We measured the orientational order parameter of $F$-actin traversing the isotropic-nematic phase transition using a combination of techniques, including fluorescence imaging, local birefringence measurements, and small-angle x-ray scattering. The order parameter approaches a saturated value of 0.75 for actin concentrations above the region of the isotropic-nematic phase transition. This result implies a significant extent of misalignment and consequently entanglement among long actin filaments, even in the nematic phase. We determine the specific birefringence of completely aligned $F$-actin to be $\Delta n_0=2.3\times {10}^{-5}\mathrm{ml}∕\mathrm{mg}$. At concentrations slightly below the isotropic-nematic transition, nonzero values of the order parameter are detected for hours following an initial alignment, indicating extremely slow rotational kinetics of $F$-actin in the entangled networks.

• Received 1 December 2005

DOI:https://doi.org/10.1103/PhysRevE.73.061901