The structures of the compact and swollen southern bean mosaic virus (SBMV) particles have been compared by X-ray diffraction and proton magnetic resonance (PMR). Small-angle X-ray scattering showed that removal of divalent cations at alkaline pH causes the particle diameter to increase from 289 A in the native SBMV by 12% in solution and by 9% in microcrystals. The swelling is fully reversible upon re-addition of Ca2+ and Mg2+ ions, as shown by the X-ray patterns at 6 A resolution and by the 270 MHz PMR spectra. Beyond 30 A resolution, X-ray patterns from the compact SBMV in solution and in microcrystals show fine fringes of similar to 1/225 A(-1) width extending to 6 A resolution, whereas patterns from the swollen SBMV in solution and in microcrystals show only broader fringes of similar to 1/90 A(-1) width, Model calculations demonstrate that the fine fringes from compact SBMV arise from regular packing of the protein subunits on the icosahedral surface lattice; the smearing of fine fringes in the swollen virus pattern can be simulated by uncorrelated displacements of pentamers and hexamers of protein subunits, with a standard deviation of 6 A from their mean locations. The PMR spectrum of compact SBMV is poorly resolved, whereas PMR spectrum of swollen SBMV shows sharp resonances in the methyl proton region. The line-narrowing for a fraction of the aliphatic protons upon swelling cannot be accounted for by rotational relaxation of the particle of 6 x 10(6) MW, but must be attributed to internal motion in small regions of the protein subunits. (c) 2017 Medical Research Council. Published by Elsevier Inc.