Isotropic to Nematic Liquid Crystalline Phase Transition of $F$-Actin Varies from Continuous to First Order

We report that the properties of the isotropic to nematic liquid crystalline phase transition of $F$-actin depend critically on the average filament length. For average filament lengths longer than $2\mu \mathrm{m}$, we confirm previous findings that the phase transition is continuous in both alignment and concentration. For average filament lengths shorter than $2\mu \mathrm{m}$, we show for the first time a first order transition with a clear discontinuity in both alignment and concentration. Tactoidal droplets of coexisting isotropic and nematic phases, differing in concentration by approximately 30%, form over the course of hours and appear to settle into near equilibrium metastable states.

Figure 1

Birefringence measurement of $F$-actin solutions at $l=11\mu \mathrm{m}$ (a),(b),(c) and at $l=1\mu \mathrm{m}$ (d),(e),(f). The $50\mu \mathrm{m}$ scale bar and the gray scale intensity bar, indicating retardance values from 0 to 10 nm, apply to all the images. The line segments represent the local direction and relative magnitude of filament alignment. (a),(b),(c) Representative samples are shown in the $I$ phase, in the $I\mathrm{\text{−}}N$ transition region for long $F$-actin, and in the $N$ phase, respectively. No discontinuity is detected in the transition region. (d),(e),(f) At $l=1\mu \mathrm{m}$ and in the $I\mathrm{\text{−}}N$ transition region, $F$-actin phase separates into tactoidal droplets: $N$ droplets in an $I$ background (d), coexistence of $N$ and $I$ droplets (e), and an $I$ droplet in a $N$ background (f).

Figure 2

A tactoid measured by fluorescence microscopy (top) and birefringence microscopy (middle) in a solution of $10.8\mathrm{mg}/\mathrm{ml}$ $F$-actin. The arrows depict line scans performed across the width of the tactoid and the $25\mu \mathrm{m}$ scale bar applies to both images. Fluorescence intensity (⋄), birefringence retardance (●), and a fit of a scaled semicircle to the birefringence data (solid line) are plotted. The plot illustrates the agreement between the two methods of imaging, both showing the three-dimensional symmetry of the tactoid. The fluorescence image reveals distinct concentrations in each region and yields a result $36\pm 5\%$ greater inside the tactoid than the surrounding solution. The birefringence image shows a clear difference in alignment inside the nematic tactoid and the surrounding solution.

Figure 3

The specific retardance of $F$-actin is plotted as a function of concentration with varying $l$. An $F$-actin solution with no added gelsolin (□) and with average filament lengths of $3.9\mu \mathrm{m}$ (○), $2.5\mu \mathrm{m}$ (▵), and $1.0\mu \mathrm{m}$ (⋄). The letter designations correspond to regions denoted in Fig. 1. The dashed lines are guides for the eye.