The time-resolved fluorescence polarization anisotropy signal has been measured from fluorescent-labeled myosin cross-bridges in single glycerinated muscle fibers in the relaxed and rigor states. In one experimental configuration, the polarization of the excitation light and the fiber axis are aligned, and the anisotropy is sensitive to rotational motions of the probes about axes other than the fiber axis. The rotational correlation times are ~1000 ns for relaxed fibers and >7000 ns for rigor fibers. In another experimental configuration, the excitation light polarization is perpendicular to the fiber axis, and its propagation vector has a component parallel to the fiber axis so that the anisotropy is sensitive to probe rotational motion about different axes, including the fiber axis. In this configuration, the rotational correlation times are ~300 ns for both relaxed and rigor fibers. The theory of rotational diffusion in a potential described in a related paper [Burghardt, T. P. (1985) Biophys. J. (in press)] is applied to the relaxed fiber data.
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