Orientational order parameter of the nematic liquid crystalline phase of F-actin.

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 Deltan(0)=2.3 x 10(-5) ml/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.