Colloidal probe study of short time local and long time reptational motion of semiflexible macromolecules in entangled networks

Abstract

We introduce a new colloidal probe technique to study local and long range motions of semiflexible actin filaments along their reptation tube axis in entangled networks. Single test filaments decorated with colloidal gold and latex beads are embedded in semidilute actin networks. The chain diffusive motion is explored by measuring the mean square displacement (MSD) of the colloidal probes in direction parallel and perpendicular to the reptation tube axis for times up to 20 min. The quasistatic constraints imposed by the neighbouring chains on the test filament are characterized in terms of an effective interaction potential V (r→) experienced by the test chain obtained from the distribution function of the transverse bead displacements. Remarkably large fluctuations of the width of the interaction potential and thus the tube diameter are found. At short times 4 × 10 -3 ≤ t ≤ 5 × 10-2 s the time dependencies of the MSD in both directions obey power laws 〈Δxi2(t)〉 ∝ t3/4 showing that the chain dynamics in this time regime can be characterized as a subdiffusive process. Between 0.5 and 1 s the MSD for both directions saturates. At times t > 1 s the parallel MSD crosses over into a linear regime following 〈Δx2(t)〉 ∝ t1 which is typical for normal Brownian diffusion. In this regime chain length dependent self-diffusivities of Drept ∼ 10-15-10-17 m2/s are measured which compare well with the value obtained by previous rheological measurements of the terminal relaxation time.