X-ray diffraction patterns from molecular arrangements with 38-nm periodicities around muscle thin filaments

Abstract

A SERIES of meridional and near-meridional reflections have been observed in X-ray diffraction patterns (ref. 1 and Fig. 1a) from the crab Plagusia leg muscle in the living, resting state, and have been attributed to troponin molecules which lie in pairs on thin filaments at 38-nm intervals. In the pattern (ref. 1 and Fig. 1b) obtained from the same muscle in rigor, the corresponding series of reflections was observed to have stronger intensities and higher order reflections. The reflections were interpreted as arising from both troponin and the cross-bridges. Electron micrograph² showed that cross-bridges occurred in symmetrical pairs around the thin filaments with a repeating distance of 38 nm in which one or a few closely spaced pairs may be included. Recently, reflections indexed as orders of 76 nm in X-ray diffraction patterns from insect flight muscle in rigor³ and those from scallop striated muscle in rigor⁴ have also been interpreted in terms of the cross-bridges attached in pairs to thin filaments with a repeating distance of 38 nm. We report here a method that enables us to analyse diffractions generated by molecular arrangements of this type. These arrangements are described by a set of identical helices, and the systematic modulation of intensities, indexed as orders of 76 nm, is interpreted in terms of displacement between these helices.