Model-independent fluorescence depolarization studies of labeled myosin cross-bridges in muscle fibers

We calculated the fluorescence depolarization signal following polarized excitation using a model-independent approach where the time development of the molecular orientation density is equal to the operation of a linear time-development operator on the initial molecular orientation density. The time-development operator, T, is not explicitly specified and the time dependent fluorescence signal can be expressed in terms of well defined matrix elements of T. We discuss two strategies with which to calculate the matrix elements of T from the experimental time-resolved signal. We applied this formalism to time-resolved studies of fluorescent labeled myosin cross-bridges in relaxed muscle fibers to measure the steady-state cross-bridge angular probability density. We find that using the model of rotational diffusion in an angular potential we can estimate the rank six order parameters of the angular probability density. The rank six order parameters are shown to make a significant contribution to the proposed cross-bridge angular distribution for relaxed muscle fibers.