Physical basis of the rheologic properties of F-actin.

The viscoelastic properties of purified rabbit skeletal muscle actin filaments (F-actin) were measured at physiologic ionic strength and pH over a range of concentrations and filament lengths. Although F-actin demonstrated transitory elastic behavior, viscous flow was observed at longer times consistent with a high degree of filament overlap. The compliance was independent of stress over a 4-fold range, implying that the measurement did not disrupt any interfilament "bonds". The dynamic storage modulus increased monotonically with frequency over the range measured, whereas the dynamic loss modulus had a relative minimum and was always less than the dynamic storage modulus. These observations are typical of topologically constrained behavior. The absolute value of the complex dynamic viscosity of F-actin, varied as the -0.8 power of the frequency and at a frequency of 0.1 radians/s was proportional to the product of the weight average filament length raised to the 0.7 power and the concentration. The experimental data agreed well with the predictions of a theory of the rheologic behavior of stiff rods in semidilute solutions. We conclude that the mechanical behavior of pure F-action solutions can be explained on the basis of the mutual topologic constraints to diffusion of long stiff rods which do not otherwise interact.