| Summary: | Understanding the fundamental principles governing the interplay of redox and molecular recognition is invaluable in the context of biochemistry and material science. Enzymes containing redox-active organic molecules such as flavins and quinones use specific enzyme-cofactor interactions to regulate the reactivity of the prosthetic group. Due to the complexity of biological systems, the effects of hydrogen bonding, π-stacking and dipolar interactions are difficult to quantify individually, and subtle interactions may go unnoticed. We have utilized synthetic model systems to parametrically probe the effect of molecular recognition on the redox behavior and physical properties of redox active organic molecules. The results are applied both to explain effects observed in biological systems and in the design of novel man-made molecular devices. We used synthetic receptors to reproduce specific hydrogen bond patterns to the flavin imide moiety present in flavoenzymes. Hydrogen bonding was found to stabilize the flavin radical by decreasing its reduction potential and suppressing proton transfer responsible for two electron reduction, analogous to the effect seen in flavoenzymes. We further observed a significant increase in the association constant to the hydrogen bonding receptor upon reduction of flavin from the oxidized form to the radical anion. Specific hydrogen bond interactions were found to redistribute the spin density of the naphthalimide radical anion, which undergoes molecular recognition analogous to flavin. Experimental hyperfine coupling constants and DFT-B3LYP calculations show an overall increase in spin polarization upon hydrogen bonding and a distortion of spin density away from the binding site. We are currently investigating a similar effect predicted for hydrogen bound flavin using electron nuclear double resonance spectroscopy. The complimentarity dependence of hydrogen bonding and π-stacking on the redox-state of a bound molecule enabled us to design a molecular device, in which the redox state of the guest switches the preference between two competing hosts, rendering the system a three component molecular switch. We developed a synthetic strategy to attach molecular recognition elements to gold surfaces through reaction of primary amines with SAMs displaying acid fluoride-headgroups, which will enable the creation of solid-supported analogs of such devices.
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