The interplay between viscoelastic and thermodynamic properties determines the birefringence of F-actin gels

F-actin gels of increasing concentrations (25-300 mu M) display in vitro a progressive onset of birefringence due to orientational ordering of actin. laments. At F-actin concentrations <100 mu M, this birefringence can be erased and restored at will by sonication and gentle flow, respectively. Hence, the orientational ordering does not result from a thermodynamic transition to a nematic phase but instead is due to mechanical stresses stored in the gels. In contrast, at F-actin concentrations >= 100 mu M, gels display spontaneous birefringence recovery, at rest, which is the sign of true nematic ordering, in good agreement with statistical physics models of the isotropic/nematic transition. Well-aligned samples of F-actin gels could be produced and their small-angle x-ray scattering patterns are quite anisotropic. These patterns show no sign of. lament positional short-range order and could be modeled by averaging the form factor with the Maier-Saupe nematic distribution function. The derived nematic order parameter S of the gels ranged from S=0.7 at 300 mu M to S=0.4 at 25 mu M. Both birefringence and small-angle x-ray scattering data indicate that, even in absence of cross-linking proteins, spontaneous cooperative alignment of actin. laments may arise in motile regions of living cells where F-actin concentrations can reach values of a few 100 mu M.