The study of nitroxide radical active esters as spin labels on muscle protein actin.

Nitroxide radical active esters were used to label the muscle protein actin and to study the orientation dependence of the EPR spectra of the labelled protein. It is concluded that the labels are located at least at two different sites of the protein with strong polar environment and different mobilities. The EPR spectrum of the strongly immobilized labels exhibits orientation dependence, the N--O bond axis of the spin labels is nearly perpendicular to the long axis of the F-actin threads, and the labels undergo a rapid rotational motion about an axis directed perpendicular to the filament axis. The application of the nitroxide radical active esters may be useful in the study of ordered systems as muscle or muscle protein systems.