Counterion-induced abnormal slowdown of F-actin diffusion across the isotropic-to-nematic phase transition.

We report an abnormal slowdown of the longitudinal diffusion of F-actin across the isotropic-to-nematic phase transition region. To probe the underlying physics of this counterintuitive discovery, we compared the diffusion of F-actin, microtubules and fd virus in F-actin solutions across the transition region and found the F-actin diffusion markedly different from the other two filament types. Also, the viscous drag probed by F-actin was found to increase sharply with [Mg(2+)] in the nematic but not in the isotropic state. Based on the experimental results, we propose that the abnormal slowdown is caused by the weak electrostatic attraction between actin filaments in the nematic phase, in which neighboring filaments in parallel associate with each other transiently as they collide due to thermal fluctuations.