Freeze-Fracture Imaging of Ordered Phases of Tobacco Mosaic Virus in Water

Aqueous solutions of Tobacco Mosaic Virus are ideal systems for visualizing "molecular" distributions in nematic and crystalline phases by freeze-fracture electron microscopy. Nematic phases of TMV in water are highly oriented with measured order parameters of 0.93. Twist deformations appear to be most common in the micrographs, confirming experimental and theoretical evidence that the twist elastic constant is smaller than either the splay or bend constants. Both edge and screw disclinations are observed in TMV nematics at molecular resolution, and their overall configurations correspond closely to those predicted by continuum theory. Disclination cores have been visualized for the first time. The edge disclination core is small, no more than a single virus length wide. The virus reorient abruptly by 90° at the core, but appear to remain in the plane of the disclination line. The screw disclination core is several virus lengths in diameter and much more disordered. The virus twist out of the plane perpendicular to the line and into the plane along the disclination line. The virus in colloidal crystal TMV solutions are more positionally ordered than the nematic samples. Although no distinct layers were visible along the virus axis in the freeze-fracture images, a sinusoidal density modulation was observed. With the experimental evidence at hand, it is possible to assign either a smectic-B liquid crystal or a true three-dimensional crystal structure to this phase. Perpendicular to the long axis of the virus, the particles appear hexagonally ordered. An average center-to-center distance of 50 nm, which agrees with that measured by X-ray diffraction, was measured from the micrographs. Freeze-fracture imaging of these anisotropic crystals is an alternative to the more common light microscopy of polymer latex colloid crystals.