Simple coacervation is the liquid-liquid phase separation (LLPS) of a single water-soluble polyelectrolyte solution into two aqueous phases, in contrast to complex coacervation, which is the LLPS of two or more oppositely charged water soluble polyelectrolytes. Protamines are small, arginine(R)-rich proteins that replace histones during spermatogenesis to compact chromatin into the sperm heads of fish, birds, and mammals. Salmine, a protamine isolated from salmon sperm, comprises 32 amino acid residues of which 21 are R, with positively charged guanidinium sidechains (guan+). There are no other ionizable or aromatic sidechains. Salmine sulfate solutions (50 mg/ml salmine, 200 mM Na2SO4) separate into two aqueous liquid phases with an upper critical solution temperature (UCST) of 45°C. The coacervate phase comprises 50 wt% water and 600 mg/ml salmine with a sulfate to guan+ stoichiometry of 1:2. Chloride ions suppress salmine sulfate coacervation. However, in 2M NaCl salmine (50 mg/ml) coacervated in 100% yield with an UCST of 40°C. A set of five synthetic salmine analogs, poly(3-guanidinopropyl methacrylamide-co-acrylamide), were polymerized with the mol% guanidinium sidechains varying from 10% to 65%. The UCST varied linearly with guan+ mol% from 10-55°C. The counterion dependence was the same as salmine; coacervation occurred with a 1:2 ratio of sulfate to guan+, or at [NaCl] greater than 1.5 M. Coacervation of synthetic polyguan+ demonstrated that coacervation of salmine was driven entirely by cation-cation pairing of the guan+ sidechains of the R residues. The chirality of the polypeptide backbone or the other amino acid sidechains had little if any effect on coacervation. Simple coacervation of an organic polyphosphate, poly(methacryloyloxyethylphosphate-co-hydroxyethylmethacrylamide) with 80 mol% phosphoester sidechains (pMOEP80) will also be described. pMOEP forms solid precipitates or gels with most multivalent metal ions. However, pMOEP/Mg2+ undergoes LLPS to form transparent coacervates within a narrow range of temperature and pH.

Simple coacervation of polyguanidiniums and organic polyphosphates

253rd National Meeting (2017)

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