The friction constants of bundle-like particles in solution

In our previous experimental work the following was found: an I-Z-I particle, having a bundle-like macromolecule composed of many thin filaments of actin, had a much larger rotational friction constant than that of a single sarcomere having a more compact bundle-like structure composed of not only actin but also myosin filaments. In this paper, the reason for such a large friction constant of I-Z-I is elucidated by a simple theory (based on the Helmholtz principle in hydrodynamics) in terms of the solvent permeability through the particles. It is demonstrated that if the filaments on the bundle surface are composed of flexible segments, each of which has a local Brownian rotation, the solvent-flow along the surface is strongly depressed because of increased energy dissipation and the solvent permeates through the bundle. The large friction constant of an I-Z-I is suggested to be due to this effect. This situation is very different from that of a single sarcomere without flexible filaments on the surface.