HIGH-FREQUENCY VISCOELASTICITY OF CROSS-LINKED ACTIN FILAMENT NETWORKS MEASURED BY DIFFUSING WAVE SPECTROSCOPY

We study the short-time relaxation dynamics of crosslinked and uncross linked networks of semi-flexible polymers using diffusing wave spectro scopy (DWS). The networks consist of concentrated solutions of actin f ilaments, crosslinked with increasing amounts of alpha-actinin. Actin filaments (F-actin) are long semi-flexible polymers with a contour len gth 1-100 mu m and a persistence length of 5-15 mu m; alpha-actinin is a small 200 kDa homodimer with two actin-binding sites. Using the lar ge bandwidth of DWS, we measure the mean-square-displacement of 0.96 m u m diameter microspheres imbedded in the polymer network, from which we extract the frequency-dependent viscoelastic moduli via a generaliz ed Langevin equation. DWS measurements yield, in a single measurement, viscoelastic moduli at frequencies up to 10(5) Hz, almost three decad es higher in frequency than probed by conventional mechanical theology . Our measurements show that the magnitude of the small-frequency plat eau modulus of F-actin is greatly enhanced in the presence of alpha-ac tinin, and that the frequency dependence of the viscoelastic moduli is much stronger at intermediate frequencies. However, the frequency-dep endence of loss and storage moduli become similar for both crosslinked and uncrosslinked networks at large frequencies, G' (omega) proportio nal to G '' (omega) proportional to omega(0.75+/-0.08). This high-freq uency behavior is due to the small-amplitude, large-frequency lateral fluctuations of actin filaments between entanglements.