Engineering Nitroxide Functional Surfaces Using Bioinspired Adhesion

Development of Universal and Clickable Film by Mimicking Melanogenesis: On-Demand Oxidation of Tyrosine-Based Azido Derivative by Tyrosinase

In this study, we synthesized a tyrosine-based azido derivative (TBAD) that permits both substrate-independent surface coating and clickable film functionalization by mimicking natural melanogenesis. In contrast to catechol derivatives, which are generally susceptible to oxidation by air under ambient conditions, the monophenol-based TBAD remains stable under alkaline and neutral conditions, and is activated to oxidized quinone in situ by tyrosinase to initiate melanin-like polymerization. The resulting poly(TBAD) film can be formed on various substrates including noble metals, metal oxides, and synthetic polymers, which can undergo click reaction with terminal alkyne moieties on the entire surface or a specific region through Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The enzyme-mediated coating can rapidly form thin films (∼10 nm) and produce a uniform film morphology, which are important aspects in surface chemistry. This on-demand, clickable coating may become a significant tool for bioconjugation, soft lithography, and labeling techniques. This article is protected by copyright. All rights reserved.

Expanding the spectrum of polydopamine antioxidant activity by nitroxide conjugation

Polydopamine (PDA) materials are important due to their unique physicochemical properties and their potential as chemopreventive agents for diseases connected with oxidative stress. Although PDA has been suggested to display antioxidant activity, its efficacy is controversial and its mechanism of action is still unclear. Herein, we report that accurately purified PDA nanoparticles in water at pH 7.4 are unable to quench alkylperoxyls (ROO˙), which are the radicals responsible for the propagation of lipid peroxidation, despite PDA reacting with the model DPPH˙ and ABTS˙⁺ radicals. PDA nanoparticles prepared by copolymerization of dopamine with the dialkyl nitroxide 4-NH₂TEMPO show instead good antioxidant activity, thanks to the ROO˙ trapping ability of the nitroxide. Theoretical calculations performed on a quinone-catechol dimer, reproducing the structural motive of PDA, indicate a reactivity with ROO˙ similar to catechol. These results suggest that PDA nanoparticles have an "onion-like" structure, with a catechol-rich core, which can be reached only by DPPH˙ and ABTS˙⁺, and a surface mainly represented by quinones. The importance of assessing the antioxidant activity by inhibited autoxidation studies is also discussed.

Nitroxide polymer gels for recyclable catalytic oxidation of primary alcohols to aldehydes

A physically crosslinked nitroxide polymer gel has been synthesized and used as a recyclable catalyst to convert alcohols to aldehydes in air.

Soft Matter Technology at KIT: Chemical Perspective from Nanoarchitectures to Microstructures

Bioinspiration has emerged as an important design principle in the rapidly growing field of materials science and especially its subarea, soft matter science. For example, biological cells form hierarchically organized tissues that not only are optimized and designed for durability, but also have to adapt to their external environment, undergo self-repair, and perform many highly complex functions. Being able to create artificial soft materials that mimic those highly complex functions will enable future materials applications. Herein, soft matter technologies that are used to realize bioinspired material structures are described, and potential pathways to integrate these into a comprehensive soft matter research environment are addressed. Solutions become available because soft matter technologies are benefitting from the synergies between organic synthesis, polymer chemistry, and materials science.

Oxidative addition of verdazyl halogenides to Pd(PPh3)4

A novel approach to the preparation of stable Pd-substituted verdazyls was developed through the direct oxidative addition of iodoverdazyls to Pd(PPh₃)₄.

Controlling biofilm formation with nitroxide functional surfaces

Nitroxide functional polymer coatings with inherent antibiofilm activity are introduced as an avenue to combat bacterial biofilm contamination of medical devices.

Dynamic Nitroxide Functional Materials

A substrate-independent and versatile coating platform for (spatially resolved) surface functionalization, based on nitroxide radical coupling (NRC) reactions and the formation of thermo-labile alkoxyamine functional groups, was introduced. Nitroxide-decorated poly(glycidyl methacrylate) (PGMA) microspheres, obtained through bioinspired copolymer surface deposition using dopamine and a nitroxide functional dopamine derivative as monomers, were conjugated with small functional groups in a rewritable process. Reversible coding of the nitroxide functional microspheres by NRC and decoding through thermal alkoxyamine fission were monitored and characterized by electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, this nitroxide coating system was exploited in "grafting-to" polymer surface ligations of poly(methyl methacrylate) (PMMA) and poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) in spatially confined areas. Polymer strands terminated with an Irgacure 2959 (2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone) photoinitiator were obtained through chain-transfer polymerization, and subsequently coupled to nitroxide-immobilized poly(dopamine) (PDA)-coated silicon substrates by using rapid photoclick NRC reactions. Light-driven polymer surface coding was visualized by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and XPS imaging.

Polymerizing Like Mussels Do: Toward Synthetic Mussel Foot Proteins and Resistant Glues

A novel strategy to generate adhesive protein analogues by enzyme-induced polymerization of peptides is reported. Peptide polymerization relies on tyrosinase oxidation of tyrosine residues to Dopaquinones, which rapidly form cysteinyldopa-moieties with free thiols from cysteine residues, thereby linking unimers and generating adhesive polymers. The resulting artificial protein analogues show strong adsorption to different surfaces, even resisting hypersaline conditions. Remarkable adhesion energies of up to 10.9 mJ m-2 are found in single adhesion events and average values are superior to those reported for mussel foot proteins that constitute the gluing interfaces.