Temperature-induced sol-gel transition and microgel formation in α-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 α-actinin. Cross-linking was regulated in a reversible way by varying the temperature through the association-dissociation equilibrium of the actin–α-actinin system. Viscoelastic parameters [shear storage modulus G′(ω), phase shift tan(Φ)(ω), 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′(ω) and tan(Φ)(ω) were studied at a frequency ω corresponding to the elastic plateau regime of the G′(ω) versus ω spectrum of the purely entangled solution. The microstructure of the networks was viewed by negative staining electron microscopy (EM). The phase shift tan(Φ) (or equivalently the viscosity η) diverges and reaches a maximum when approaching the apparent gel point from lower and higher temperatures, and the maximum defines the gel point (temperature [Formula Presented]). The elastic plateau modulus [Formula Presented] diverges at temperatures beyond this gel point T<[Formula Presented] but increases only very slightly at T>[Formula Presented]. 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 [Formula Presented] at T<[Formula Presented] is analyzed by a power law of the form [Formula Presented]∼[p(T)-[Formula Presented][Formula Presented] where p(T) is the temperature dependent fraction of crosslinks formed. A power of γ=1.5–1.8 is found. Negative staining EM shows (1) that the gel is essentially homogeneous above the cross-linking transition (T>[Formula Presented]), (2) that microscopic segregation takes place at T⩽[Formula Presented] leading to local formation of clusters (a state termed microgel), and (3) that at low actin–α-actinin ratios ([Formula Presented]≤10) and low temperatures (T≤10 °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.