Reversible voltammograms and a Pourbaix diagram for a protein tyrosine radical.

Abstract

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 α(3)Y 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(•)/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(•) 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(•)/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(•)/Y32-OH redox pair is measured between pH 4.7 and 9.0. These results are described and analyzed.