Electrode Potentials of l‑Tryptophan, l‑Tyrosine, 3‑Nitro‑l‑tyrosine, 2,3-Difluoro‑l‑tyrosine, and 2,3,5-Trifluoro‑l‑tyrosine

Electrode potentials for aromatic amino acid radical/amino acid couples were deduced from cyclic voltammograms and pulse radiolysis experiments. The amino acids investigated were l-tryptophan, l-tyrosine, <i>N</i>-acetyl-l-tyrosine methyl ester, <i>N</i>-acetyl-3-nitro-l-tyrosine ethyl ester, <i>N</i>-acetyl-2,3-difluoro-l-tyrosine methyl ester, and <i>N</i>-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester. Conditional potentials were determined at pH 7.4 for all compounds listed; furthermore, Pourbaix diagrams for l-tryptophan, l-tyrosine, and <i>N</i>-acetyl-3-nitro-l-tyrosine ethyl ester were obtained. Electron transfer accompanied by proton transfer is reversible, as confirmed by detailed analysis of the current waves, and because the slopes of the Pourbaix diagrams obey Nernst’s law. <i>E</i>°′(Trp^•,H⁺/TrpH) and <i><i>E</i>°</i>′(TyrO^•,H⁺/TyrOH) at pH 7 are 0.99 ± 0.01 and 0.97 ± 0.01 V, respectively. Pulse radiolysis studies of two dipeptides that contain both amino acids indicate a difference in <i><i>E</i>°</i>′ of approximately 0.06 V. Thus, in small peptides, we recommend values of 1.00 and 0.96 V for <i>E</i>°′(Trp^•,H⁺/TrpH) and <i><i>E</i>°</i>′(TyrO^•,H⁺/TyrOH), respectively. The electrode potential of <i>N</i>-acetyl-3-nitro-l-tyrosine ethyl ester is higher, while because of mesomeric stabilization of the radical, those of <i>N</i>-acetyl-2,3-difluoro-l-tyrosine methyl ester and <i>N</i>-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester are lower than that of tyrosine. Given that the electrode potentials at pH 7 of <i>E</i>°′(Trp^•,H⁺/TrpH) and <i><i>E</i>°</i>′(TyrO^•,H⁺/TyrOH) are nearly equal, they would be, in principle, interchangeable. Proton-coupled electron transfer pathways in proteins that use TrpH and TyrOH are thus nearly thermoneutral

Why Selenocysteine Replaces Cysteine in Thioredoxin Reductase: A Radical Hypothesis

Thioredoxin reductases, important biological redox mediators for two-electron transfers, contain either 2 cysteines or a cysteine (Cys) and a selenocysteine (Sec) at the active site. The incorporation of Sec is metabolically costly, and therefore surprising. We provide here a rationale: in the case of an accidental one-electron transfer to a S–S or a S–Se bond during catalysis, a thiyl or a selanyl radical, respectively would be formed. The thiyl radical can abstract a hydrogen from the protein backbone, which subsequently leads to the inactivation of the protein. In contrast, a selanyl radical will not abstract a hydrogen. Therefore, formation of Sec radicals in a GlyCysSecGly active site will less likely result in the destruction of a protein compared to a GlyCysCysGly active site

Redox Properties and Activity of Iron–Citrate Complexes: Evidence for Redox Cycling

Iron in iron overload disease is present as non-transferrin-bound iron, consisting of iron, citrate, and albumin. We investigated the redox properties of iron citrate by electrochemistry, by the kinetics of its reaction with ascorbate, by ESR, and by analyzing the products of reactions of ascorbate with iron citrate complexes in the presence of H₂O₂ with 4-hydroxybenzoic acid as a reporter molecule for hydroxylation. We report −0.03 V < <i>E</i>°′ > +0.01 V for the (Fe³⁺–cit/Fe²⁺–cit) couple. The first step in the reaction of iron citrate with ascorbate is the rapid formation of mixed complexes of iron with citrate and ascorbate, followed by slow reduction to Fe²⁺–citrate with <i>k</i> = ca. 3 M^–1 s^–1. The ascorbyl radical is formed by iron citrate oxidation of Hasc^– with <i>k</i> = ca. 0.02 M^–1 s^–1; the majority of the ascorbyl radical formed is sequestered by complexation with iron and remains EPR silent. The hydroxylation of 4-hydroxybenzoic acid driven by the Fenton reduction of iron citrate by ascorbate in the presence of H₂O₂ proceeds in three phases: the first phase, which is independent of the presence of O₂, is revealed as a nonzero intercept that reflects the rapid reaction of accumulated Fe²⁺ with H₂O₂; the intermediate oxygen-dependent phase fits a first-order accumulation of product with <i>k</i> = 5 M^–1 s^–1 under aerobic and <i>k</i> = 13 M^–1 s^–1 under anaerobic conditions; the slope of the final linear phase is ca. <i>k</i> = 5 × 10^–2 M^–1 s^–1 under both aerobic and anaerobic conditions. Product yields under aerobic conditions are greater than predicted from the initial concentration of iron, but they are less than predicted for continuous redox cycling in the presence of excess ascorbate. The ongoing formation of hydroxylated product supports slow redox cycling by iron citrate. Thus, when H₂O₂ is available, iron–citrate complexes may contribute to pathophysiological manifestations of iron overload diseases