Abstract
Catechols are abundant in nature and are believed to perform diverse biological functions that include photoprotection (e.g., melanins), molecular signaling (e.g., catecholamine neurotransmitters), and mechanical adhesion (e.g., mussel glue). Currently, the structure-property-function relationships for catechols remain poorly resolved, and this is especially true for redox-based properties (e.g., antioxidant, pro-oxidant, and radical scavenging activities). Importantly, there are few characterization methods available to probe the redox properties of materials. In this review, we focus on recent studies with redox-active catechol-chitosan films. First, we describe film fabrication methods to oxidatively-graft catechols to chitosan through chemical, enzymatic, or electrochemical methods. Second, we discuss a new experimental characterization method to probe the redox properties of catechol-functionalized materials. This mediated electrochemical probing (MEP) method probes the redox-activities of catechol-chitosan films by: (i) employing diffusible mediators to shuttle electrons between the electrode and grafted catechols; (ii) imposing tailored sequences of input voltages to “tune” redox probing; and (iii) analyzing the output current response characteristics to infer properties. Finally, we demonstrate that the redox properties of catechol-chitosan films enable them to perform antioxidant radical scavenging functions, as well as a pro-oxidant (reactive oxygen-generation) antimicrobial functions. In summary, our increasing knowledge of catechol-chitosan films is enabling us to better-understand the functions of catechols in biology as well as enhancing our capabilities to create advanced functional materials.
Highlights
Catechol can be oxidatively-grafted to chitosan using various chemical, electrochemical (Wu et al, 2005, 2006), or enzymatic (Sun et al, 1992; Muzzarelli et al, 1994) approaches. In all these fabrication approaches the catechol is oxidized to a reactive o-quinone intermediate that grafts to the chitosan presumably through Schiff-base or Michael-type adduct formation
A more subtle point is that these results suggest that the E0 values for the Ru3+ (−0.2 V vs. Ag/AgCl) and ferrocene dimethanol mediator (Fc)+ (+0.25 V) mediators bracket the E0 value for catechol-chitosan film
The oxidative grafting of catechols to chitosan confers redox activities that can be used to perform either protective antioxidant radical-scavenging functions, or prooxidant antimicrobial functions
Summary
Catechols are believed to confer diverse functions to materials: the photo and free radical protection of melanins (Seagle et al, 2005; Dadachova and Casadevall, 2008; Panzella et al, 2013); the hardening of the insect cuticle through crosslinking (Andersen, 2010; Sugumaran and Barek, 2016; Whitten and Coates, 2017); the adhesion of the mussel glue to surfaces (Ryu et al, 2015; Waite, 2017); the virulence of fungal pathogens (Jacobson, 2000; Nosanchuk and Casadevall, 2006); and the innate immunity of insects (Christensen et al, 2005; Nakhleh et al, 2017). In. Redox-Active Catechol-Chitosan Films many cases, the synthesis of catecholic materials is initiated by oxidative reactions that yield a reactive intermediate (e.g., the tyrosinase-generated of o-quinones) that can undergo subsequent non-enzymatic reactions. Often the final product has catecholic moieties that could retain redox activity. An overarching hypothesis of our work is that catecholic materials play important roles in redox biology but these roles are under-appreciated, in part, because of the absence of simple characterization methods. We summarize recent research on the biomimetic oxidative-grafting of catechols onto films of the aminopolysaccharide chitosan. To characterize the redox properties of these catechol-chitosan films, we are developing an electrochemical reverse engineering method which is briefly described. We discuss two applications that highlight the unique technological capabilities for such redox-active catecholchitosan films
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