Reversible voltammograms and a Pourbaix diagram for a protein tyrosine radical

Reversible voltammograms and a voltammetry half-wave potential versus solution pH diagram are described for a protein tyrosine radical. This work required a de novo designed tyrosine-radical protein displaying a unique combination of structural and electrochemical properties. The alpha Y-3 protein is structurally stable across a broad pH range. The redox-active tyrosine Y32 resides in a desolvated and well-structured environment. Y32 gives rise to reversible square-wave and differential pulse voltammograms at alkaline pH. The formal potential of the Y32-O-center dot/Y32-OH redox couple is determined to 918 +/- 2 mV versus the normal hydrogen electrode at pH 8.40 +/- 0.01. The observation that Y32 gives rise to fully reversible voltammograms translates into an estimated lifetime of >= 30 ms for the Y32-O-center dot state. This illustrates the range of tyrosine-radical stabilization that a structured protein can offer. Y32 gives rise to quasireversible square-wave and differential pulse voltammograms at acidic pH. These voltammograms represent the Y32 species at the upper edge of the quasirevesible range. The square-wave net potential closely approximates the formal potential of the Y32-O center dot/Y32-OH redox couple to 1,070 +/- 1 mV versus the normal hydrogen electrode at pH 5.52 +/- 0.01. The differential pulse voltammetry half-wave potential of the Y32-O-center dot/Y32-OH redox pair is measured between pH 4.7 and 9.0. These results are described and analyzed.