[Spin label study of slow molecular rotations of globular proteins using microwave saturation effects in electron paramagnetic resonance].

Viscosity, temperature and ionic strength dependences of ESR microwave saturation parameters of spin labelled human oxyhemoglobin (Hb) and bovine serum albumin (BSA) have been studied. The piperidine and pyrrolidine nitroxyl derivatives of maleimide were used as covalent SH reagents for Hb and BSA and the same two derivatives of gamma-benzocarboline and spin labelled stearic acid were used as noncovalent spin probes for BSA. The effects of label binding tightness on ESR spectral parameters were considered. The rotational correlation times were determined using viscosity dependences of the separation of the outer hyperfine extrema and Stokes extrapolations at high viscosities. The ESR microwave saturation parameters of the spin labels were shown to depend just weakly on temperature (at constant eta/t) over the range 0-25 degrees and on g, A values but to be sensitive to protein rotational correlation times up to 10(-4) sec and also to the rotational anisotropy and to the relative motion of the spin label.