Biomacromolecules rely on the precise placement of monomers to encode information for structure, function, and physiology. Efforts to emulate this complexity via the synthetic control of chemical sequence in polymers are finding success; however, there is little understanding of how to translate monomer sequence to physical material properties. Here we establish design rules for implementing this sequence-control in materials known as complex coacervates. These materials are formed by the associative phase separation of oppositely charged polyelectrolytes into polyelectrolyte dense (coacervate) and polyelectrolyte dilute (supernatant) phases. We demonstrate that patterns of charges can profoundly affect the charge–charge associations that drive this process. Furthermore, we establish the physical origin of this pattern-dependent interaction: there is a nuanced combination of structural changes in the dense coacervate phase and a 1D confinement of counterions due to patterns along polymers in the supernatant phase.
Charged polymers
are ubiquitous in biological systems because electrostatic
interactions can drive complicated structure formation and respond
to environmental parameters such as ionic strength and pH. In these
systems, function emerges from sophisticated molecular design; for
example, intrinsically disordered proteins leverage specific sequences
of monomeric charges to control the formation and function of intracellular
compartments known as membraneless organelles. The role of a charged
monomer sequence in dictating the strength of electrostatic interactions
remains poorly understood despite extensive evidence that sequence
is a powerful tool biology uses to tune soft materials. In this article,
we use a combination of theory, experiment, and simulation to establish
the physical principles governing sequence-driven control of electrostatic
interactions. We predict how arbitrary sequences of charge give rise
to drastic changes in electrostatic interactions and correspondingly
phase behavior. We generalize a transfer matrix formalism that describes
a phase separation phenomenon known as “complex coacervation”
and provide a theoretical framework to predict the phase behavior
of charge sequences. This work thus provides insights into both how
charge sequence is used in biology and how it could be used to engineer
properties of synthetic polymer systems.
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