Modelling the molecular structure of muscle with comparison to X-ray diffraction data

Changes in the molecular structure of striated muscle during contraction can be determined owing to corresponding changes in X-ray diffraction data. The work presented in this thesis analyses X-ray diffraction patterns obtained from frog sartorius muscles using Synchrotron Radiation at two distinctly different stages of the contractile cycle; rest and peak of isometric plateau.;Two-dimensional theoretical diffraction patterns were produced from a three-dimensional computer model of striated muscle and compared with experimental X-ray diffraction data. Modifications were made to both the thick and thin filaments situated within a hexagonal lattice, within the model, however the emphasis of the work was upon the protein myosin. In particular, the configuration, orientation and axial distribution of the myosin heads were analysed until the meridional intensity profile was comparable with experiment. Additionally, the dominant equatorial and row line reflections were modelled to fully define the disposition of the heads in space for every pair.;At rest, the results of this study indicate the myosin heads prefer a flexed configuration about a point defined by the S1-S2 junction. Both myosin heads within a pair adopted similar axial orientations, and one head in each pair favoured a small rotation about the normal to thick filament axis. Azimuthally all myosin heads preferred to project perpendicular from the backbone.;At plateau, the results suggest that a configuration closely mirroring the crystalline myosin head structure at rigor was favoured. Each myosin head within a pair preferred to adopt distinctly different axis orientations with one head perpendicular to the thick filament axis and the other head more inclined to the backbone. The implication that there are two distinct populations of myosin heads suggests that approximately 50% of myosin heads form a stereospecific actomyosin complex at plateau. The axis distribution of the myosin heads at plateau was also investigated. A less compressed regular axial distortion was required in comparison to modelling at rest. Furthermore, two myosin heads periodicities were studied in conjunction with an extension to the thin filament.