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

Ruthenium dinitrosyl complexes – computational characterization of structure and reactivity

<div><p>Elucidation of the electronic structure of a dinitrosyl dithiolate ruthenium complex in several formal oxidation states ranging from Ru(I) to Ru(III) has been undertaken. DFT and <i>ab initio</i> molecular dynamics simulations have shown clear evidence of asymmetry within the dinitrosyl moieties in all models though most noticeably in the excited states. The reaction pathway of a hyponitrite adduct formation was also examined and found to be more feasible in the excited states. These results, along with the recently reported study on the dinitrosyl dithiolate iron analog of these complexes, provide insight toward the mechanism of NO donation by dinitrosyl metal complexes.</p></div

Decarbonylation Products of Binuclear Methylphosphinidene Complexes of Cyclopentadienyliron Carbonyls: Triplet and Quintet Structures Are Favored Energetically over Singlet Structures with Iron–Iron Multiple Bonding

The structures, energetics, and energetically preferred spin states of methylphosphinidene-bridged binuclear cyclopentadienyliron carbonyl complexes MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) related to the experimentally known (μ-RP)Fe2(μ-CO)(CO)2Cp2 (R = cyclohexyl, phenyl, mesityl, and 2,4,6-tBu3C6H2) complexes have been investigated by density functional theory. Singlet structures having a pyramidal pseudotetrahedral phosphorus environment with 18-electron iron configurations are energetically preferred in the tricarbonyl and tetracarbonyl systems MePFe2(CO)nCp2 (n = 4 and 3) with the lowest energy structures of the tricarbonyl very closely resembling the experimentally determined structures. For the more unsaturated dicarbonyl and monocarbonyl systems MePFe2(CO)nCp2 (n = 2 and 1), higher spin state triplet and quintet structures are energetically preferred over singlet structures. These more highly unsaturated structures can be derived from the lowest energy singlet MePFe2(CO)nCp2 (n = 4, 3) by the removal of carbonyl groups. The iron atoms giving up carbonyl groups in their 16- and 14-electron configurations bear the spin density of the unpaired electrons in the higher spin states. The lowest energy singlet structure of the monocarbonyl MePFe2(CO)Cp2, although a relatively high energy isomer, is unusual among the collection of MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) structures by having both the formal FeFe double bond and the four-electron donor MeP unit with the planar phosphorus coordination required to allow each of its iron atoms to attain the favored 18-electron configuration

Decarbonylation Products of Binuclear Methylphosphinidene Complexes of Cyclopentadienyliron Carbonyls: Triplet and Quintet Structures Are Favored Energetically over Singlet Structures with Iron–Iron Multiple Bonding

The structures, energetics, and energetically preferred spin states of methylphosphinidene-bridged binuclear cyclopentadienyliron carbonyl complexes MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) related to the experimentally known (μ-RP)Fe2(μ-CO)(CO)2Cp2 (R = cyclohexyl, phenyl, mesityl, and 2,4,6-tBu3C6H2) complexes have been investigated by density functional theory. Singlet structures having a pyramidal pseudotetrahedral phosphorus environment with 18-electron iron configurations are energetically preferred in the tricarbonyl and tetracarbonyl systems MePFe2(CO)nCp2 (n = 4 and 3) with the lowest energy structures of the tricarbonyl very closely resembling the experimentally determined structures. For the more unsaturated dicarbonyl and monocarbonyl systems MePFe2(CO)nCp2 (n = 2 and 1), higher spin state triplet and quintet structures are energetically preferred over singlet structures. These more highly unsaturated structures can be derived from the lowest energy singlet MePFe2(CO)nCp2 (n = 4, 3) by the removal of carbonyl groups. The iron atoms giving up carbonyl groups in their 16- and 14-electron configurations bear the spin density of the unpaired electrons in the higher spin states. The lowest energy singlet structure of the monocarbonyl MePFe2(CO)Cp2, although a relatively high energy isomer, is unusual among the collection of MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) structures by having both the formal FeFe double bond and the four-electron donor MeP unit with the planar phosphorus coordination required to allow each of its iron atoms to attain the favored 18-electron configuration

Copolymerization of recombinant <i>Phascolopsis</i> <i>gouldii</i> hemerythrin with human serum albumin for use in blood substitutes

<p>Hemerythrin is an oxygen-carrying protein found in marine invertebrates and may be a promising alternative to hemoglobin for use in blood substitutes, primarily due to its negligible peroxidative toxicity. Previous studies have shown that glutaraldehyde-induced copolymerization of hemoglobin with bovine serum albumin increases the half-life of the active <i>oxy</i> form of hemoglobin (i.e. decreases the auto-oxidation rate). Here, we describe a protocol for glutaraldehyde copolymerization of Hr with human serum albumin and the dioxygen-binding properties of the co-polymerized products. The copolymerization with HSA results in alteration of hemerythrin’s dioxygen-binding properties in directions that may be favorable for use in blood substitutes.</p

Antioxidant activity evaluation by physiologically relevant assays based on haemoglobin peroxidase activity and cytochrome <i>c</i>-induced oxidation of liposomes

<p>Two new protocols for exploring antioxidant-related chemical composition and reactivity are described: one based on a chronometric variation of a haemoglobin ascorbate peroxidase assay and one based on cytochrome <i>c</i>-induced oxidation of lecithin liposomes. Detailed accounts are given on their design, application, critical correlations with established methods and mechanisms. These assays are proposed to be physiologically relevant and bring new information regarding a real sample, both qualitative and quantitative. The well-known assays used for evaluation of antioxidant (re)activity are revisited and compared with these new methods. Extracts of the <i>Hedera helix</i> L. are examined as test case, with focus on seasonal variation and on leaf, fruit and flower with respect to chromatographic, spectroscopic and reactivity properties. According to the set of assays performed, winter are the most antioxidant, followed by summer leaves, and then by flowers and fruits.</p

Studies of reaction of tetramethylthiourea with hydrogen peroxide: evidence of formation of tetramethylthiourea monoxide as a key intermediate of the reaction

<p>The reaction between tetramethylthiourea (TMTU) and hydrogen peroxide was studied by UV–VIS spectroscopy, ESI mass spectrometry, ¹H NMR, cyclic voltammetry and surface-enhanced Raman scattering. We found that the reaction includes two consecutive steps, that is, (i) an oxidation of TMTU to TMTU monoxide (TMTMO) and (ii) further oxidation of TMTU monoxide to tetramethylurea (TMU) and sulfate. The second step is complex and seems to include extrusion of sulfur monoxide (SO). Density functional theory calculations were employed on tetramethylthiourea oxide models in order to underline differences between monoxide and di- and trioxides. Calculations predict that the TMTMO structure can be best explained as an adduct of thiourea with an oxygen atom (═S–>O).</p

Iron histochemical analyses.

<p>Iron histochemical analyses.</p

Comparison of the iron deposits in liver, spleen and kidney in Control and experimental groups.

<p>The structures were scored for their intensities of iron deposition in the range of + (normal aspects of the analyzed structures) and ++++ (the most intensely stained areas). The absence of iron deposits was marked with–(negative).</p

Blood ion concentration of control and experimental groups (mean ± SEM).

<p>Blood ion concentration of control and experimental groups (mean ± SEM).</p