Counterion-dependent microrheological properties of F-actin solutions across the isotropic-nematic phase transition.

We studied microrheological properties of F-actin across the isotropic-nematic phase transition region by video particle tracking (VPT) and by laser deflection particle tracking (LDPT). Both methods track the motion of thermally driven micron-sized beads, and convert the temporal mean square displacement (MSD) to shear moduli. The two methods give consistent results for the elastic modulus G' and less so for the loss modulus G'' . As the nematic order parameter increases with actin concentration, G'|| (measured parallel to the nematic director) and G' perpendicular (perpendicular to the director) grow apart, with G' perpendicular larger than G'||. The moduli scale with actin concentration as G'|| approximately c 0.54+/-0.13 and G' perpendicular approximately c 1.38+/-0.15. Furthermore, G' and G'' dependence on [Mg2+] were measured and compared for 1mg/ml isotropic and 4 mg/ml nematic F-actin solutions, respectively. In the isotropic phase, G' increases with [Mg2+] up to 6mM and then plateaus. In the nematic phase, G' perpendicular is larger than G'||, and both G' perpendicular and G'|| increase with [Mg2+] progressively up to 16 mM , above which F-actin form large bundles. In both isotropic and nematic phases, G'' only weakly depends on [Mg2+] . In conclusion, particle tracking microrheology reveals rich rheological features of F-actin affected by the isotropic-nematic phase transition and by tuning weak electrostatic interactions among the protein filaments.