Local Water Dynamics in Coacervated Polyelectrolytes Monitored through Dynamic Nuclear Polarization-Enhanced ¹H NMR

Abstract

We present the first study of quantifying the diffusion coefficient of interfacial water on polyelectrolyte surfaces of systems fully dispersed in bulk water under ambient conditions. Such measurements were made possible through the implementation of a recently introduced Dynamic Nuclear Polarization (DNP) technique to selectively amplify the nuclear magnetic resonance (NMR) signal of hydration water that is interacting with specifically located spin labels on polyelectrolyte surfaces. The merit of this novel capability is demonstrated in this report through the measurement of solvent microvisosity on the surface of two types of oppositely charged polyelectrolytes, when freely dissolved versus when complexed to form a liquid-liquid colloidal phase called complex coacervates. These complex coacervates were formed through electrostatic complexation between the imidazole-based cationic homopolymer poly(N-vinylimidazole) (PVIm), and anionic polypeptide polyaspartate (PAsp) in the pH range of 4.5 -6.0, under which conditions the coacervate droplets are highly fluidic yet densely packed with polyelectrolytes. We also investigated the rotational diffusion coefficients of the spin labels covalently bound to the polyelectrolyte chains for both PVIm and PAsp, showing a 5 fold change in the rotational correlation time as well as anisotropy parameter upon coacervation, which represents a surprisingly small decrease given the high polymer concentration inside the dense microdroplets. For both DNP and ESR experiments, the polymers were covalently tagged with stable nitroxide radical spin labels (∼1 wt %) to probe the local solvent and polymer segment dynamics. We found that the surface water diffusion coefficients near uncomplexed PVIm and PAsp at pH 8 differ, and are around D∼1.3×10 −9 m 2 / s. In contrast, inside the complex coacervate phase, the water diffusion coefficient in the immediate vicinity of either polyelectrolyte was D∼ 0.25×10 −9 m 2 / s, which is about an order of magnitude smaller than the bulk water self diffusion coefficient, and yet orders of magnitude greater than that of associated, bound, hydration water. This observation suggests the existence of measurable water inside complex coacervates with relatively high diffusion and exchange dynamics, implying that water moves in nanometer-scale pore spaces as opposed to being structurally bound or even absent. We infer from our observation that the PVIm and PAsp chains are undergoing roughly pairwise association, so that largely charge neutralized species compose the concentrated, yet fluidic and partially hydrated coacervate cores.

Simple peptide coacervates adapted for rapid pressure-sensitive wet adhesion

“…ODNP data thus reveal that the mobility of hydration water surrounding the peptide constituents is significantly restricted in the mfp-3S-pep SCC phase that we had confirmed to be composed of highly confined and motionally restricted peptide molecules. This property is in stark contrast to the wet and highly dynamic environment found within the mfp-3F-pep/HA CC phase that correspond to characteristics found in several other CC systems made of intrinsically disordered and charged biopolymers 19,39,40 . Based on this, it is easy to suggest that the mfp-3S-pep SCC phase must be virtually dry, and on its way to forming a condensed solid gel.…”

Investigation of the interfacial tension of complex coacervates using field-theoretic simulations

“…Such association can take the form of layer-bylayer assembly of polymers on a charged substrate, 1,2 or it could involve charge complexation in solution. [3][4][5] Examples of emerging technologies that rely on these and related phenomena include wet adhesives for medical, construction, consumer, and military use, 6,7 advanced biosensors, drug and gene delivery vehicles, [8][9][10] and responsive and switchable surfaces. 11,12 When the association takes place in solution, depending on the strength of the polyelectrolyte and the solvent conditions, the resulting polyelectrolyte complex can either be a solid precipitate or a polyelectrolyte-rich liquid phase termed a "complex coacervate."…”

“…Many of the applications exploiting complex coacervation that are currently under exploration depend critically on the interfacial properties of the coacervate phase with its supernatant, notably the ultra-low interfacial tension, yet the characterization of the interfacial properties of these systems has been limited. 5,13 It would be advantageous to employ modeling techniques to investigate the interfacial properties of coacervate systems, but it is a significant challenge to simulate such phases; accurate descriptions of the coacervate phase must take into account both the long-range electrostatic effects as well as a) Electronic mail: ghf@mrl.ucsb.edu.…”

Liquid state DNP for water accessibility measurements on spin-labeled membrane proteins at physiological temperatures

“…The DNP parameter therefore contains 'water accessibility' information in terms of local dynamics of water-label collisions (n) and in terms of exchange between local water and bulk water (f). Up to date, biochemical DNP studies focused on monitoring protein aggregation [22] or hydration dynamics in synthetic soft matter [23][24][25][26] and water soluble proteins [27,28]. The hydration dynamics studies made use of the theoretical force-free hardsphere model [29] to extract absolute values of water diffusivity.…”