3 research outputs found
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<sup>•</sup>,H<sup>+</sup>/TrpH) and <i><i>E</i>°</i>′(TyrO<sup>•</sup>,H<sup>+</sup>/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<sup>•</sup>,H<sup>+</sup>/TrpH) and <i><i>E</i>°</i>′(TyrO<sup>•</sup>,H<sup>+</sup>/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<sup>•</sup>,H<sup>+</sup>/TrpH) and <i><i>E</i>°</i>′(TyrO<sup>•</sup>,H<sup>+</sup>/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 H<sub>2</sub>O<sub>2</sub> with 4-hydroxybenzoic acid as a reporter
molecule for hydroxylation. We report −0.03 V < <i>E</i>°′ > +0.01 V for the (Fe<sup>3+</sup>–cit/Fe<sup>2+</sup>–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 Fe<sup>2+</sup>–citrate with <i>k</i> = ca. 3 M<sup>–1</sup> s<sup>–1</sup>. The ascorbyl radical is formed by iron citrate
oxidation of Hasc<sup>–</sup> with <i>k</i> = ca.
0.02 M<sup>–1</sup> s<sup>–1</sup>; 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 H<sub>2</sub>O<sub>2</sub> proceeds in three phases: the
first phase, which is independent of the presence of O<sub>2</sub>, is revealed as a nonzero intercept that reflects the rapid reaction
of accumulated Fe<sup>2+</sup> with H<sub>2</sub>O<sub>2</sub>; the
intermediate oxygen-dependent phase fits a first-order accumulation
of product with <i>k</i> = 5 M<sup>–1</sup> s<sup>–1</sup> under aerobic and <i>k</i> = 13 M<sup>–1</sup> s<sup>–1</sup> under anaerobic conditions; the slope of the
final linear phase is ca. <i>k</i> = 5 × 10<sup>–2</sup> M<sup>–1</sup> s<sup>–1</sup> 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 H<sub>2</sub>O<sub>2</sub> is available, iron–citrate complexes may contribute
to pathophysiological manifestations of iron overload diseases