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-tetramethyldine)-N1-oxyl at His-15 and examined by EPR. It was shown that the rotational correlation time for the carrier is determined by the mobility of the histidine-containing domain rather than the structure of the protein monomer (pH 4.7) and dimer (pH 7.1). The rotational correlation time for the domain was determined from the viscosity dependence 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 temperature, the spectrum contained a tripler, while;at 1 degrees C it was immobilized. In EPR experiments, stochastic reorientation of the spin label with respect to the lysozyme domain was represented as two model stochastic processes, namely axial rotation of the nitroxyl group about the preferable rotation axis and its angular oscillation with respect to the molecular axes. The preferable rotaion axis had a two-dimensional Gauss distribution in the angle space. The experimental EPR data were consistent with the theoretical spectra.