The temperature dependences of the nanosecond dynamics of different chemical classes of amino acid residue have been analyzed by combining elastic incoherent neutron scattering experiments with molecular dynamics simulations on cytochrome P450cam. At T = 100-160 K, anharmonic motion in hydrophobic and aromatic residues is activated, whereas hydrophilic residue motions are suppressed because of hydrogen-bonding interactions. In contrast, at T = 180-220 K, water-activated jumps of hydrophilic side chains, which are strongly coupled to the relaxation rates of the hydrogen bonds they form with hydration water, become apparent. Thus, with increasing temperature, first the hydrophobic core awakens, followed by the hydrophilic surface.
The effect of surface hydration water on internal protein motion is of fundamental interest in molecular biophysics. Here, by decomposing the picosecond to nanosecond atomic motion in molecular dynamics simulations of lysozyme at different hydration levels into three components--localized single-well diffusion, methyl group rotation, and nonmethyl jumps--we show that the effect of surface hydration is mainly to increase the volume of the localized single-well diffusion. These diffusive motions are coupled in such a way that the hydration effect propagates from the protein surface into the dry core.
The effect of variation of the water model on temperature dependent protein and hydration water dynamics is examined by performing molecular dynamics simulations of myoglobin with the TIP3P, TIP4P, and TIP5P water models and the CHARMM protein force field at temperatures between 20 and 300 K. The atomic mean-square displacements, solvent reorientational relaxation times, pair angular correlations between surface water molecules and time-averaged structures of the protein are all found to be similar and the protein dynamical transition is described almost indistinguishably for the three water potentials. The results provide further evidence that variation of the water model without loss of accuracy can be considered, opening up the possibility of choosing the water model employed in protein simulations according to the properties to be investigated.
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