Abstract

Multivalent interactions play a leading role in biological processes such as the inhibition of inflammation or virus internalization. The multivalent interactions show enhanced strength and better selectivity compared to monovalent interactions, but they are much less understood due to their complexity. Here, we detect molecular interactions in the range of a few piconewtons to several nanonewtons and correlate them with the formation and subsequent breaking of one or several bonds and assign these bonds. This becomes possible by performing atomic force microcopy (AFM)-based single molecule force spectroscopy of a multifunctional polymer covalently attached to an AFM cantilever tip on a substrate bound polymer layer of the multifunctional polymer. Varying the pH value and the crosslinking state of the polymer layer, we find that bonds of intermediate strength (non-covalent), like coordination bonds, give the highest multivalent bond strength, even outperforming strong (covalent) bonds. At the same time, covalent bonds enhance the polymer layer density, increasing in particular the number of non-covalent bonds. In summary, we can show that the key for the design of stable and durable polymer coatings is to provide a variety of multivalent interactions and to keep the number of non-covalent interactions at a high level.

Highlights

  • One important amino acid used in nature to form multivalent bonds is the catecholic 3,4-dihydroxy-L-phenylalanine (DOPA)

  • The oxidation of catechol to semi-quinone and to quinone is a crucial step for crosslinking of adhesives based on catechol rich compounds

  • The following processes have been optimized to ensure a proper passivation of the cantilever tip surface with 5 kDa silane-polyethylene glycol (PEG) and the attachment of PG110-b-P(Cat5-Ph5-A2) to obtain detachment events in the Single molecule force spectroscopy (SMFS) experiment

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Summary

Introduction

One important amino acid used in nature to form multivalent bonds is the catecholic 3,4-dihydroxy-L-phenylalanine (DOPA) It is for example used by blue mussels, which can bind in wet and salty conditions to inorganic as well as organic surfaces.[2] Catechols can reversibly be oxidized to form quinones, which is highly electrophilic and can irreversibly crosslink with other catechol moieties.[3,4] Crosslinking can occur via Michael addition of the free amine with the quinone group, via Schiff Base formation of the amine with the carbonyl group of the quinone or via quinone-quinone coupling.[5,6] The oxidation of catechol to semi-quinone and to quinone is a crucial step for crosslinking of adhesives based on catechol rich compounds. This evidence can be used to synthesize and tailor mussel-inspired and catechol-based polymers with strong adhesion to surfaces in order to obtain universal/substrate-independent coatings with self-healing or antifouling properties under mild conditions.[7,8,9,10]

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