Redox Activation of Small Molecules at Biological Metal Centers

Adduct of Aquacobalamin with Hydrogen Peroxide

Aquacobalamin binds hydrogen peroxide reversibly to form a cobalt(III) hydroperoxo adduct with a 0.25 mM dissociation constant, as evidenced by UV-vis absorption spectroscopy and corroborated by NMR, Raman spectroscopy, stopped-flow UV-vis measurements, and density functional theory calculations.

Cyclopentadienylmetal group 6 metal carbonyl derivatives with 2-propanoneoximato and related ligands

The experimental NO-dihapto (Me2CNO)Mo(CO)2Cp dicarbonyl structure is found by density functional theory to be the lowest energy isomer by a substantial margin. The lowest energy structures for the corresponding monocarbonyls are of the type (Me2CN)M(O)(CO)Cp with separate dimethylimino and oxo ligands.

Redox control and autoxidation of class 1, 2 and 3 phytoglobins from Arabidopsis thaliana

Despite a recent increase in interest towards phytoglobins and their importance in plants, much is still unknown regarding their biochemical/biophysical properties and physiological roles. The present study presents data on three recombinant Arabidopsis phytoglobins in terms of their UV-vis and Raman spectroscopic characteristics, redox state control, redox potentials and autoxidation rates. The latter are strongly influenced by pH for all three hemoglobins - (with a fundamental involvement of the distal histidine), as well as by added anion concentrations - suggesting either a process dominated by nucleophilic displacement of superoxide for AtHb2 or an inhibitory effect for AtHb1 and AtHb3. Reducing agents, such as ascorbate and glutathione, are found to either enhance- (presumably via direct electron transfer or via allosteric regulation) or prevent autoxidation. HbFe3+ reduction was possible in the presence of high (presumably not physiologically relevant) concentrations of NADH, glutathione and ascorbate, with differing behaviors for the three globins. The iron coordination sphere is found to affect the autoxidation, redox state interconversion and redox potentials in these three phytoglobins.

A mononuclear non-heme-iron dioxygen-carrying protein?

The ability of mononuclear non-heme iron complexes to function as molecular oxygen transporters is investigated by density functional theory. The factors governing the efficiency of the reversible binding of dioxygen at the active site of the dinuclear non-heme iron enzyme hemerythrin, including antiferromagnetic coupling and the conversion of dioxygen to hydroperoxo by a proton coupled 2-electron transfer mechanism, are revisited and considered as possible tools in mononuclear non-heme complexes. Several mononuclear non-heme model complexes, including active sites of enzymes already known to interact with dioxgenic ligands, are constructed and the molecular oxygen transportation capabilities of these complexes are examined computationally. The high-spin nature of the ground state of these complexes implies an intrinsic kinetic lability of the oxy structures, as also evident from potential energy surface calculations towards iron-dioxygen cleavage. Proton affinities as calibrated with reference compounds showed that these complexes are highly unlikely to undergo protonation to form hydroperoxo-like adducts. Mixed superoxo descriptions of the dissociated dioxygenic ligands in all complexes add to the overall conclusion that these model structures are significantly disadvantaged in any attempt to be employed for molecular oxygen transportation.

Computational investigation of the initial two-electron, two-proton steps in the reaction mechanism of hydroxylamine oxidoreductase

Reported here is a computational study based on density functional theory that presents the first attempt to investigate the 2-electron 2-proton reaction of Fe(III)-H2NOH to Fe(III)-HNO in the catalytic cycle of hydroxylamine oxidoreductase-a multiheme-containing enzyme that catalyzes the conversion of hydroxylamine (HA) to nitrite in nitrifying bacteria. Two subsequent protonation events are proposed to initiate the process, of which the second is suggested to be concerted with a one-electron oxidation. The final one-electron oxidation is further proposed to be accompanied by a third deprotonation process, suggesting that Fe(III)-HNO may not be an isolable intermediate in the HAO catalytic cycle. Further explorations are suggested to be focused on the following steps in the catalytic cycle, the influence of the lateral substituents of the heme (and especially of the Cys and Tyr cross-links), the comparative study of hydrazine oxidation, the proton delivery network in the distal site and, possibly, on linkage isomerism.