Spectroscopic probes of muscle cross-bridge rotation.

Recent progress in the quantitative study of the mechanics, biochemistry, and molecular structure of muscle has resulted in the formulation of detailed models for the molecular mechanism of force production (reviewed in Refer­ ences 18 and 31). Most of these models involve the motions of myosin cross-bridges, which are coupled to the actin-activated ATPase cycle of myosin. Spectroscopic probes play an essential role in testing these models. In general, spectroscopic methods permit studies to be carried out under physiological or near-physiological conditions that are often inaccessible to more invasive analytical techniques such as electron microscopy or chemical analysis. The detection of site-specific signals, particularly from extrinsically introduced molecular probes attached to myosin heads, offers the kind of selectivity that is not usually possible with mechanical or x-ray diffraction studies. Most importantly, each spectroscopic technique is sensitive to its own specific physical properties, so a judicious choice of technique permits a relatively direct selection of physical parameters that are particularly relevant to contraction models. Techniques sensitive to orientation and rotational motion are particularly important, since cross-bridge rotations are the key events according to most models (18, 32). The present review surveys the recent literature on spectroscopic studies of cross-bridge orientation and rotational motion. Emphasis is placed on applica­ tions to intact or skinned fibers, although studies on myofibrils or purified myosin are also included in cases where they are of particular relevance to fiber work or where extensions of the techniques to fibers are likely. The review is divided into two main sections, optical probes and spin probes.