Temperature-induced sol-gel transition and microgel formation in alpha-actinin cross-linked actin networks: A rheological study

We have studied the sol-gel transition, the viscoelastic and the structural properties of networks constituted of semiflexible actin filaments cross-linked by alpha-actinin. Cross-linking was regulated in a reversible way by varying the temperature through the association-dissociation equilibrium of the actin-alpha-actinin system. Viscoelastic parameters [shear storage modulus G'(omega), phase shift tan(phi)(omega), creep compliance J(t)] were measured as a function of temperature and actin-to-cross-linker ratio by a magnetically driven rotating disc rheometer. G'(omega) and tan(phi)(omega) were studied at a frequency omega corresponding to the elastic plateau regime of the G'(omega) versus omega spectrum of the purely entangled solution. The microstructure of the networks was viewed by negative staining electron microscopy (EM). The phase shift tan(phi) (or equivalently the viscosity eta) diverges and reaches a maximum when approaching the apparent gel point from lower and higher temperatures, and the maximum defines the gel point (temperature T-g). The elastic plateau modulus G(N)' diverges at temperatures beyond this gel point T<T-g but increases only very slightly at T>T-g. The cross-linking transition (corresponding to a sol-gel transition at zero frequency) is interpreted in terms of a percolation model and the divergence of G(N)' at T<T-g is analyzed by a power law of the form G(N)'similar to[p(T)-p(c)](gamma) where p(T) is the temperature dependent fraction of crosslinks formed. A power of gamma=1.5-1.8 is found. Negative staining HM shows (1) that the gel is essentially homogeneous above the cross-linking transition (T>T-g), (2) that microscopic segregation takes place at T<T-g leading to local formation of clusters (a state termed microgel), and (3) that at low actin-alpha-actinin ratios (r(A alpha)less than or equal to 10) and low temperatures (T less than or equal to 10 degrees C) macroscopic segregation into bundles of cross-linked actin filaments and a diluted solution of actin filaments is observed. The three regimes of network structure are represented by an equivalent phase diagram.