On the intensity reversal of the "tropomyosin reflexions" in X-ray diffraction patterns from crab striated muscle.

X-ray diffraction patterns have been recorded from skinned single fibres obtained from crab leg muscle, and the outer parts of layer-lines indexed as orders of 38.5 nm, which have been assigned to tropomyosin and actin, were examined. Fibres at normal length in the presence of ATP (gamma-S), and over-stretched fibres in rigor solutions, show no intensity reversal of the 2nd and the 3rd layer-lines when the Ca2+ concentration is raised. The results are discussed with reference to the mechanism of Ca2+ regulation. Fibres in the presence of Mg-ADP and vanadate ion (Vi) and fibres pretreated at low pH, though generating no substantial tension at high Ca2+ concentration, give rise to rigor-like patterns which are dependent on Ca2+ concentration.