water HB network in this hydration shell is challenging, because the water-water HB lifetime is very short (typically 1 ps) (38).…”

Mapping hydration dynamics around a protein surface

“…These processes represent the dynamic exchange of hydration layer water with outside bulk water via thermal fluctuations. Femtosecond-resolved spectroscopic studies of protein solvation (19)(20)(21)(22)(23)(24)(25)(26) recently have shown the dynamics of surface hydration on picosecond time scales with a biphasic distribution. We attributed the first ultrafast solvation to water local relaxation and the second longer-time dynamics to coupled water-protein fluctuations (25,27).…”