Effect of Mg Ions on Microrheological Properties of F-actin Solution across Isotropic-Nematic Phase Transition*

We studied microrheological properties of F-actin across the isotropic-nematic phase transition region by video particle tracking and laser deflection particle tracking methods. The two methods give consistent results. As the nematic order parameter increases with the actin concentration, G$'_{x}$ (along the alignment) and G$'_{y}$ grow apart, with G$'_{y}$ larger than G$'_{x}$. The moduli scale with actin concentration as $G'_{x}\propto c^{0.54 \pm0.13}$ and $G'_{y}\propto c^{1.38\pm0.15}$. Furthermore, G$'$ and G$''$ dependence on [Mg$^{2+}$] were measured and compared for 1 mg/ml isotropic and 4 mg/ml nematic F-actin solutions. For isotropic phase, G$'$ increase with [Mg$^{2+}$] up to 6 mM and then plateaus; for nematic phase, G$'_{y}$ is larger and both G$'_{x}$ and G$'_{y}$ increase with [Mg$^{2+}$] monotonically all the way up to 16 mM, above which F-actin bundle formation occurs. In both isotropic and nematic phases, G$''$ only weakly depends on [Mg$^{2+}$]. In conclusion, particle tracking microrheology reveals rich rheological features of F-actin affected by I-N phase transition and by tuning weak electrostatic interactions among the filaments.