The electrochemistry of 2,6-dimethylbenzoquinone (DMBQ) has been characterized for three different systems: DMBQ freely solvated in aqueous buffer; DMBQ bound to a neutral, blocked cysteine (N-acetylL-cysteine methyl ester) and the resulting DMBQ-bCys compound solvated in aqueous buffer; and DMBQ bound to a small model protein denoted α3C. The goal of this study is to detect and characterize differences in the redox properties of the protein-ligated DMBQ relative to the solvated quinones. The α3C protein used here is a tryptophan-32 to cysteine-32 variant of the structurally defined α3W de novo protein (Dai et al. J. Am. Chem. Soc. 2002, 124, 10952-10953). The properties of α3C were recently described (Hay et al. Biochemistry 2005, 44, 11891-11902). DMBQ was covalently bound to bCys and α3C through a sulfur substitution reaction with the cysteine thiol. In contrast to the solvated DMBQ and DMBQ-bCys compounds, diffusion controlled electrochemistry of DMBQ-α3C showed well-behaved and fully reversible n = 2 oxidation/reduction with a peak separation of ∼30 mV between pH 5 and 9. DMBQ-α3C could also be immobilized on a gold electrode modified with a self-assembled monolayer of 3-mercaptopropionoic acid, allowing the measurement, by cyclic voltammetry, of an apparent rate of electron transfer of 22 s-1. The (cysteine) sulfur substitution significantly lowers one of the hydroquinone PK A'S from 10.4 in DMBQ to 6.8 in DMBQ-bCys. This pKA is slightly elevated in DMBQ-α3C to 7.0 and the E1/2 at pH 7i0 is raised by 110 mV from +190 mV in DMBQ-bCys to +297 mV in DMBQ-α3C. © 2007 American Chemical Society.