Mechanistic understanding of catechols and integration into an electrochemically cross-linked mussel foot inspired adhesive hydrogel

Alexander M. Imre,Markus Valtiner,Julia Appenroth,Laura L. E. Mears,Laila Moreno Ostertag

doi:10.1116/6.0001609

Abstract

Catechol reaction mechanisms form the basis of marine mussel adhesion, allowing for bond formation and cross-linking in wet saline environments. To mimic mussel foot adhesion and develop new bioadhesive underwater glues, it is essential to understand and learn to control their redox activity as well as their chemical reactivity. Here, we study the electrochemical characteristics of functionalized catechols to further understand their reaction mechanisms and find a stable and controllable molecule that we subsequently integrate into a polymer to form a highly adhesive hydrogel. Contradictory to previous hypotheses, 3,4-dihydroxy-L-phenylalanine is shown to follow a Schiff-base reaction whereas dopamine shows an intramolecular ring formation. Dihydrocaffeic acid proved to be stable and was substituted onto a poly(allylamine) backbone and electrochemically cross-linked to form an adhesive hydrogel that was tested using a surface forces apparatus. The hydrogel's compression and dehydration dependent adhesive strength have proven to be higher than in mussel foot proteins (mfp-3 and mfp-5). Controlling catechol reaction mechanisms and integrating them into stable electrochemically depositable macroscopic structures is an important step in designing new biological coatings and underwater and biomedical adhesives.

Highlights

Catechols are a central chemical motif in a variety of biochemical processes
We study the electrochemical characteristics of functionalized catechols to further understand their reaction mechanisms and find a stable and controllable molecule that we subsequently integrate into a polymer to form a highly adhesive hydrogel
We first study the redox response of L-DOPA and related molecules shown in Fig. 1, and we aim to understand the electrochemical oxidation and potential subsequent degradation reactions of catechols

Summary

Introduction

Catechols are a central chemical motif in a variety of biochemical processes These include electrochemical signaling, adhesion promotion, and redox mediation in both the respiratory and photosynthetic cycles. They are found in all living organisms and a wide variety of natural environments. Certain marine organisms, such as mussels, have developed sophisticated adhesive proteins with a high (up to 30%) content of the redox-active 3,4-dihydroxy-L-phenylalanine (L-DOPA) amino acid in the sequence to increase their adhesive power to organic and inorganic surfaces in wet and high salinity environments.[1,2,3] The oxidation/ reduction behavior of catechols has been found to support a precisely controlled reactive environment, mediating interfacial adhesion and cross-linking, and, cohesion in mussel foot structures.[4]. There are extensive studies capitalizing on L-DOPA’s redox chemistry with the goal of producing biomimetic adhesives for medical use and underwater adhesives in general,[6,7] where technical adhesives are often lacking, in environmental and biocompatibility.[8]

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