The sensitivity of saturation-transfer electron paramagnetic resonance to rotational motion in the submillisecond range provides a means of studying rotational motions of the myofibrillar proteins of muscle and segments of these proteins. Estimates of these rotational motions have been made using purified proteins and supramolecular complexes that have been formed by assembly of purified proteins or isolated as such by partially disrupting the intact muscle cell. These motions are of particular relevance to an understanding of the molecular basis of energy transduction in muscle, where the force-generating step is generally believed to involve rotation of the myosin crossbridge about some point on the actin filament. By selectively introducing rigidly bound spin labels into specific muscle proteins and into specific regions of these proteins it has been possible to obtain insights into the molecular motions of myosin and actin, and into the nature of interaction between these proteins.
John C. Seidel, "Rotational Motions of Muscle Proteins Studied by Saturation-Transfer Electron Paramagnetic Resonance," Appl. Spectrosc. 34, 280-288 (1980)