Salt Triggers the Simple Coacervation of an Underwater Adhesive When Cations Meet Aromatic π Electrons in Seawater

Abstract

Adhesive systems in many marine organisms are postulated to form complex coacervates (liquid-liquid phase separation) through a process involving oppositely charged polyelectrolytes. Despite this ubiquitous speculation, most well-characterized mussel adhesive proteins are cationic and polyphenolic, and the pursuit of the negatively charged proteins required for bulk complex coacervation formation internally remains elusive. In this study, we provide a clue for unraveling this paradox by showing the bulky fluid/fluid separation of a single cationic recombinant mussel foot protein, rmfp-1, with no additional anionic proteins or artificial molecules, that is triggered by a strong cation-π interaction in natural seawater conditions. With the similar condition of salt concentration at seawater level (>0.7 M), the electrostatic repulsion between positively charged residues of mfp-1 is screened significantly, whereas the strong cation-π interaction remains unaffected, which leads to the macroscopic phase separation (i.e., bulky coacervate formation). The single polyelectrolyte coacervate shows interesting mechanical properties including low friction, which facilitates the secretion process of the mussel. Our findings reveal that the cation-π interaction modulated by salt is a key mechanism in the mussel adhesion process, providing new insights into the basic understanding of wet adhesion, self-assembly processes, and biological phenomena that are mediated by strong short-range attractive forces in water.

Neutralization of a Distributed Coulombic Switch Precisely Tunes Reflectin Assembly

“…interactions and tunable longer-range repulsive ones, which we experimentally tuned by histidine titration and manipulation of charges through mutation. This suggested mechanism is similar to one observed for some mussel foot proteins that are similarly rich in both cationic and aromatic residues, which have been shown to undergo LLPS upon an increase in ionic strength, postulated to act by screening the repulsive longer-range cationic interactions, while attractive shorter range cationic-pi interactions remain unaffected 50 . It remains unclear which aspects of the reflectin sequence and composition might enable the rapid solidification of condensates suggested here, although recent work with other proteins suggests that serine and glutamine content may be an important factor.…”

Hierarchically-structured metalloprotein composite coatings biofabricated from co-existing condensed liquid phases

“…The repetitive region of mefp-1 possesses a hydrophilic character due to the high content of conserved Lysine (K) residues. Recombinant expression of truncated mfp-1 consisting of 12 or 22 repeats of the hydrophilic decapeptide have been demonstrated to undergo spontaneous LLPS under high salt conditions mediated via pi-cation interactions 37 , offering support to the coacervate-like nature in the vesicles. However, at the N-terminus of native mefp-1 and mgfp-1 from M. galloprovincialis is an 60-80 amino acid non-repetitive domain that is markedly less hydrophilic, giving the overall protein an amphiphilic profile (i.e.…”

Bioinspired Underwater Adhesives by Using the Supramolecular Toolbox

“…Besides electrostatic interactions, additional interactions, such as hydrophobic, cation–π, or π–π interactions, may take place . These interactions have not been identified in the adhesive of sandcastle worms, but hydrophobic interactions may increase the driving force for coacervation, and cation–π interactions may relate to the cationic character of Pc1 and Pc4, which remains unexplained so far …”

“…From the mfps, mfp‐3s was shown to coacervate before full charge neutralization was obtained, suggesting that ionic coacervation of mfp‐3s is enhanced by additional interactions such as hydrophobic, cation–π, or π–π interactions . Cation–π interactions were shown to induce a liquid–liquid phase separation of recombinant mfp‐1 from solution . At constant pH (7.2), phase separation was induced by increasing the salt concentration till 0.7 m , which is equal to the salt concentration in seawater.…”