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.2013 May 13;14(5):1395-402.
doi: 10.1021/bm4000579. Epub 2013 Apr 19.

Asymmetric collapse in biomimetic complex coacervates revealed by local polymer and water dynamics

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Asymmetric collapse in biomimetic complex coacervates revealed by local polymer and water dynamics

Julia H Ortony et al. Biomacromolecules..

Abstract

Complex coacervation is a phenomenon characterized by the association of oppositely charged polyelectrolytes into micrometer-scale liquid condensates. This process is the purported first step in the formation of underwater adhesives by sessile marine organisms, as well as the process harnessed for the formation of new synthetic and protein-based contemporary materials. Efforts to understand the physical nature of complex coacervates are important for developing robust adhesives, injectable materials, or novel drug delivery vehicles for biomedical applications; however, their internal fluidity necessitates the use of in situ characterization strategies of their local dynamic properties, capabilities not offered by conventional techniques such as X-ray scattering, microscopy, or bulk rheological measurements. Herein, we employ the novel magnetic resonance technique Overhauser dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP), together with electron paramagnetic resonance (EPR) line shape analysis, to concurrently quantify local molecular and hydration dynamics, with species- and site-specificity. We observe striking differences in the structure and dynamics of the protein-based biomimetic complex coacervates from their synthetic analogues, which is an asymmetric collapse of the polyelectrolyte constituents. From this study we suggest charge heterogeneity within a given polyelectrolyte chain to be an important parameter by which the internal structure of complex coacervates may be tuned. Acquiring molecular-level insight to the internal structure and dynamics of dynamic polymer complexes in water through the in situ characterization of site- and species-specific local polymer and hydration dynamics should be a promising general approach that has not been widely employed for materials characterization.

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Figures

Figure 1
Figure 1
(a) mfp151 sequence, where mfp1 segments (AKPSYPPTYK)6 flank the mfp5 mid-segment. Cationic residues are indicated by red and anionic residues are indicated by blue. The single cysteine is indicated in green. Mfp151 is spin labeled (mfp151SL) by covalent functionalization of the single cysteine by the spin label MTSL; (b) Hyaluronic acid (35 kDa) is spin labeled (HA-SL) by addition of 4-amino-TEMPO in the presence of EDC; (c) Turbidity of mfp151 / HA shows maximum coacervation occurs at 60 % mfp151 by mass; (d) Differential interference contrast (DIC) micrograph of mfp151 / HA complex coacervate suspension in water.
Figure 2
Figure 2
(a) EPR of mfp151SL alone (red) and after coacervation with HA (black); (b) EPR of HA-SL alone (blue) and after coacervation with mfp151 (black).
Figure 3
Figure 3
Water diffusion coefficients, determined by dynamic nuclear polarization enhanced NMR spectroscopy (DNP), show that under optimal coacervation conditions (0.6 fraction mfp151) water diffusion is minimized around mfp151SL and maximized around HA-SL.
Figure 4
Figure 4
(a) The mfp1 protein mutated at three select sites is then spin labeled (SL) by addition of MTSL, yielding mfp1SL-N, mfp1SL-m, and mfp1SL-C. Positively charged residues are designated in red and the single cysteine site at which the spin label is installed is indicated in green. No negatively charged residues occur in the mfp1 sequence. (b) EPR of each mfp1SL species indicates similar local environments when the polymer chains are freely dissolved in water; (c) Synthetic routes to poly(aspartic acid) (PAsp) and spin labeled poly(aspartic acid) (PAsp-SL).
Figure 5
Figure 5
Water diffusion coefficients determined by DNP show the collapse of mfp1 upon complex coacervation with HA at each of three selected sites, near the N-terminus, the middle of the protein chain, and the C-terminus. In contrast, the water diffusion coefficient of HA-SL is shown to increase upon complex coacervation with mfp1.
Figure 6
Figure 6
Illustration of the collapse of cationic protein (red) and the liberation of anionic biopolymer (blue) that occur upon complex coacervation of biomimetic polyelectrolytes.
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