Myoglobin hydration water

Figure 20. The time-resolved MSD ( x (t)) vs. t) of bulk water and of myoglobin hydration water h = 0.4), measured by neutron for 180K < T < 320K [71].

Molecular dynamics (MD) simulations [29-31], coupled with experimental observations, have played an important role in the understanding of protein hydration. They predicted that the dynamics of ordered water molecules in the surface layer is ultrafast, typically on the picosecond time scales. Most calculated residence times are shorter than experimental measurements reported before, in a range of sub-picosecond to 100 ps. Water molecules at the surface are very mobile and are in constant exchange with bulk water. For example, the trajectory study of myoglobin hydration revealed that among 294 hydration sites, the residence times at 284 sites (96.6% of surface water molecules) are less than lOOps [32]. Furthermore, the population time correlation functions... 

The effect of dipole-dipole interaction between the Fe3+ heme group of myoglobin and water protons was used to study heme hydration and displacement in the pre-denaturational conformational transition of the molecule (Derzhansci et al. 1970). 

Mossbauer spectroscopic measurements suggest that the hydration water of myoglobin and the internal motions of the protein are coupled. [ Fe]Ferricyanide diffused into the solvent of myoglobin crystals exhibits (x ) values equal to those for the heme iron for temperatures below 250 K, and greater than those for the heme iron at higher temperatures (50% greater at 300 K) (Parak, 1986). The [ Fe]ferricyanide in the crystal monitors motions of the hydration water [ Fe]ferricyanide in bulk water shows no Mossbauer spectrum. 

Other measurements also suggest that the hydration water of myoglobin and the internal motions of the protein are coupled. For example, the 10 GHz dielectric response of the water of myoglobin crystals has a temperature dependence close to that of the heme iron (Singh et al., 1981). The O-D stretching band (Doster et al, 1986) is also correlated with the above properties (Fig. 26). The temperature dependence of the infrared properties and of the heat capacity (Doster et al., 1986) were interpreted as indicating that the hydration water melts at 190 K and... 

Fig. 27. Mean square displacement averaged over all atoms of myoglobin, but corrected for the water content, determined from Rayleigh scattering of Mossbauer radiation. (Sample a) , Lyophilized sperm whale myoglobin hydrated for 3 days at 0.37 relative humidity. (Sample b) O, Hydrated at 0.94 relative humidity. (Sample c) A, 29.6 wt% solution. (Sample d) 9, Myoglobin crystals. From Krupyanskii etal. (1982).

Since the values of the thermal diffusivity for myoglobin and water are close, the discrepancy between their thermal conductivities is largely due to differences in their respective heat capacities. At 300 K the specific heat of water is 4.2 J K 1 g-1, whereas we calculate the specific heat of myoglobin to be 1.0 J K 1 g 1 at 300 K. This latter value is reasonably close to measured specific heats for proteins. For example, the specific heat of lysozyme in dilute aqueous solution is 1.5 J K-1 g , and it is 1.3 J K-1 g-1 for the dry protein [154,155]. Still, we should bear in mind that at 300 K the partial specific heat of a hydrated protein may be some 50% larger than the specific heat of a dry protein [156], so that the coefficient of thermal conductivity may be correspondingly larger. 

The amount of tightly bound water (water of hydration) present in the myoglobin solution can be calculated from the amplitude of the S dispersion using a method previously described (1,10). From the value of 3.6 for this parameter (Table I) a value of hydration of 0.15-0.02 unit mass of water per unit mass of myoglobin is obtained. Considered as a volume fraction of the total water content this would amount to about 4%, which compares well with the figure of 5% recently proposed for muscle fibres by Foster, Schepps and Schwan (16). 

Ruggiero et al. (1986) measured the ESR spectra of samples of lysozyme, myoglobin, and hemoglobin with covalently bound spin labels and noncovalently bound spin probes, in solution and in the partially hydrated powder, over the temperature range 120—260 K. The several proteins behaved similarly. The solution samples differed from the powders in showing a change in spectrum shape at 210 K, understood to represent freezing of water in the hydration shell. 

This more sophisticated way shows a [arge distribution of residence times for water molecules in the cage formed by the neighboring molecules, which is a more realistic view than the sharp separation of water molecules into two classes, according to their mobility [49]. Short time dynamics resuits about hydrated myoglobin have recently been interpreted by using this same theory of kinetic glass transition in dense supercooled liquids [73]. 

The protein-solvent interface was studied in an explicit solvent environment of 3182 water molecules by MD simulations performed on metmyoglobin [31].Both the structure and dynamics of the hydrated surface of myoglobin are similar to that obtained by experimental methods calculating three-dimensional density distributions, temperature factors and occupancy weights of the solvent molecules. On the basis of trajectories they identified multiple solvation layers around the protein surface including more than 500 hydration sites. Properties of theoretically calculated hydration clusters were compared to that obtained from neutron and X-ray data. This study indicates that the simulation unified the hydration picture provided by X-ray and neutron diffraction experiments. 

Figure 11. Relaxation, 4>(r), of the center of energy is plotted for wave packets propagated by the normal modes of cytochrome c hydrated by 400 water molecules (circles) and myoglobin (squares). Curve is a stretched exponential, Eq. (33), with p = 2v = 0.52, the value fit to the computed energy diffusion data for cytochrome c plotted in Fig. 10, and time constant, t — 11 ps.

Makarov V, Andrews BV, Smith PE, Pettitt BM (2000) Residence times of water molecules in the hydration sites of myoglobin. Biophys J 79 2966-2974... 

Settles M, Doster W Anomalous diffusion of adsorbed water A neutron scattering study of hydrated myoglobin. Farar/ay Dwcmm. 1996, 103 269-279. 

---