[Dynamic mobility of the histidine-containing domain of spin-labeled lysozyme]

The hen egg-white lysozyme was modified by the spin label (2,2,6,6-tetramethylpiperidine-N1-oxyl-4-iodacetamide) at the single histidine residue His-15. The rotational correlation time of the molecular carrier was found to be defined by the mobility of the histidine-bearing domain and not influenced by the protein monomer shape at pH 4.7 and dimer shape at pH 7.1. The dependence of viscosity at 1 degree C on the distance between outer wide peaks in the immobilized EPR spectra enabled us to evaluate rotational correlation time of the domain. The molecular mass of the latter was close to the data obtained by X-ray analysis. The spin label was highly mobile at room temperature, as the EPR spectrum displayed the triple shape; at 1 degree C it was immobilized. The new general approach to the EPR spectra simulation was applied to all experimental EPR spectra. This approach is based on a substitution of an undefined stochastic process of the spin label reorientation relative to the lysozyme domain by the defined modelled stochastic processes: axial rotation of the nitroxide relative to the preferable axis and angular oscillations of the nitroxide relative to axes of the molecular coordinate system. Each of the modelled stochastic processes leads to a relative partial averaging of the magnetic tensor components. A set of discrete partially averaged states is introduced with the relative cluster of the spin-labelled molecules. The resulting EPR spectrum is assumed to be the sum of EPR spectra from all the clusters. A good fitting of all simulated EPR spectra is obtained.