DYNAMIC MOBILITY OF THE HISTIDINE-CONTAINING DOMAIN IN SPIN-LABELED LYSOZYME

Hen egg-white lysozyme was spin-labled with (4-iodoacetamide-2,4,6,6-t etramethyldine)-N1-oxyl at His-15 and examined by EPR. It was shown th at the rotational correlation time for the carrier is determined by th e mobility of the histidine-containing domain rather than the structur e of the protein monomer (pH 4.7) and dimer (pH 7.1). The rotational c orrelation time for the domain was determined from the viscosity depen dence of the distance between the outer wide peaks in the EPR spectrum . Its molecular weight coincided with that obtained by X-ray analysis. The mobility of the spin label was found to be very high; at room tem perature, the spectrum contained a tripler, while;at 1 degrees C it wa s immobilized. In EPR experiments, stochastic reorientation of the spi n label with respect to the lysozyme domain was represented as two mod el stochastic processes, namely axial rotation of the nitroxyl group a bout the preferable rotation axis and its angular oscillation with res pect to the molecular axes. The preferable rotaion axis had a two-dime nsional Gauss distribution in the angle space. The experimental EPR da ta were consistent with the theoretical spectra.