Two-state protein model with water interactions: influence of temperature on the intrinsic viscosity of myoglobin.

We describe a single-domain protein as a two-state system with water interactions. Around the unfolded apolar parts of the protein we incorporate the hydration effect by introducing hydrogen bonds between the water molecules in order to mimic the "icelike" shell structure. Intrinsic viscosity, proportional to the effective hydrodynamic volume, for sperm whale metmyoglobin is assigned from experimental data in the folded and in the denaturated state. By weighing statistically the two states against the degree of folding, we express the total intrinsic viscosity. The temperature dependence of the intrinsic viscosity, for different chemical potentials, is in good correspondence with experimental data [P. L. Privalov et al., J. Mol. Biol. 190, 487 (1986)]. Cold and warm unfolding, common to small globular proteins, is also a result of the model.