Diffusion of actin filaments within a thin layer between two walls

Diffusion of the protein filaments F-actin confined in a thin layer between two walls is studied using the methods of single filament fluorescence imaging and particle tracking. The translational and rotational diffusion coefficients are measured for F-actin of lengths in the range of 1.5-5 mum. The length dependence of the measured diffusion coefficients is consistent with the predicted two-dimensional projection of the diffusion of a cylinder in an unbounded fluid. Fits based on the formulas for diffusion in the bulk fluid yield higher apparent viscosity values than that of the buffer solution by a factor of 2 for a layer thickness between 0.7 and 1.6 mum. We show that the measured results can be accounted for by correction based on the hydrodynamic theory of a long cylinder between confining walls.