X-ray equatorial analysis of crab striated muscle in the relaxed and rigor states.

The structure of muscle projected along the fiber axis was studied by equatorial X-ray diffraction. The electron-density distributions in axial projection of muscle were derived by the Fourier syntheses to a resolution of 7 nm in the relaxed and rigor states. The structure of the thick filament backbone (diameter about 21.5 nm) has a nearly smooth cylindrical surface and a low electron-density core (diameter about 7 nm) in the center. In the relaxes state, the center of gravity of th myosin heads is situated at a radius of 19.6 nm from the center of thick filament, lying just between the surface of the thick filament backbone and the surface of the thin filament (diameter about 8.4 nm). From the electron-density distributions in two states, the amount of mass transfer from the thick filament to the thin filament was estimated. It was in accordance with that predicted from the structure derived by the X-ray layer-line analyses.