Rotational and Translational Diffusion of Tobacco Mosaic Virus in Extended and Globular Polymer Solutions

Abstract

Depolarized dynamic light scattering is used to measure the translational and rotational diffusion of a rodlike probe, tobacco mosaic virus, in matrix solutions of dextran, an extended polymer with some branching, and globular Ficoll. Translation and rotation both decline almost exponentially as the concentration of either matrix rises. The ratio of rotational to translational diffusion is similar in dextran or Ficoll solutions and close to the values expected from continuum theories of the friction of rodlike particles. Reinforcing a continuum picture in which hydrodynamic effects surpass any due to topological constraints, the declines in rotational and translational motion are almost inversely proportional to the solution viscosity. Only modest and gradual deviations from Stokes−Einstein behavior are observed, even at high matrix concentrations. This stands in stark contrast to an earlier study by this group [Macromolecules 1997, 30, 4920−4926]. The difference may be traced to the subtle effects of optical rotation (dextran and Ficoll are chiral) on instrument alignment, coupled to the weak depolarization of the strongly scattering tobacco mosaic virus and the very slow rotations encountered at high matrix content. In optically inactive solutions, and even in optically active ones studied with the correct and tedious alignment, a particle the shape and size of tobacco mosaic virus can serve as an effective microrheological probe. Confirming this conclusion, the apparent microviscosity obtained by inverting rotational or translational diffusion coefficients reflected the molecular weight trend, at a particular concentration, of shear viscosity measured in a cone and plate device.