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Reversible Phenol Oxidation and Reduction in the Structurally Well-Defined 2‑Mercaptophenol‑α3C Protein

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journal contribution
posted on 19.02.2016, 21:25 by Cecilia Tommos, Kathleen G. Valentine, Melissa C. Martínez-Rivera, Li Liang, Veronica R. Moorman
2-Mercaptophenol-α3C serves as a biomimetic model for enzymes that use tyrosine residues in redox catalysis and multistep electron transfer. This model protein was tailored for electrochemical studies of phenol oxidation and reduction with specific emphasis on the redox-driven protonic reactions occurring at the phenol oxygen. This protein contains a covalently modified 2-mercaptophenol-cysteine residue. The radical site and the phenol compound were specifically chosen to bury the phenol OH group inside the protein. A solution nuclear magnetic resonance structural analysis (i) demonstrates that the synthetic 2-mercaptophenol-α3C model protein behaves structurally as a natural protein, (ii) confirms the design of the radical site, (iii) reveals that the ligated phenol forms an interhelical hydrogen bond to glutamate 13 (phenol oxygen–carboxyl oxygen distance of 3.2 ± 0.5 Å), and (iv) suggests a proton-transfer pathway from the buried phenol OH (average solvent accessible surface area of 3 ± 5%) via glutamate 13 (average solvent accessible surface area of the carboxyl oxygens of 37 ± 18%) to the bulk solvent. A square-wave voltammetry analysis of 2-mercaptophenol-α3C further demonstrates that (v) the phenol oxidation–reduction cycle is reversible, (vi) formal phenol reduction potentials can be obtained, and (vii) the phenol-O state is long-lived with an estimated lifetime of ≥180 millisecond. These properties make 2-mercaptophenol-α3C a unique system for characterizing phenol-based proton-coupled electron transfer in a low-dielectric and structured protein environment.