pH-dependent cross-linking of catechols through oxidation via Fe³⁺and potential implications for mussel adhesion

Abstract

The mussel byssus is a remarkable attachment structure that is formed by injection molding and rapid in-situ hardening of concentrated solutions of proteins enriched in the catecholic amino acid 3,4-dihydroxy-L-phenylalanine (DOPA). Fe3+, found in high concentrations in the byssus, has been speculated to participate in redox reactions with DOPA that lead to protein polymerization, however direct evidence to support this hypothesis has been lacking. Using small molecule catechols, DOPA-containing peptides, and native mussel foot proteins, we report the first direct observation of catechol oxidation and polymerization accompanied by reduction of Fe3+ to Fe2+. In the case of the small molecule catechol, we identified two dominant dimer species and characterized their connectivities by nuclear magnetic resonance (NMR), with the C6-C6 and C5-C6 linked species as the major and minor products, respectively. For the DOPA-containing peptide, we studied the pH dependence of the reaction and demonstrated that catechol polymerization occurs readily at low pH, but is increasingly diminished in favor of metal-catechol coordination interactions at higher pH. Finally, we demonstrate that Fe3+ can induce cross-links in native byssal mussel proteins mefp-1 and mcfp-1 at acidic pH. Based on these findings, we discuss the potential implications to the chemistry of mussel adhesion.

Siderophores and mussel foot proteins: the role of catechol, cations, and metal coordination in surface adhesion

“…Fe(III)-dependent dimer formation decreases with increasing pH. Dimerization occurs most readily at pH 2, can be detected at pH 5, and is absent at pH 7 and 9, where Fe(III) coordination is favored over Fe(III) induced oxidation [86]. A Dopa autoxidation product was observed at pH 9 in the absence of Fe(III) and this product is not detected in the presence of Fe(III), demonstrating that Fe(III) coordination protects Dopa against autoxidation [86].…”

“…. Dopa-containing small molecules and model peptides also form covalently crosslinked aryl dimers at pH 2 in the presence of Fe(III) [86]. Fe(III)-dependent dimer formation decreases with increasing pH.…”

“…Dimerization occurs most readily at pH 2, can be detected at pH 5, and is absent at pH 7 and 9, where Fe(III) coordination is favored over Fe(III) induced oxidation [86]. A Dopa autoxidation product was observed at pH 9 in the absence of Fe(III) and this product is not detected in the presence of Fe(III), demonstrating that Fe(III) coordination protects Dopa against autoxidation [86]. 24 Collectively, these results imply that a significant number of Fe(III)-induced covalent crosslinks may initially form within the mfps in the plaque in the low pH environment of the distal depression before mussel foot removal allows equilibration to oceanic pH.…”

The Effects of Oxidation States, Spin States and Solvents on Molecular Structure, Stability and Spectroscopic Properties of Fe-Catechol Complexes: A Theoretical Study

“…However, spectroscopic evidence has linked the presence of Fe 3+ to catechol oxidation for decades [16] [17] [18]. It has been reported that catecholmodified polyethylene glycol (PEG) polymers could be covalently cross-linked under acidic pH conditions in the presence of Fe 3+ [19] [20]. Therefore, the mechanism of interactions between Fe 3+ and catechols must be extensively investigated.…”

Mussel-mimetic self-healing polyaspartamide derivative gel via boron-catechol interactions

“…An inexpensive and biocompatible self-healing hydrogel for injectable cell therapy carrier was also investigated by Yang et al [14]. Recently, some significant work has focused on non-covalent hydrogels, which are cross-linked by reversible interactions based on metal-ligand coordination in the polymer chains; these can provide stimuli responsiveness, magnetic self-healing and good mechanical properties [8][9][10][15][16][17][18][19][20]. Because of the thermodynamics and kinetic properties of metal-ligand coordination bonds, metal-ligand interactions can provide stable, reversible crosslinking points between polymers [21].…”