Hydration layer dynamics

In sharp contrast to the large number of experimental and computer simulation studies reported in literature, there have been relatively few analytical or model dependent studies on the dynamics of protein hydration layer. A simple phenomenological model, proposed earlier by Nandi and Bagchi [4] explains the observed slow relaxation in the hydration layer in terms of a dynamic equilibrium between the bound and the free states of water molecules within the layer. The slow time scale is the inverse of the rate of bound to free transition. In this model, the transition between the free and bound states occurs by rotation. Recently Mukherjee and Bagchi [14] have numerically solved the space dependent reaction-diffusion model to obtain the probability distribution and the time dependent mean-square displacement (MSD). The model predicts a transition from sub-diffusive to super-diffusive translational behaviour, before it attains a diffusive nature in the long time. However, a microscopic theory of hydration layer dynamics is yet to be fully developed. 

In order to understand the role of water in the glass transition of the protein and its subsequent loss of functionality, we have performed a temperature dependent study of the hydration layer dynamics. The study shows that at low temperatures the bound to free conversion rates reduce drastically and this leads to a non-diffusive motion of the bound water. The mean square displacement obtained after 10 ps and the diffusion coefficient... 

Stadler AM, Embs JP, Digel 1, Artmann GM, Unruh T, Biildt G, Zaccai G Cytoplasmic water and hydration layer dynamics in human red blood cells. J. Am. Chem. Soc. 2008, 130 16852-16853. 

Protein-glass transition and hydration-layer dynamics... 

To summarize, all the above studies clearly indicate the existence of multiple timescales in the hydration-layer dynamics. While a large fraction of hydration-layer water remains almost as fast as its bulk counterpart, a sizable fraction is slow. It is conceivable that the slow water molecules reside near the hydrophilic residues that provide stability to the enzymes, while the fast water molecules participate in the biological activities. For example, in adenylate kinase catalysis, one finds that water molecules play an important functional role, which has been discussed earlier in Chapter 7, section 7.2. 

In the present chapter we discussed quantitative aspects of protein hydration-layer dynamics. Because of the complexity of the problem and perhaps due to a certain lack of concerted effort, there are still many issues remained to be settled. 

Li TP, Hassanali AA, Singer SJ (2008) Origin of slow relaxation following photoexcitation of W7 in myoglobin and the dynamics of its hydration layer. J Phys Chem B 112(50) 16121-16134... 

Fig. 6. Hydration surface dynamics of the protein (enzyme) Substilisin Carlsberg, and for comparison that of bulk water, and a probe outside the water layer...

The response range of the local environment to the excited Trp-probe is mainly within 10 A because the dipole-dipole interaction at 10 A to that at —3.5 A of the first solvent shell drops to 4.3%. This interaction distance is also confirmed by recent calculations [151]. Thus, the hydration dynamics we obtained from each Trp-probe reflects water motion in the approximately three neighboring solvent shells. About seven layers of water molecules exist in the 50-A channel, and we observed three discrete dynamic structures. We estimated about four layers of bulk-like free water near the channel center, about two layers of quasi-bound water networks in the middle, and one layer of well-ordered rigid water at the lipid interface. Because of lipid fluctuation, water can penetrate into the lipid headgroups, and one more trapped water layer is probably buried in the headgroups. As a result, about two bound-water layers exist around the lipid interface. The obtained distribution of distinct water structures is also consistent with —15 A of hydration layers observed by X-ray diffraction studies from White and colleagues [152, 153], These discrete water stmctures in the nanochannel are schematically shown in Figure 21, and these water molecules are all in dynamical equilibrium. 

The robust observation of surface hydration dynamics on two time scales and a series of correlations with protein properties provides a molecular picture of water motions and their coupling with protein fluctuations in the layer, as shown in Fig. 46. The dynamic exchange of hydration layer water with outside bulk... 

These CMD results are still qualitative and somewhat conflicting with the available experimental data [55], largely because of the simplified models used for the surfaces and more certainly due to difficulties in choosing suitable potential functions for the simulations. However, recently, molecular dynamic simulations of the hen egg-white lysozyme-Fab D1.3 complex have been reported both the crystal state and the complex in solution were studied [35]. The findings are consistent with the observation by various experimentalists of a reduced water mobility in a region extending several angstroms beyond the first hydration layer [54-57], as reported also from CMD simulations [60]. 

In order to understand the effect of temperature on the water dynamics and how it leads to the glass transition of the protein, we have performed a study of a model protein-water system. The model is quite similar to the DEM, which deals with the collective dynamics within and outside the hydration layer. However, since we want to calculate the mean square displacement and diffusion coefficients, we are primarily interested in the single particle properties. The single particle dynamics is essentially the motion of a particle in an effective potential described by its neighbors and thus coupled to the collective dynamics. A schematic representation of the d)mamics of a water molecule within the hydration layer can be given by ... 

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