Low-angle X-ray Diffraction from Concentrated Sols of F-Actin

Abstract

X-RAY diffraction maxima from an ordered array of identical particles can arise either from the internal structure of each particle or from an interparticle repeat. If the Bragg spacing varies with the concentration of particles in the preparation, it is generally safe to assume that the spacing is an interparticle one. In the case of F-actin, previous workers^1,2 who studied dried fibres by X-ray diffraction observed an equatorial reflexion with a spacing of 55 Å, which increased in spacing as the relative humidity was raised². It was therefore concluded that this represented an interfilament repeat. It has also been deduced³ that the 120 Å equatorial reflexion from living molluscan smooth muscle arises from the side-by-side packing of actin filaments: this reflexion is obtained as well from vertebrate smooth muscle, and the spacing changes with the tonicity of the medium^4,5. In view of these results it is puzzling that when Vazina et al.⁶ studied concentrated sols of F-actin the positions of their low-angle reflexions did not change with protein concentration; they concluded, in fact, that the diffraction maxima (at 230 and 110 Å) arose from the axial periodicity of the filaments rather than from an interfilament repeat.