Spectroscopic, Electrochemical, and Kinetic Trends in Fe(III)–Thiolate Disproportionation Near Physiological pH

In addition to its primary oxygen-atom-transfer function, cysteamine dioxygenase (ADO) exhibits an understudied disproportionation reaction (ADO-Fe(III)-SR → ADO-Fe(II) + ½ RSSR) with its native substrates. Inspired by ADO disproportionation reactivity, we employ [Fe(tacn)Cl3] (tacn = 1,4,7-triazacyclononane) as a precursor for generating Fe(III)–thiolate intermediates in buffered aqueous media. A series of Fe(III)–thiolate intermediates are generated in situ using aqueous [Fe(tacn)Cl3] and thiol-containing ligands cysteamine, penicillamine, mercaptopropionate, cysteine, cysteine methyl ester, N-acetylcysteine, and N-acetylcysteine methyl ester. We observe trends in UV–vis and electron paramagnetic resonance (EPR) spectra, disproportionation rate constants, and cathodic peak potentials as a function of thiol ligand. These trends will be useful in rationalizing substrate-dependent Fe(III)–thiolate disproportionation reactions in metalloenzymes

The [4Fe4S] Cluster of Yeast DNA Polymerase ϵ Is Redox Active and Can Undergo DNA-Mediated Signaling

Many DNA replication and DNA repair enzymes have been found to carry [4Fe4S] clusters. The major leading strand polymerase, DNA polymerase ε (Pol ε) from Saccharomyces cerevisiae, was recently reported to have a [4Fe4S] cluster located within the catalytic domain of the largest subunit, Pol2. Here the redox characteristics of the [4Fe4S] cluster in the context of that domain, Pol2CORE, are explored using DNA electrochemistry, and the effects of oxidation and rereduction on polymerase activity are examined. The exonuclease deficient variant D290A/E292A, Pol2COREexo–, was used to limit DNA degradation. While no redox signal is apparent for Pol2COREexo– on DNA-modified electrodes, a large cathodic signal centered at −140 mV vs NHE is observed after bulk oxidation. A double cysteine to serine mutant (C665S/C668S) of Pol2COREexo–, which lacks the [4Fe4S] cluster, shows no similar redox signal upon oxidation. Significantly, protein oxidation yields a sharp decrease in polymerization, while rereduction restores activity almost to the level of untreated enzyme. Moreover, the addition of reduced EndoIII, a bacterial DNA repair enzyme containing [4Fe4S]2+, to oxidized Pol2COREexo– bound to its DNA substrate also significantly restores polymerase activity. In contrast, parallel experiments with EndoIIIY82A, a variant of EndoIII, defective in DNA charge transport (CT), does not show restoration of activity of Pol2COREexo–. We propose a model in which EndoIII bound to the DNA duplex may shuttle electrons through DNA to the DNA-bound oxidized Pol2COREexo– via DNA CT and that this DNA CT signaling offers a means to modulate the redox state and replication by Pol ε

Spectroscopic Characterization of the 3+ and 2+ Oxidation States of Europium in a Macrocyclic Tetraglycinate Complex

The 3+ and 2+ oxidation states of europium have drastically different magnetic and spectroscopic properties. Electrochemical measurements are often used to probe Eu^III/II oxidation state changes, but a full suite of spectroscopic characterization is necessary to demonstrate conversion between these two oxidation states in solution. Here, we report the facile conversion of an europium­(III) tetraglycinate complex into its Eu^II analogue. We present electrochemical, luminescence, electron paramagnetic resonance, UV–visible, and NMR spectroscopic data demonstrating complete reversibility from the reduction and oxidation of the 3+ and 2+ oxidation states, respectively. The Eu^II-containing analogue has kinetic stability within the range of clinically approved Gd^III-containing complexes using an acid-catalyzed dissociation experiment. Additionally, we demonstrate that the 3+ and 2+ oxidation states provide redox-responsive behavior through chemical-exchange saturation transfer or proton relaxation, respectively. These results will be applicable to a wide range of redox-responsive contrast agents and Eu-containing complexes