The structural dynamics of myosin during muscle contraction can be discerned in situ through site directed spin labeling of myosin and electron paramagnetic resonance (EPR) spectroscopy. To achieve in situ measurements, spin labeled myosin regulatory light chain (RLC) is exchanged for endogenous RLC in rabbit psoas fiber bundles. In order to ensure the structural integrity of exchanged muscle fibers, functional measurements must be done before and after exchange. After verifying function after RLC exchange, we can use EPR to measure the orientational dynamics of the RLC in a variety of states during muscle contraction.
