When oppositely charged polyelectrolytes are mixed below a critical salt concentration, their mixtures show macroscopic phase separation into a dilute and a dense, polyelectrolyte complex phase. Binodal compositions of the polyelectrolyte complexes have been measured experimentally using fluorescently labeled polyelectrolytes. We used fluorescein-labeled poly(acrylic acid) (PAA) of four different chain lengths (N = 20, 50, 150, and 510) to determine the binodal compositions of polyelectrolyte complexes of PAA and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) of similar chain lengths. The water content of polyelectrolyte complexes obtained has a lower limit of about 65%, practically independent of chain length, and increases with increasing salt concentration. We interpret our results on binodal compositions, water content and critical salt concentration as a function of chain length using the mean-field model of Voorn and Overbeek and find good quantitative agreement with our experiments using only one adjustable parameter. We believe that such a model can be used to predict equilibrium concentrations also for other strongly charged flexible polyelectrolytes.

Two flexible, oppositely charged polymers can form liquid-like complex coacervate phases with rich but poorly understood viscoelastic properties. They serve as an experimental model system for many biological and man-made materials made from oppositely charged macromolecules. We use rheology to systematically study the viscoelastic properties as a function of salt concentration, chain length, chain length matching, and mixing stoichiometry of model complex coacervates of poly(N,N-dimethylaminoethyl methacrylate), PDMAEMA, and poly(acrylic acid), PAA. The dynamics of making and breaking ionic bonds between the oppositely charged chains underlie all linear viscoelastic properties of the complex coacervates. We treat (clusters of) ionic bonds as sticky points and find that there is a remarkable resemblance between the relaxation spectra of these complex coacervates and the classical sticky Rouse model for single polymer systems. Salt affects all relaxation processes in the same way, giving rise to a widely applicable time–salt superposition principle. The viscoelastic properties of the complexes are very different from those of the individual components. In the complexes with a chain length mismatch, the effect of the mismatch on the viscoelastic properties is not trivial: changing the length of the polycation affects the relaxation behavior differently from changing the length of the polyanion.

In this Letter we show that in the rheology of electrostatically assembled soft materials, salt concentration plays a similar role as temperature for polymer melts, and as strain rate for soft solids. We rescale linear and nonlinear rheological data of a set of model electrostatic complexes at different salt concentrations to access a range of time scales that is otherwise inaccessible. This provides new insights into the relaxation mechanisms of electrostatic complexes, which we rationalize in terms of a microscopic mechanism underlying salt-enhanced activated processes.

Liquid-liquid phase transitions in complex mixtures of proteins and other molecules produce crowded compartments supporting in vitro transcription and translation. We developed a method based on picoliter water-in-oil droplets to induce coacervation in Escherichia coli cell lysate and follow gene expression under crowded and noncrowded conditions. Coacervation creates an artificial cell-like environment in which the rate of mRNA production is increased significantly. Fits to the measured transcription rates show a two orders of magnitude larger binding constant between DNA and T7 RNA polymerase, and five to six times larger rate constant for transcription in crowded environments, strikingly similar to in vivo rates. The effect of crowding on interactions and kinetics of the fundamental machinery of gene expression has a direct impact on our understanding of biochemical networks in vivo. Moreover, our results show the intrinsic potential of cellular components to facilitate macromolecular organization into membranefree compartments by phase separation.microdroplets | macromolecular crowding P rotocells are minimal compartmentalized systems exhibiting key characteristics of cellular function, including metabolism and replication (1, 2). Lipid vesicles are considered the prototypical protocell as they can form functional microscopic spherical assemblies suited for in vitro gene expression (3, 4). Compartmentalization via lipid bilayers is considered essential for the emergence of cells (4), but there are alternative models based on liquid-liquid phase transitions that lead to the emergence of compartments (5, 6). Compartmentalization is but one characteristic, as protocells ideally also mimic the highly crowded interior of living cells, which have total macromolecule concentrations in excess of 300 g/L (7). Examples in which compartmentalization and high local concentrations are obtained concurrently, include DNA brushes (8), aqueous two-phase systems (9), and liquid coacervates (10). Phase separation or coacervation occurs in a wide range of polymer and protein solutions, often triggered by changes in temperature or salt concentration, or by the addition of coacervating agents (11). The (complex) coacervate droplets that are formed in such systems present macromolecularly crowded, aqueous, physical compartments, 1-100 μm in diameter (12). Recent work has identified similar liquid phase transitions in vivo in the formation of intracellular non-membrane-bound compartments exhibiting liquid-like properties, slowed down diffusion, and strongly interacting macromolecular components (13,14). Well-studied examples are the intracellular localization of DNA or RNA and proteins in Cajal bodies, P granules, and nucleoli (15-17), which can contain over 100 components. Such complexity has not been achieved in two-phase systems in vitro (18,19). Although the physics of coacervates is well understood, progress in their development as protocell models has stalled, because of the lack of demonstrations of complex biochemical proce...

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