Evidence for tilted smectic liquid crystalline packing of fd Inovirus from X-ray fiber diffraction

Fibers of the fd strain of Inovirus (filamentous bacteriophage) prepared above its isoelectric point (about pH 4) give X-ray diffraction patterns, ''higher pH patterns'', that differ from the patterns of fd fibers below the isoelectric point, ''lower pH patterns''. The overall distribution of intensity on higher and lower pH patterns is substantially the same, indicating that the virion structure is substantially the same, but the crystalline reflections that define the packing of the virions in crystallites are different. For the lower pH patterns, the crystalline reflections can be indexed on a conventional hexagonal lattice. However, for the higher pH patterns, the crystalline reflections on the equator and first layer line are spread out in a broad, angular, but specific fashion (''layer-line fanning''), unlike disorientation spreading. We interpret this observation to mean that the crystallites in the higher pH fiber are tilted with respect to the fiber axis. This interpretation is supported by simulated fiber diffraction patterns calculated from models of the packing of tilted virions. The diffraction patterns from fibers of some mutants of fd, and from other wild-type strains of Inovirus (M13, IKe, If1), do not show layer-line fanning. We propose that the tilting of crystallites that gives rise to layer-line fanning in fd fibers is due both to a specific pattern of charge on the surface of the fd virion at higher pH and to the presence of a specific inter-subunit hydrogen bond. Apparently these features of the fd virion structure require a strict 2-fold screw axis along the virion axis that affects the packing between neighboring virions, resulting in the formation of a tilted smectic liquid crystal phase.