Boronate Complex Formation with Dopa Containing Mussel Adhesive Protein Retards pH-Induced Oxidation and Enables Adhesion to Mica

Abstract

The biochemistry of mussel adhesion has inspired the design of surface primers, adhesives, coatings and gels for technological applications. These mussel-inspired systems often focus on incorporating the amino acid 3,4-dihydroxyphenyl-L-alanine (Dopa) or a catecholic analog into a polymer. Unfortunately, effective use of Dopa is compromised by its susceptibility to auto-oxidation at neutral pH. Oxidation can lead to loss of adhesive function and undesired covalent cross-linking. Mussel foot protein 5 (Mfp-5), which contains ∼30 mole % Dopa, is a superb adhesive under reducing conditions but becomes nonadhesive after pH-induced oxidation. Here we report that the bidentate complexation of borate by Dopa to form a catecholato-boronate can be exploited to retard oxidation. Although exposure of Mfp-5 to neutral pH typically oxidizes Dopa, resulting in a>95% decrease in adhesion, inclusion of borate retards oxidation at the same pH. Remarkably, this Dopa-boronate complex dissociates upon contact with mica to allow for a reversible Dopa-mediated adhesion. The borate protection strategy allows for Dopa redox stability and maintained adhesive function in an otherwise oxidizing environment.

Mussel adhesion – essential footwork

“…If bidentate H-bonding is the only possible interaction with a given surface, does adhesion increase or decrease with increasing pH? This was tested on mica by protecting Dopa in Mfp-5 against oxidation at pH 7-8 by moderate complexation with borate (Kan et al, 2014). Adhesion of borate-protected Mfp-5 to mica at pH 3 and pH 8 remained about the same.…”

The Chemistry behind Catechol‐Based Adhesion

“…Researchers have also developed different strategies to cross-link polymers at acidic pH values by introducing additional moieties [80a] or diminishing the pK a value of the catecholic hydroxy groups. [130] Other studies have described an intelligent approach based on the formation of aboronatecatechol complex and its pH-dependent reversibility.W aite and co-workers [136] followed this strategy with at wofold objective:1 )retard DOPAo xidation at an eutral pH value and 2) maintain adhesive properties under oxidising conditions through the dissociation of the complex. In another study,L ee and co-workers [137] described the preparation of pH-responsive adhesive hydrogels by copolymerisation of dopamine methacrylamide and 3-(acrylamido)phenylboronic acid.…”

Bioinspired Underwater Adhesives by Using the Supramolecular Toolbox

“…As reported by Kan et al, the presence of borates at pH 7.5 can retard catechol oxidation. The adhesion between two symmetric mica surfaces bridged by mfp‐5 was fully maintained as compared to that at pH 3.0 . Narkar et al designed a copolymer containing catechol and borax functional groups, i.e., a copolymer composed of dopamine methacrylamide and 3‐(acrylamido)phenylboronic acid.…”