The liquid-liquid phase separation to form droplets from intrinsically disordered proteins (IDPs) is driven in many cases by the complexation of oppositely charged polyelectrolytes, followed by the further association of these polyelectrolyte complexes into extended assemblies that are held together by weak electrostatic interactions. This process known as complex coacervation in the biomaterials community has been suggested to facilitate bio-inspired coating and wet adhesion, and to be dominantly entropy-driven as facilitated by the release of salt and water from the IDP surface. A consensus property of the complex coacervate phase is high fluid dynamics, rooted in low cohesive energy between the polyelectrolyte complexes and weakly bound water constituting the fluid. This in turn is consistent with the low interfacial tension typically found between the complex coacervate fluid and water, and the tendency for the complex coacervate droplets to fuse and coalesce, as well as engulf exogenous molecules or surfaces. The question we ask is what are the molecular-level consequences for the polyelectrolytes undergoing complex coacervation? Based on electron paramagnetic resonance (EPR) and Overhauser dynamic nuclear polarization (ODNP) measurements using polyelectrolyte-tethered spin labels, we interrogate the conformational and dynamic state of the IDP constituents within the  highly condensed and fluidic complex coacervate phase, and find that the IDP is dynamic, fully hydrated and populates a conformational ensemble resembling a dilute solution. We will discuss the potential functional role of the droplet state for IDPs.

Complex coacervation as a dynamic reservoir of intrinsically disordered proteins

253rd National Meeting (2017)

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