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

Abstract

We have studied the sol-gel transition, the viscoelastic and the structural properties of networks constituted of semiflexible actin filaments cross-linked by \ensuremath{\alpha}-actinin. Cross-linking was regulated in a reversible way by varying the temperature through the association-dissociation equilibrium of the actin--\ensuremath{\alpha}-actinin system. Viscoelastic parameters [shear storage modulus G\ensuremath{'}(\ensuremath{\omega}), phase shift tan(\ensuremath{\Phi})(\ensuremath{\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\ensuremath{'}(\ensuremath{\omega}) and tan(\ensuremath{\Phi})(\ensuremath{\omega}) were studied at a frequency \ensuremath{\omega} corresponding to the elastic plateau regime of the G\ensuremath{'}(\ensuremath{\omega}) versus \ensuremath{\omega} spectrum of the purely entangled solution. The microstructure of the networks was viewed by negative staining electron microscopy (EM). The phase shift tan(\ensuremath{\Phi}) (or equivalently the viscosity \ensuremath{\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 ${\mathit{T}}_{\mathit{g}}$). The elastic plateau modulus ${\mathit{G}}_{\mathit{N}}^{\ensuremath{'}}$ diverges at temperatures beyond this gel point T${\mathit{T}}_{\mathit{g}}$ but increases only very slightly at T>${\mathit{T}}_{\mathit{g}}$. 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 ${\mathit{G}}_{\mathit{N}}^{\ensuremath{'}}$ at T${\mathit{T}}_{\mathit{g}}$ is analyzed by a power law of the form ${\mathit{G}}_{\mathit{N}}^{\ensuremath{'}}$\ensuremath{\sim}[p(T)-${\mathit{p}}_{\mathit{c}}$${]}^{\ensuremath{\gamma}}$ where p(T) is the temperature dependent fraction of crosslinks formed. A power of \ensuremath{\gamma}=1.5--1.8 is found. Negative staining EM shows (1) that the gel is essentially homogeneous above the cross-linking transition (T>${\mathit{T}}_{\mathit{g}}$), (2) that microscopic segregation takes place at T\ensuremath{\leqslant}${\mathit{T}}_{\mathit{g}}$ leading to local formation of clusters (a state termed microgel), and (3) that at low actin--\ensuremath{\alpha}-actinin ratios (${\mathit{r}}_{\mathit{A}\mathrm{\ensuremath{\alpha}}}$\ensuremath{\le}10) and low temperatures (T\ensuremath{\le}10 \ifmmode^\circ\else\textdegree\fi{}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. \textcopyright{} 1996 The American Physical Society.