Residual motion of hemoglobin-bound spin labels as a probe for protein dynamics

The residual motion of spin labels bound to cysteine beta 93 of methemoglobin and oxyhemoglobin has been analyzed as a function of temperature and hydration. The rotational diffusion of the whole protein molecule has been prevented or restricted by crystallization, lyophilization or by high viscosity of the solution. The residual motion of the labels is characterized by an angle of the limited motion cone and their rotational correlation time using computer simulations of the EPR spectra. Two types of motion can be separated due to different correlation times and different dependences on temperature and hydration. One of these motional mechanisms can be shown to be determined by protein fluctuations. Correlation times of these fluctuations decrease from 2 X 10(-8) s at T = 220 K to 10(-9) s at T = 300 K in the samples of high water concentration. Strong correlation between the properties of the hydration shell and these fluctuations are observed.