Mechanism of O2 generation in reduction and oxidation cycle of ubiquinones in a model of mitochondrial electron transport systems

O⁻₂ generation in mitochondrial electron transport systems, especially the NADPH-coenzyme Q₁₀ oxidoreductase system, was examined using a model system, NADPH-coenzyme Q₁-NADPH-dependent cytochrome P-450 reductase. One electron reduction of coenzyme Q₁ produces coenzyme Q₁ and O⁻₂ during enzyme-catalyzed reduction and O₂ + coenzyme Q₁ are in equilibrium with O⁻₂ + coenzyme Q₁ in the presence of enough O₂. The coenzyme Q₁ produced can be completely eliminated by superoxide dismutase, identical to bound coenzyme Q₁₀ radical produced in a succinate/fumarate couple-KCN-submitochondrial system in the presence of O₂. Superoxide dismutase promotes electron transfer from reduced enzyme to coenzyme Q₁ by the rapid dismutation of O₂⁻ generated, thereby preventing the reduction of coenzyme Q₁ by O⁻₂. The enzymatic reduction of coenzyme Q₁ to coenzyme Q₁H₂ via coenzyme Q₁ is smoothly achieved under anaerobic conditions. The rate of coenzyme Q₁H₂ autoxidation is extremely slow, i.e., second-order constant for [O₂][coenzyme Q₁H₂] = 1.5 M⁻¹ · s⁻¹ at 258 μM O₂, pH 7.5 and 25°C.     

Spectroscopic and electrical properties of the [W(C3Se5)3] anion complex (C3Se5 = 1,3-diselenole-2-selone-4,5-diselenolate) and the oxidized species

[NBuⁿ₄]₂[W(C₃Se₅)₃] (C₃Se₅²⁻ = 1,3-diselenole-2-selone-4,5- diselenolate(2−)) was prepared by the reaction of Na₂[C₃Se₅] with WCl₆ in ethanol, followed by addition of [NBuⁿ₄]Br. The cyclic voltammogram in dichloromethane exhibits two oxidation peaks at −0.04 and +0.03 V (versus SCE). The complex reacted with [Fe(C₅Me₅)₂][BF₄], iodine or [TTF]₃[BF₄]₂ (TTF^·+ = the tetrathiafulvalenium radical cation) in acetonitrile to afford the oxidized complexes [Fe(C₅Me₅)₂]_0.5[W(C₃Se₅)₃], [NBuⁿ₄]_0.1[W(C₃Se₅)₃] and [TTF]_0.5[W(C₃Se₅)₃], respectively. Current-controlled electrochemical oxidation of the complex in acetonitrile gave [NBuⁿ₄]_0.6[W(C₃Se₅)₃]. The oxidized complexes exhibit electrical conductivities of 4.7×10 ⁻⁵−1.5×10⁻³ S cm⁻¹ at room temperature measured for compacted pellets. Electronic absorption, IR and ESR spectra of these complexes are discussed.     

Bradykinin dilates rat middle cerebral artery and its large branches via endothelial B2 receptors and release of nitric oxide

Ring segments of rat middle cerebral artery (MCA) were prepared for measurement of isometric force and precontracted with 10⁻⁴ M uridine triphosphate (UTP). Concentration-effect curves (CEC) were constructed for bradykinin (BK, 10⁻⁸–10⁻⁵ M) in segments with functionally intect (E+) or denuded (E−) endothelium. E− segments did not dilate to BK. The BK receptor was characterized by application of specific B₁ or B₂ antagonists [des-Arg⁹-Leu⁸] BK (10⁻⁵ M) and [ -Arg⁰-Hyp³-Thi⁵- -Tic⁷-Oic⁸] BK (HOE140,3 × 10⁻⁷ M), respectively, or B₁ agonist [des-Arg⁹] BK (10⁻⁸–10⁻⁴ M). Involvement of nitric oxide (NO) was tested with N^G-nitro- -arginine (LNNA, 10⁻⁴ M). BK induced concentration-dependent relaxation with a maximal effect (E_max) of 40.86 ± 1.50% at 10⁻⁶ M and a pD₂ (−log₁₀ EC₅₀) of 6.818 ± 0.044. This relaxation could be prevented with HOE140 or LNNA, but was not influenced by [des-Arg⁹-Leu⁸] BK. [des-Arg⁹] BK did not induce any effect. These results demonstrate that BK induced relaxation via endothelial B₂ receptors and release of NO in isolated rat MCA.     

Synthesis and structures of the cuboidal iron-sulfur-nitrosyl-phosphine clusters [Fe4S3(NO)4(PR3)3] (R = Et, Pr, C6H11)

A series of cuboidal iron-sulfur clusters [Fe₄S₃(NO)₄(PR₃)₃]^0,1+ (R = Et, Prⁱ, Cy) were synthesized by two routes: reductive desulfurization of [Fe₄S₄(NO)₄] by tertiary phosphines, and substitution of triphenylphosphine in [Fe₄4S₃(NO)₄(PPh₃)₃] by a more basic phosphine. The structures of 3[Fe₄S₃(NO)₄(PEt₃)₃] · 0.5Et₂O, [Fe₄S₃(NO)₄(PEt₃)₃] [Fe₄S₃(NO)₇] and partially substituted [Fe₄S₃(NO)₄(PPh₃)_2⁻ (PPrⁱ₃)] have been determined by X-ray diffraction in order to define the cuboidal Fe₄S₃ core, previously known only in Roussin's black anion and its reduced form, [Fe₄S₃(NO)₇7]^1−,2−, and as a part of the iron-molybdenum cofactor of nitrogenase.     

Structure of Co2(CO)6(dppm) and Co2(CO)5(CHCO2Et)(dppm) (dppm = Ph2PCH2PPh2) and exchange reaction with CO: An experimental and computational study

Crystal structures of Co₂(CO)₆(dppm) (1) and Co₂(CO)₅(CHCO₂Et)(dppm) (2) (dppm = Ph₂PCH₂PPh₂) show asymmetry with respect to the orientation of the phenyl groups in 1 and owing to the bridging ethoxycarbonylcarbene ligand in 2. The effect of this asymmetry was recognized in the solid-state ³¹P NMR spectra of 1 and 2 and in the solid-state and solution ¹³C NMR spectra of 2 as well, but not in the solid-state and solution ¹³C NMR spectra of 1. In CH₂Cl₂ solution under an atmosphere of ¹³CO, the CO ligands of both complexes exchange with ¹³CO. The overall rate of ¹³CO exchange at 10 °C was found to be k_obs = 0.107 × 10⁻³ s⁻¹ for 1 and k_obs = 0.243 × 10⁻³ s⁻¹ for 2. Two-layered ONIOM(B3LYP/6-31G(d):LSDA/LANL2MB) studies revealed fluxional behavior of 1 with rather small barriers of activation of the rearrangements. Four possible isomers have been computed for 2, close to each other energetically.     

Production d'NH4 par la crevette Crangon crangon L. dans deux écosystémes côtiers. approche expérimentale et étude de l'influence du sédiment sur le taux d'excrétion

Estimation of the ammonia production of the shrimp C. crangon in two littoral ecosystems (oligotrophic sand and eutrophic mud) was determined in winter and summer conditions from laboratory observations in experimental microcosms. The ammonia excretion rate of C. crangon was not influenced by either the sediment type or the ammonia concentration of the overlying water; on the other hand, the mean excretion rate and the response to initial handling stress increased markedly as shrimp were deprived of soft substratum.

The daily ammonia production of C. crangon was 16 μmol NH₃ · g ⁻¹ wet wt · day ⁻¹ in winter and 40 μmol in summer. A gross production of 12 μmol NH₃ · m⁻² · day ⁻¹ and 300–700 μmol μ m⁻² · day⁻¹, respectively, could be expected in the two ecosystems studied. This would account for 5% (winter) and 2–4% (summer) of the total NH⁺₄ flux at the sediment-water interface. The contribution of the excretion of all macrofauna to the NH⁺₄ flux from the sediment is discussed.     

The versatile coordination chemistry of tricyanoethenolate, [(NC)2C=C(CN)O]

The reaction of meso-tetrakis (4-dimethoxyphenyl) porphinatomanganese(II), MnTP_OMeP, with TCNE (TCNE = tetracyanoethylene) leads to the formation of [MnTP_OMeP]⁺ [TCNE]⁻ and [MnTP_OMeP]⁺[OC(CN)C(CN)₂]⁻. The single-crystal X-ray structures of the latter as well as [Cu(bipy)₂Cl]⁺ [OC(CN)C(CN)₂]⁻ were determined. The former has a disordered [OC(CN)C(CN)₂]⁻ bridging via C and O between a pair of Mn^III sites, whereas the latter has an isolated [OC(CN)C(CN)₂]⁻ unbound to Cu^II. The IR characterization for μ₂-C,O bound [OC(CN)C(CN)₂]⁻ is at 2219m and 2196s (νCN) cm⁻¹ and at 1558s (νCO) cm⁻¹ while for unbound [OC(CN)C(CN)₂]⁻ it is at 2210m, 2203m, 2181m (νCN) cm⁻¹ and at 1583s (νCO) cm⁻¹.     

Ligand induced P---H bond formation and a comparison of the reactivity of two isomers of the carbonyl hydride [Pt2(μ-PBu2)2(H)(CO)(PBu2H)]CF3SO3

The Pt₂ ^(II) isomeric terminal hydrides [(CO)(H)Pt(μ-PBu^t ₂)₂Pt(PBu^t ₂H)]CF₃SO₃ (1a), and [(CO)Pt(μ-PBu^t ₂)₂Pt(PBu^t ₂H)(H)]CF₃SO₃ (1b), react rapidly with 1 atm of carbon monoxide to give the same mixture of two isomers of the Pt₂ ^(I) dicarbonyl [Pt₂(μ-PBu^t ₂)(CO)₂(PBu^t ₂H)₂]CF₃SO₃ (3-Pt); the solid state structure of the isomer bearing the carbonyl ligands pseudo-trans to the bridging phosphide was solved by X-ray diffraction. A remarkable difference was instead found between the reactivity of 1a and 1b towards carbon disulfide or isoprene. In both cases 1b reacts slowly to afford [Pt₂(μ-PBu^t ₂)(μ,η²,η²-CS₂)(PBu^t ₂H)₂]CF₃SO₃ (4-Pt), and [Pt₂(μ-PBu^t ₂)(μ,η²,η²-isoprene) (PBu^t ₂H)₂]CF₃SO₃ (6-Pt), respectively. In the same experimental conditions, 1a is totally inert. A common mechanism, proceeding through the preassociation of the incoming ligand followed by the P---H bond formation between one of the bridging P atoms and the hydride ligand, has been suggested for these reactions.     

The effect of hydrogen peroxide on CO2 fixation of isolated intact chloroplasts

Low concentrations of hydrogen peroxide strongly inhibit CO₂ fixation of isolated intact chloroplasts (50% inhibition at 10⁻⁵ M hydrogen peroxide). Addition of catalase to a suspension of intact chloroplasts stimulates CO₂ fixation 2–6 fold, indicating that this process is partially inhibited by endogenous hydrogen peroxide formed in a Mehler reaction.

The rate of CO₂ fixation is strongly increased by addition of Calvin cycle intermediates if the catalase activity of the preparation is low. However, at high catalase activity addition of Calvin cycle intermediates remains without effect. Obviously the hydrogen peroxide formed at low catalase activity leads to a loss of Calvin cycle substrates which reduces the rate of CO₂ fixation.

3-Phosphoglycerate-dependent O₂-evolution is not influenced by hydrogen peroxide at a concentration (5 · 10⁻⁴ M) which inhibits CO₂ fixation almost completely. Therefore the inhibition site of hydrogen peroxide cannot be at the step of 3-phosphoglycerate reduction. Dark CO₂ fixation of lysed chloroplasts in a hypotonic medium is not or only slightly inhibited by hydrogen peroxide (2.5 · 10⁻⁴ M), if ribulose-1,5-diphosphate, ribose 5-phosphate or xylulose 5-phosphate were added as substrates. However, there is a strong inhibition of CO₂ fixation by hydrogen peroxide, if fructose 6-phosphate together with triose phosphate are used as substrates. This indicates that hydrogen peroxide interrupts the Calvin cycle at the transketolase step, leading to a reduced supply of the CO₂-acceptor ribulose 1,5-diphosphate.     

Differentiation induction of human leukemia cells (HL60) by a combination of 1,25-dihydroxyvitamin D3 and retinoic acid (all trans or 9-cis)

1,25(OH)₂D₃ and two stereoisomers of retinoic acid, all trans and 9-cis retinoic acid, are regulators of cell proliferation and differentiation. The aim of this study was to evaluate the effects of a combination of 1,25(OH)₂D₃ and retinoic acid (all trans or 9-cis) on proliferation and cell differentiation of the human promyelocytic leukemia cell line HL60, and to test the reversibility of the induced differentiation. Cell proliferation was inhibited as expected by 1,25(OH)₂D₃ and all trans retinoic acid alone (IC₅₀ of cell survival was 4 × 10⁻⁷ M, 9 × 10⁻⁶ M and 9 × 10⁻⁷ M for 1,25(OH)₂D₃, all trans and 9-cis retinoic acid, respectively). Combination of 1,25(OH)₂D₃ and either form of retinoic acid resulted in a partially additive decrease in cell proliferation. 1,25(OH)₂D₃ induced a monocytic differentiation (100% CD14+ cells with 10⁻⁷ M 1,25(OH)₂D₃), while retinoic acid led to a predominantly granulocytic differentiation (36 and 42% CD67+ cells with 10⁻⁶ M all trans and 9-cis retinoic acid, respectively). Additive effects on differentiation were observed upon combination of subtherapeutical doses of the drugs, achieving a mainly monocytic population, demonstrating the dominant role of 1,25(OH)₂D₃ in determining the direction of differentiation. The effects on proliferation and differentiation of the solitary drugs were reversible, while the proliferation arrest and differentiation induced by the combination persisted and even progressed after withdrawal of the drugs. We conclude that 1,25(OH)₂D₃ and retinoic acid (all trans or 9-cis) exert additive effects on inhibition of proliferation and induction of cell differentiation of HL60 cells, leading to a persistent differentiation, even after drug withdrawal.     

Characterization of monoglucuronides of vitamin D2 and 25-hydroxyvitamin D2 in rat bile using high-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

The characterization of vitamin D₂ 3-glucuronide, 25-hydroxyvitamin D₂ 3-glucuronide and 25-hydroxyvitamin D₂ 25-glucuronide, biliary metabolites obtained from rats dosed with vitamin D₂ and 25-hydroxyvitamin D₂ per os, was carried out using HPLC-atmospheric pressure chemical ionization (APCI)-MS. The glucuronide obtained from bile specimens was identified by comparison of its chromatographic behaviour with an authentic sample using HPLC—APCI-MS operating in the negative-ion mode. Methylation of the respective fraction with diazomethane gave the methyl ester, which was also confirmed by HPLC—APCI-MS operating in the positive-ion mode. The (M-M)⁻ and (M+NH₄)⁺ ions were monitored in the selected-ion monitoring mode.     

New herbicide-binding site in the photosynthetic electron-transport chain: Competitive herbicide binding at the photosystem I phylloquinone-(vitamin K1)-binding site

The binding of herbicides to the phylloquinone-(primary electron acceptor A₁)-binding site in green plant photosystem (PS) I reaction centers is shown. Dissociation constants (K_d) of various herbicides to the phylloquinone-binding site were estimated by analyzing their competitive inhibition of the reconstitution of the phylloquinone analogue, menadione (vitamin K₃), to the phylloquinone-extracted spinach PS I particles. The phylloquinone-binding site was found to bind o-phenanthroline (K_d = 1.2 × 10⁻⁴ M), but only weak binding was observed with atrazine (K_d > 10⁻² M), although both are known to bind specifically to the quinone-(Q_B)-binding site in reaction centers of purple photosynthetic bacteria or PS II. The inhibitors of the cytochrome b/c₁(ƒ) complex, myxothiazol (K_d=9.5 × 10⁻⁶ M) or antimycin A (K_d = 2.8 × 10⁻⁶ M), also strongly bound to the phylloquinone site. This is the first report showing that the PS I reaction center complex also has a herbicide-binding site, although the site is probably not sensitive in vivo to these herbicides due to its higher affinity for phylloquinone than herbicides. The inhibitor specificity of the PS I phylloquinone site is different from that of the other quinone-functioning sites in the photosynthetic or respiratory electron-transfer chain, suggesting it to have a unique structure.     

Kinetics and mechanism of the stoichiometric oxygenation of [Cu(fla)(idpa)]ClO4 [fla=flavonolate, IDPA=3,3′-imino-bis(N,N-dimethylpropylamine)] and the [Cu(fla)(idpa)]ClO4-catalysed oxygenation of flavonol

Oxygenation of [Cu^II(fla)(idpa)]ClO₄ (fla=flavonolate; IDPA=3,3′-iminobis(N,N-dimethylpropylamine)) in dimethylformamide gives [Cu^II(idpa)(O-bs)]ClO₄ (O-bs=O-benzoylsalicylate) and CO. The oxygenolysis of [Cu^II(fla)(idpa)]ClO₄ in DMF was followed by electronic spectroscopy and the rate law −d[{Cu^II(fla)(idpa)}ClO₄]/dt=k_obs[{Cu^II(fla)(idpa)}ClO₄][O₂] was obtained. The rate constant, activation enthalpy and entropy at 373 K are k_obs=6.13±0.16×10⁻³ M⁻¹ s⁻¹, ΔH^‡=64±5 kJ mol⁻¹, ΔS^‡=−120±13 J mol⁻¹ K⁻¹, respectively. The reaction fits a Hammett linear free energy relationship and a higher electron density on copper gives faster oxygenation rates. The complex [Cu^II(fla)(idpa)]ClO₄ has also been found to be a selective catalyst for the oxygenation of flavonol to the corresponding O-benzoylsalicylic acid and CO. The kinetics of the oxygenolysis in DMF was followed by electronic spectroscopy and the following rate law was obtained: −d[flaH]/dt=k_obs[{Cu^II(fla)(idpa)}ClO₄][O₂]. The rate constant, activation enthalpy and entropy at 403 K are k_obs=4.22±0.15×10⁻² M⁻¹ s⁻¹, ΔH^‡=71±6 kJ mol⁻¹, ΔS^‡=−97±15 J mol⁻¹ K⁻¹, respectively.     

Synthesis, solution behaviour and molecular structures of 16-electron closo-2-(Ph3P)-1-N,2-[μ-(η-CH2CH=Ch2)]-1-N-(σ-CH2CH=CH2)-2,1- RhCB10H10, the first η-olefinic monocarbon metallacarborane

The first η²-olefinic monocarbon metallacarbone closo-2-(Ph₃P)-1-N,2-[μ-(η²-CH₂CH=Ch₂)]-1-N-(σ-CH₂CH=CH₂)-2,1- RhCB₁₀H₁₀ has been prepared by the reaction of the dimeric anion {[Ph₃PRhB₁₀H₁₀CNH₂]₂-μ-H}⁻[PPN]⁺ with allyl bromide and characterized by a combination of spectroscopic methods and a single-crystal X-ray diffraction study. The variable temperature ¹H and ¹³C NMR studies revealed the fluxional behavior of the η²-olefinic complex in CD₂Cl₂ solution which is associated with the allyl side-chain exchange process.     

Mixed-metal butterfly acetamidediato clusters derived from a highly reactive ruthenium oxo cluster: synthesis and characterization of [(PPh3)2N][MRu3(CO)12(η-μ3-NC(μ-O)CH3)], (M = Mn or Re)

Analogy with the isolable oxo cluster [Fe₃(CO)₉(μ₃-O)]²⁻, which is structurally interesting and synthetically useful, prompted the present attempt to synthesize its ruthenium analog. Although the high reactivity of [Ru₃(CO)₉(μ₃-O)]²⁻ (I) prevented its isolation, the reaction of this species with [M(CO)₃(NCCH₃)]⁺, where M = Mn or Re, yields [PPN][MRu₃(CO)₁₂(η²-μ₃-NC(μ-O)CH₃]⁻. The high nucleophilicity of the oxo ligand in [Ru₃(CO)₉(μ₃-O)]²⁻ (I) appears to be responsible for the conversion of acetonitrile to an acetamidediato ligand and for the instability of I. The crystal structure of [PPN][MnRu₃(CO)₁₂(η²-μ₃-NC(μ-O)CH₃)]] reveals a hinged butterfly array of metal atoms in which the acetamidediato ligand bridges the two wings with μ₃-N bonding to an Mn and two Ru atoms, and μ-O bonding to an Ru atom.     

Nitric oxide stimulates prostaglandin synthesis in cultured rabbit gastric cells

Both prostaglandins (PGs) and nitric oxide (NO) have cytoprotective and hyperemic effects in the stomach. However, the effect of NO on PG synthesis in gastric mucosal cells is unclear. We examined whether sodium nitroprusside (SNP), a releaser of NO, stimulates PG synthesis in cultured rabbit gastric mucus-producing cells. These cells did not release NO themselves. Co-incubation with SNP (2 × 10⁻⁴, 5 × 10⁻⁴, 10⁻³ M) increased PGE₂ synthesis, and SNP (10⁻³ M) increased PGI₂ synthesis in these cells. Hemoglobin, a scavenger of NO, (10⁻⁵ M) eliminated the increase in PGE₂ synthesis by SNP, but methylene blue, an inhibitor of soluble guanylate cyclase, (5 × 10⁻⁵ M) did not affect the increase in PGE₂ synthesis by SNP. 8-bromo guanosine 3′ : 5′-cyclic monophosphate (8-bromo cGMP), a cGMP analogue, (10⁻⁶, 10⁻⁵, 10⁻⁴, 10⁻³ M) did not affect PGE₂ synthesis. These findings suggest that NO increased PGE₂ and PGI₂ synthesis via a cGMP-independent pathway in cultured rabbit gastric cells.     

A series of heteroleptic complexes of the type fac-[MnL2] [H2L=derivatives of N-(2-hydroxybenzyl)glycine or N-(5-nitro-2-hydroxybenzyl)sarcosine] possessing unusual Mn(III) co-ordination spheres

The mononuclear manganese(III) complexes [C₅H₁₀NH₂][MnL₂] [L²⁻=a substituted N-(2-hydroxybenzyl)glycinate (hbg²⁻) viz. 3,5-dibromo- (3,5-Br-hbg²⁻), 3,5-dichloro- (3,5-Cl-hbg²⁻), 3-methyl-5-chloro- (3,5-Me,Cl-hbg²⁻), 5-bromo- (5-Br-hbg²⁻), 5-chloro- (5-Cl-hbg²⁻), 5-nitro- (5-NO₂-hbg²⁻) or N-(5-nitro-2-hydroxybenzyl)sarcosine (5-NO₂-hbs²⁻)] have been synthesised by reaction of the appropriate ligand with manganese(II) perchlorate under ambient conditions in a 2:1 molar ratio using piperidine as base. The structures of three of these complexes, [C₅H₁₀NH₂][Mn(3,5-Cl-hbg)₂] (2), [C₅H₁₀NH₂][Mn(5-NO₂-hbg)₂] (6) and [C₅H₁₀NH₂][Mn(5-NO₂-hbs)₂] (7) have been elucidated by single-crystal X-ray crystallography and each displays two similar, independent [MnL₂]⁻ ions in the asymmetric unit linked via piperidinium cations through hydrogen bonding. The ligands co-ordinate in a facial tridentate fashion with the three donor atoms being the phenolate and carboxylate oxygens and the amine nitrogen. The geometry at the Mn centres is compressed rhombic octahedral consistent with a pseudo-Jahn–Teller compression along the Mn–O(phenolate) axis. Mean bond lengths are in the ranges 1.886–1.889 Å for the Mn–O(phenolate), 2.062–2.125 Å for the Mn–O(carboxylate) and 2.091–2.184 Å for the Mn–N(amine) distances. The magnetic susceptibility and electronic and IR spectroscopic data are discussed with reference to the crystal structures.     

Kinetics and mechanism of iron exchange in hydroxamate siderophores: Catalysis of the iron(III) transfer from ferrioxamine B to ethylenediaminetetraacetic acid

The oxalate catalyzed iron(III) transfer from a trihydroxamate siderophore ferrioxamine B, [Fe(Hdfb)⁺], to ethylenediaminetetraacetic acid (H₄edta) has been studied spectro-photometrically in weakly acidic aqueous solutions at 298 K and a constant 2.0 M ionic strength maintained by NaClO₄. The results reveal that oxalate is a more efficient catalyst than the so far studied synthetic monohydroxamic acids. Any role of reduction of Fe(Hdfb)⁺ by oxalate in the catalysis has been rejected by the experimentally observed preservation of the oxalate concentration during the reaction time. Therefore, catalysis has been proposed to be a substitution based process. Under our experimental conditions Fe(Hdfb)⁺ is hexacoordinated and addition of oxalate results in the formation of Fe(H₂dfb)(C₂O₄), Fe(H₃dfb)(C₂O₄)⁻₂ and Fe(C₂O₄)³⁻₃. Therefore, catalysis was proposed to be accomplished by the intermediate formation of the ternary and tris(oxalato) complexes. All three complexes react with H₂edta²⁻ to form thermodynamically stable Fe(edta)⁻ as a final reaction product. Whereas the formation of the ternary complexes is fast enough to feature a pre-equilibrium process to the iron exchange reaction, the formation of Fe(C₂O₄)³⁻₃ is slow and is directly involved in the rate determining step of the Fe(edta)⁻ formation. Nonlinear dependencies of the rate constant on the oxalate and the proton concentrations have been observed and a four parallel path mechanism is proposed for the exchange reaction. The rate and equilibrium constants for the various reaction paths were determined from the kinetic and equilibrium study involving the desferrioxamine B- (H₄dfb⁺), oxalate- and proton-concentration variations. The observed proton catalysis was attributed to the fast monoprotonation of ferrioxamine B as well as of the oxalate ligand. The observed catalysis of iron dissociation from the siderophore has been discussed in view of its significance with respect to in vivo microbial iron transport.     

Electrospray mass spectrometry of trans-[Ru(NO)Cl(dpaH)2] (dpaH=2,2′-dipyridylamine) and ‘caged NO’, [RuCl3(NO)(H2O)2]: loss of HCl and NO from positive ions versus NO and Cl from negative ions

The positive ion electrospray mass spectrometry (ESI-MS) of trans-[Ru(NO)Cl)(dpaH)₂]Cl₂ (dpaH=2,2′-dipyridylamine), obtained from the carrier solvent of H₂O–CH₃OH (50:50), revealed 1+ ions of the formulas [Ru^II(NO⁺)Cl(dpaH)(dpa)]⁺ (m/z=508), [Ru^IIICl(dpaH)(dpa⁻)]⁺ (m/z=478), [Ru^II(NO⁺)(dpa)₂]⁺ (m/z=472), [Ru^III(dpa)₂]⁺ (m/z=442), originating from proton dissociation from the parent [Ru^II(NO⁺)Cl(dpaH)₂]²⁺ ion with subsequent loss of NO (17.4% of dissociative events) or loss of HCl (82.6% of dissociative events). Further loss of NO from the m/z=472 fragment yields the m/z=442 fragment. Thus, ionization of the NH moiety of dpaH is a significant factor in controlling the net ionic charge in the gas phase, and allowing preferential dissociation of HCl in the fragmentation processes. With NaCl added, an ion pair, {Na[Ru^II(NO)Cl(dpa)₂]}⁺ (m/z=530; 532), is detectable. All these positive mass peaks that contain Ru carry a signature ‘handprint’ of adjacent m/z peaks due to the isotopic distribution of ¹⁰⁴Ru, ¹⁰²Ru, ¹⁰¹Ru, ⁹⁹Ru, ⁹⁸Ru and ⁹⁶Ru mass centered around ¹⁰¹Ru for each fragment, and have been matched to the theoretical isotopic distribution for each set of peaks centered on the main isotope peak. When the starting complex is allowed to undergo aquation for two weeks in H₂O, loss of the axial Cl⁻ is shown by the approximately 77% attenuation of the [Ru^II(NO⁺)Cl(dpaH)(dpa)]⁺ ion, being replaced by the [Ru^II(NO⁺)(H₂O)(dpa)₂]⁺ (m/z=490) as the most abundant high-mass species. Loss of H₂O is observed to form [Ru^II(NO⁺)(dpa)₂]⁺ (m/z=472). No positive ion mass spectral peaks were observed for RuCl₃(NO)(H₂O)₂, ‘caged NO’. Negative ions were observed by proton dissociation forming [Ru^II(NO)Cl₃(H₂O)(OH)]⁻ in the ionization chamber, detecting the parent 1− ion at m/z=274, followed by the loss of NO as the main dissociative pathway that produces [Ru^IIICl₃(H₂O)(OH)]⁻ (m/z=244). This species undergoes reductive elimination of a chlorine atom, forming [Ru^IICl₂(H₂O)(OH)]⁻ (m/z=208). The ease of the NO dissociation is increased for the negative ions, which should be more able to stabilize a Ru^III product upon NO loss.     

Studies of the oxovanadium(IV)—oxalate system: syntheses and crystal structures of binuclear (Ph4P)2[(VO)2(H2O)2(C2O4)3]·4H2O and of the one-dimensional chain material, (Ph4P)[VOCl(C2O4)]

The hydrothermal reactions of (Ph₄P)[VO₂Cl₂] and H₂C₂O₄ at 150 and 125°C yield (Ph₄P)₂[V₂O₂(H₂O)₂(C₂O₄)₃]·4H₂O (1) and (Ph₄P)[VOCl(C₂O₄)] (2), respectively. The structure of the molecular anion of 1 consists of a binuclear unit of oxovanadium(IV) octahedra bridged by a bisbidentate oxalate group. The VO₆ coordination geometry at each vanadium site is defined by a terminal oxo group, an aquo ligand, and four oxygen donors — two from the bisbidentate bridging oxalate and two from the terminal bidentate oxalate. The structure of 2 consists of discrete Ph₄P⁺ cations occupying regions between [VOCl(C₂O₄)]⁻_∞ spiral chains. The structure of the one-dimensional anionic chain exhibits V(IV) octahedra bridged by bisbidentate oxalate groups. Crystal data: 1·4H₂O, monoclinic P2₁/n, A = 12.694(3), B = 12.531(3), C = 17.17(3) Å, β = 106.32(2)°, V = 2621.3(13) ų, Z = 2, D_calc = 1.501 g cm⁻³, structure solution and refinement converged at a conventional residual of 0.0518; 2, tetragonal P4₃, A = 12.145(2), C = 15.991(3) Å, V = 2358.7(12) ų, Z = 4, R = 0.0452.