ROTATIONAL AND TRANSLATIONAL DIFFUSION OF A RODLIKE VIRUS IN RANDOM COIL POLYMER-SOLUTIONS

Depolarized dynamic light scattering was used to measure the translati onal and rotational diffusion of tobacco mosaic virus, TMV, in aqueous solutions of dextran (M similar to 505 000). TMV is an electrically c harged, nucleoprotein assembly with the shape of a stiff, rigid rod. D extran is an uncharged, flexible carbohydrate polymer. The TMV was hel d at a fixed, dilute concentration (0.5 mg/mL), while the concentratio n of dextran spanned both dilute and semidilute regimes (0-14.5% by we ight). There was no evidence of phase separation or strong aggregation of the TMV particles in the presence of the dextran. The TMV particle s dominated the depolarized scattering at all dextran concentrations. The angular variation of the decay rates of the autocorrelation functi ons always followed the form expected for symmetric top molecules in t he absence of translational-rotational coupling. Nevertheless, transla tional and rotational motions are almost surely coupled in most dextra n-containing solutions. The apparent translational and rotational diff usion rates decreased with added dextran, but not exactly according to the rise in macroscopic solution viscosity. A transition occurred at about 6.5% dextran. Beyond this concentration, pronounced failures of the continuum (Stokes-Einstein) relation between diffusion and viscosi ty were found. Translational diffusion continued more rapidly than exp ected on the basis of the macroscopic viscosity, while rotational diff usion fell sharply below expectation. The quotient D-r/D-t of rotation al and translational diffusion, which presumably cancels effects due t o viscosity, also dropped suddenly above the transition point. These f indings are consistent with a sudden onset of topological constraints to rotational motion of the TMV, without onset of severe constraints t o translational motion. Temperature dependent studies showed that eith er the solution or the solvent viscosity can describe translation and rotation fairly well, at least at concentrations below the transition. Energies of activation for translational and rotational diffusion of TMV were similar and not strongly dependent on dextran concentration i n this regime.