Reaction Of Dioxygen Research Articles

Role of hemoglobin oxygenation in the modulation of red blood cell mechanical properties by nitric oxide

It has been previously demonstrated that both externally generated and internally synthesized nitric oxide (NO) can affect red blood cell (RBC) deformability. Further studies have shown that the RBC has active NO synthesizing mechanisms and that these mechanisms may play role in maintaining normal RBC mechanical properties. However, hemoglobin within the RBC is known to be a potent scavenger of NO; oxy-hemoglobin scavenges NO faster than deoxy-hemoglobin via the dioxygenation reaction to nitrate. The present study aimed at investigating the role of hemoglobin oxygenation in the modulation of RBC rheologic behavior by NO. Human blood was obtained from healthy volunteers, anticoagulated with sodium heparin (15 IU/mL), and the hematocrit was adjusted to 0.4 L/L by adding or removing autologous plasma. Several two mL aliquots of blood were equilibrated at room temperature (22 ± 2 °C) with moisturized air or 100% nitrogen by a membrane gas exchanger, The NO donor sodium nitroprusside (SNP), at a concentration range of 10 −7–10 −4 M, was added to the equilibrated aliquots which were maintained under the same conditions for an additional 60 min. The effect of the non-specific NOS inhibitor l-NAME was also tested at a concentration of 10 −3 M. RBC deformability was measured using an ektacytometer with an environment corresponding to that used for the prior incubation (i.e., oxygenated or deoxygenated). Our results indicate an improvement of RBC deformability with the NO donor SNP that was much more pronounced in the deoxygenated aliquots. SNP also had a more pronounced effect on RBC aggregation for deoxygenated RBC. Conversely, l-NAME had no effect on deoxygenated blood but resulted in impaired deformability, with no change in aggregation for oxygenated blood. These findings can be explained by a differential behavior of hemoglobin under oxygenated and deoxygenated conditions; the influence of oxygen partial pressure on NOS activity may also play a role. It is therefore critical to consider the oxygenation state of intracellular hemoglobin while studying the role of NO as a regulator of RBC mechanical properties.

Evidence for the formation of a mononuclear ferric–hydroperoxo complex via the reaction of dioxygen with an (N4S(thiolate))iron(ii) complex

Addition of O(2)(g) at low-temperature to a mononuclear, nonheme iron(ii) complex comprising a tetraazamacrocyclic N(4) donor and an arylthiolate S donor leads to the generation of a deep red complex assigned as a low-spin Fe(III)?OOH complex, formed via metal- and ligand-assisted oxidation.

A Putative Fe2+-Bound Persulfenate Intermediate in Cysteine Dioxygenase

The common reactions of dioxygen, superoxide, and hydroperoxides with thiolates are thought to proceed via persulfenate intermediates, yet these have never been visualized. Here we report a 1.4 A resolution crystal structure of the Fe(2+)-dependent enzyme cysteine dioxygenase (CDO) containing this putative intermediate trapped in its active site pocket. The complex raises the possibility that, distinct from known dioxygenases and proposed CDO mechanisms, the Fe-proximal oxygen atom may be involved in the primary oxidation event yielding a unique three-membered Fe-S-O cyclic intermediate. A nonpolar environment of the distal oxygen would facilitate isomerization of the persulfenate to the sulfinate product.

His-311 and Arg-559 Are Key Residues Involved in Fatty Acid Oxygenation in Pathogen-inducible Oxygenase

Pathogen-inducible oxygenase (PIOX) oxygenates fatty acids into 2R-hydroperoxides. PIOX belongs to the fatty acid alpha-dioxygenase family, which exhibits homology to cyclooxygenase enzymes (COX-1 and COX-2). Although these enzymes share common catalytic features, including the use of a tyrosine radical during catalysis, little is known about other residues involved in the dioxygenase reaction of PIOX. We generated a model of linoleic acid (LA) bound to PIOX based on computational sequence alignment and secondary structure predictions with COX-1 and experimental observations that governed the placement of carbon-2 of LA below the catalytic Tyr-379. Examination of the model identified His-311, Arg-558, and Arg-559 as potential molecular determinants of the dioxygenase reaction. Substitutions at His-311 and Arg-559 resulted in mutant constructs that retained virtually no oxygenase activity, whereas substitutions of Arg-558 caused only moderate decreases in activity. Arg-559 mutant constructs exhibited increases of greater than 140-fold in Km, whereas no substantial change in Km was observed for His-311 or Arg-558 mutant constructs. Thermal shift assays used to measure ligand binding affinity show that the binding of LA is significantly reduced in a Y379F/R559A mutant construct compared with that observed for Y379F/R558A construct. Although Oryza sativa PIOX exhibited oxygenase activity against a variety of 14-20-carbon fatty acids, the enzyme did not oxygenate substrates containing modifications at the carboxylate, carbon-1, or carbon-2. Taken together, these data suggest that Arg-559 is required for high affinity binding of substrates to PIOX, whereas His-311 is involved in optimally aligning carbon-2 below Tyr-379 for catalysis.

Pd-Catalyzed Homocoupling Reaction of Arylboronic Acid: Insights from Density Functional Theory

The key step in the mechanism of the Palladium-catalyzed homocoupling of arylboronic acids ArB(OH)(2)(Ar = 4-Z-C(6)H(4) with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been elucidated by using density functional theory. In particular, by starting from the peroxo complex O(2)PdL(2)(L = PPh(3)), generated in the reaction of dioxygen with the Pd(0) catalyst, the fundamental role played by an intermediate formed by coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid has been pointed out. This adduct reacts with a second molecule of arylboronic acid to generate a cis-Ar-Pd(OOB(OH)(2))L(2) complex that can form the stable intermediate trans-Ar-Pd(OH)L(2) (experimentally characterized) through a sequence of hydrolysis and isomerization reactions. All theoretical insights are in agreement and do substantiate the experimentally postulated mechanism. Furthermore, direct comparison of experimental and computed spectroscopic parameters (here, (31)P chemical shifts) allows us to confirm the formation of the intermediate.

Metal Ion-coupled Electron-transfer Reduction of Dioxygen

Abstract Metal ion-coupled and proton-coupled electron-transfer reactions of dioxygen is described for one-electron, two-electron, and four-electron reduction of dioxygen by one-electron reductants such as ferrocene derivatives as well as by two-electron reductants such as NADH analogs. The catalytic mechanism of the four-electron reduction of dioxygen with cofacial dicobalt porphyrins has been clarified based on the detailed kinetic study and the detection of the intermediate.

Dioxygen Activation at a Mononuclear Cu(I) Center Embedded in the Calix[6]arene-Tren Core

The reaction of a cuprous center coordinated to a calix[6]arene-based aza-cryptand with dioxygen has been studied. In this system, Cu(I) is bound to a tren unit that caps the calixarene core at the level of the small rim. As a result, although protected from the reaction medium by the macrocycle, the metal center presents a labile site accessible to small guest ligands. Indeed, in the presence of O2, it reacts in a very fast and irreversible redox process, leading, ultimately, to Cu(II) species. In the coordinating solvent MeCN, a one electron exchange occurs, yielding the corresponding [CalixtrenCu-MeCN](2+) complex with concomitant release of superoxide in the reaction medium. In a noncoordinating solvent such as CH2Cl2, the dioxygen reaction leads to oxygen insertions into the ligand itself. Both reactions are proposed to proceed through the formation of a superoxide-Cu(II) intermediate that is unstable in the Calixtren environment due to second sphere effects. The transiently formed superoxide ligand either undergoes fast substitution for a guest ligand (in MeCN) or intramolecular redox evolutions toward oxygenation of Calixtren. Interestingly, the latter process was shown to occur twice on the same ligand, thus demonstrating a possible catalytic activation of O2 at a single cuprous center. Altogether, this study illustrates the oxidizing power of a [CuO2](+) adduct and substantiates a mechanism by which copper mono-oxygenases such as DbetaH and PHM activate O2 at the Cu(M) center to produce such an intermediate capable of C-H breaking before the electron input provided by the noncoupled Cu(H) center.

Nitrite reductase activity of hemoglobin as a systemic nitric oxide generator mechanism to detoxify plasma hemoglobin produced during hemolysis

Hemoglobin (Hb) potently inactivates the nitric oxide (NO) radical via a dioxygenation reaction forming nitrate (NO(3)(-)). This inactivation produces endothelial dysfunction during hemolytic conditions and may contribute to the vascular complications of Hb-based blood substitutes. Hb also functions as a nitrite (NO(2)(-)) reductase, converting nitrite into NO as it deoxygenates. We hypothesized that during intravascular hemolysis, nitrite infusions would limit the vasoconstrictive properties of plasma Hb. In a canine model of low- and high-intensity hypotonic intravascular hemolysis, we characterized hemodynamic responses to nitrite infusions. Hemolysis increased systemic and pulmonary arterial pressures and systemic vascular resistance. Hemolysis also inhibited NO-dependent pulmonary and systemic vasodilation by the NO donor sodium nitroprusside. Compared with nitroprusside, nitrite demonstrated unique effects by not only inhibiting hemolysis-associated vasoconstriction but also by potentiating vasodilation at plasma Hb concentrations of <25 muM. We also observed an interaction between plasma Hb levels and nitrite to augment nitroprusside-induced vasodilation of the pulmonary and systemic circulation. This nitrite reductase activity of Hb in vivo was recapitulated in vitro using a mitochondrial NO sensor system. Nitrite infusions may promote NO generation from Hb while maintaining oxygen delivery; this effect could be harnessed to treat hemolytic conditions and to detoxify Hb-based blood substitutes.

Modeling tyrosinase activity. Effect of ligand topology on aromatic ring hydroxylation: An overview

Synthetic modeling of tyrosinase (o-phenol ring hydroxylation) has emerged as a novel class of successful biomimetic studies. It is a well-established fact that the reaction of dioxygen with copper(I) complexes of m-xylyl-based ligands generate putative copper-oxygen intermediate species such as side-on peroxo {CuII2(mu-O2)}2+ [in some cases bis-oxo {CuIII2(mu-O)2}2+ in equilibrium with isomeric side-on peroxo], due to oxygen activation. Electrophilic attack of such species brings about monooxygenase activity by incorporating one of the oxygens to m-xylyl ring of the ligand and the other oxygen is reduced to hydroxide ion. The goal of this review is to provide a concise overview of the present day knowledge in this field of research to emphasize the important role the designed ligands play in eliciting the desired tyrosinase-like chemistry.

The interaction of hydroxymandelate synthase with the 4-hydroxyphenylpyruvate dioxygenase inhibitor: NTBC

Hydroxymandelate synthase (HMS) catalyzes the committed step in the formation of para-hydroxyphenylglycine, a recurrent substructure of polycyclic non-ribosomal peptide antibiotics such as vancomycin. HMS uses the same substrates as 4-hydroxyphenylpyruvate dioxygenase (HPPD), 4-hydroxyphenylpyruvate (HPP) and O 2, and also conducts a dioxygenation reaction. The difference between the two lies in the insertion of the second oxygen atom, HMS directing this atom onto the benzylic carbon of the substrate while HPPD hydroxylates the aromatic C1 carbon. We have shown that HMS will bind NTBC, a herbicide/therapeutic whose mode of action is based on the inhibition of HPPD. This occurs despite residue differences at the active site of HMS from those known to contact the inhibitor in HPPD. Moreover, the minimal kinetic mechanism for association of NTBC to HMS differs only slightly from that observed with HPPD. The primary difference is that three charge-transfer species are observed to accumulate during association. The first reversible complex forms with a weak dissociation constant of 520 μM, the subsequent two charge-transfer complexes form with rate constants of 2.7 s −1 and 0.67 s −1. As was the case for HPPD, the final complex has the most intense charge-transfer, is not observed to dissociate, and is unreactive towards dioxygen.

Syntheses, characterization, and reactivity of copper complexes with tridentate N-donor ligands

The reaction of dioxygen with the copper(I) complex of the tridentate ligand 1,1,4,7,7-pentamethyldiethylenetriamine (Me 5dien) has been investigated using low-temperature stopped-flow techniques. The formation of a bis(μ-oxo)copper(III) complex as a reactive intermediate could be detected spectroscopically at low temperatures and a quantitative kinetic analysis was performed for this system. Crystal structures of the copper(II) complexes [(Me-bpa)Cu(Cl) 2] ( 1), [{(Me-bpa)Cu(Cl)(ClO 4)} 2] ( 2), [{(MeL)Cu(Cl)(ClO 4)} 2] ( 3), and [(MeL)Cu(NCS) 2] ( 4) (Me-bpa = N-methyl-[bis(2-pyridyl)methyl]amine; MeL = N-methyl-[(2-pyridyl)ethyl(2-pyridyl)methyl]amine) are reported.

An Isolable Silanoic Ester by Oxygenation of a Stable Silylene

The first silicon analogue of a carbonic ester, compound 2, and the novel cyclodisiloxane 4 have been synthesized by remarkably clean mono- and dioxygenation reactions of the stable siloxysilylene 1 with N2O (CO2) and O2, respectively. Ester 2 represents a unique donor-supported Si═O complex with an intriguing thermal stability up to 200 °C. The addition of MeOH at the Si═O bond in 2 furnishes the corresponding adduct 3.

Radical Intermediates in Monooxygenase Reactions of Rieske Dioxygenases

Rieske dioxygenases catalyze the cis-dihydroxylation of a wide range of aromatic compounds to initiate their biodegradation. The archetypal Rieske dioxygenase naphthalene 1,2-dioxygenase (NDOS) catalyzes dioxygenation of naphthalene to form (+)-cis-(1R,2S)-dihydroxy-1,2-dihydronaphthalene. NDOS is composed of three proteins: a reductase, a ferredoxin, and an α3β3 oxygenase (NDO). In each α subunit, NDO contains a Rieske Fe2S2 cluster and a mononuclear iron site where substrate dihydroxylation occurs. NDOS also catalyzes monooxygenase reactions for many substrates. The mechanism of the reaction is unknown for either the mono- or dioxygenase reactions but has been postulated to involve direct reaction of either a structurally characterized Fe(III)−hydroperoxy intermediate or the electronically equivalent Fe(V)−oxo−hydroxo intermediate formed by O−O bond cleavage before reaction with substrate. The reaction for the former intermediate is expected to proceed through cationic intermediates, while the latter is anticipated to initially form a radical intermediate. Here the monooxygenation reactions of the diagnostic probe molecules, norcarane and bicyclohexane, are investigated. In each case, a significant amount of the rearrangement product derived from a radical intermediate (lifetime of 11−18 ns) is observed, while little or no ring expansion product from a cationic intermediate is formed. Thus, monooxygenation of these molecules appears to proceed via the Fe(V)−oxo−hydroxo intermediate. The formation of this high-valent intermediate shows that it must also be considered as a possible participant in the dioxygenation reaction, in contrast to computational studies but in accord with previous biomimetic studies.

Copper(I) Complexes of Tripodal Tris(imidazolyl) Ligands: Potential Mimics of the Cu(A) Site of Hydroxylase Enzymes

Copper(I) complexes of tripodal tris(N-methyl-4,5-diphenyl-imidazolyl)methane ligands, N3CR (1a-c, R = OH, OMe, H), have been prepared as models for the Cu(A) site of copper hydroxylase enzymes. In the absence of additional donors, the ligands 1 react with [Cu(CH3CN)4]PF6 (2) to produce dinuclear complexes [(N3CR)2Cu2](PF6)2 (3) in which the tripodal ligands bridge two trigonal Cu centers; the structures of 3b and 3c are established by X-ray diffraction. Mononuclear adducts [(N3CR)CuL]Z are produced with L = acetonitrile (4), carbon monoxide (5), and t-BuNC (6, 7). The carbonyl complexes 5 are in dynamic equilibrium with the dimeric complexes 3, but 5c (R = H) can be isolated. The structures of the isocyanide derivatives depend critically on the tripod methane substituent, R. Thus, the X-ray structures of 6 (R = OMe) and 7 (R = H) show trigonal and tetrahedral geometries, respectively, with bi- or tridentate coordination of the tripod. A trinuclear complex [Cu3(N3COH)2(t-BuNC)2](PF6)3 (8) is formed from N3COH (1a) which features both three-coordinate and two-coordinate Cu atoms and bidentate tripod coordination. Reactions of dioxygen with dinuclear 3c or mononuclear [(N3CR)CuL]Z are sluggish, producing from the latter in acetone [(N3CH)CuII(L)(L')](PF6)2 (9, L = acetone, L' = H2O).

Purification, characterization, and crystallization of the components of a biphenyl dioxygenase system from Sphingobium yanoikuyae B1

Sphingobium yanoikuyae B1 initiates the catabolism of biphenyl by adding dioxygen to the aromatic nucleus to form (+)-cis-(2R, 3S)-dihydroxy-1-phenylcyclohexa-4,6-diene. The present study focuses on the biphenyl 2,3-dioxygenase system, which catalyzes the dioxygenation reaction. This enzyme has been shown to have a broad substrate range, catalyzing the dioxygenation of not only biphenyl, but also three- and four-ring polycyclic aromatic hydrocarbons. Extracts prepared from biphenyl-grown B1 cells contained three protein components that were required for the oxidation of biphenyl. The genes encoding the three components (bphA4, bphA3 and bphA1f,A2f) were expressed in Escherichia coli. Biotransformations of biphenyl, naphthalene, phenanthrene, and benzo[a]pyrene as substrates using the recombinant E. coli strain resulted in the formation of the expected cis-dihydrodiol products previously shown to be produced by biphenyl-induced strain B1. The three protein components were purified to apparent homogeneity and characterized in detail. The reductase component (bphA4), designated reductase(BPH-B1), was a 43 kD monomer containing one mol FAD/mol reductase(BPH-B1). The ferredoxin component (bphA3), designated ferredoxin(BPH-B1), was a 12 kD monomer containing approximately 2 g-atoms each of iron and acid-labile sulfur. The oxygenase component (bphA1f,A2f), designated oxygenase(BPH-B1), was a 217 kD heterotrimer consisting of alpha and beta subunits (approximately 51 and 21 kD, respectively). The iron and acid-labile sulfur contents of oxygenase(BPH-B1) per alphabeta were 2.4 and 1.8 g-atom per mol, respectively. Reduced ferredoxin(BPH-B1) and oxygenase(BPH-B1) each gave EPR signals typical of Rieske [2Fe-2S] proteins. Crystals of reductase(BPH-B1), ferredoxin(BPH-B1) and oxygenase(BPH-B1 )diffracted to 2.5 A, 2.0 A and 1.75 A, respectively. The structures of the three proteins are currently being determined.

Molecular Oxygen to Regenerate PdII Active Species

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Synthetic Uses of Peroxides

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Molecular Oxygen To Regenerate PdII Active Species

Palladium(II) catalysis allows various aerobic oxidation reactions, but the mechanism of the regeneration of the active catalytic species remains, in many cases, undetermined. In recent years, considerable effort has been directed toward the comprehension of the reaction of dioxygen with hydridopalladium(II) and palladium(0) complexes. This Focus Review highlights the results of these experimental and theoretical studies that can contribute to the exploitation of the powerful nature of Pd(II) catalysis.

Synthesis and Characterization of a Series of Thiol‐Derivatized Porphyrins and Electrocatalytic Reduction of Dioxygen by Cobalt(II)‐tetra‐[p‐(mercaptoprophyloxy)‐phenyl] Porphyrin (CoTMPP) on a Gold Support, Self‐Assembled Monolayers (SAMs)

A series of thiol‐derivatized porphyrins with one to four [p‐(3‐mercaptopropyloxy)‐phenyl] groups appended to the meso‐position of the porphyrins, have been synthesized and shown to form molecular monolayers via irreversible chemical adsorption on the surface of gold via S‐Au linkage to form a highly ordered surface with fewer defects and exhibit a high degree of orientation, molecular ordering, packing density and thus as new chemically modified electrodes to study their electrochemical properties. Electrochemical surface coverage determinations, although not conclusive, suggested near monolayer coverage with preferential binding of the porphyrin ring perpendicular or coplanar to the gold surface. With post‐assembly insertion of Cobalt(II) into tetra‐[p‐(mercaptoprophyloxy)‐phenyl] porphyrin (H2TMPP) monolayers on gold electrode, molecular films of CoTMPP reduce dioxygen at potentials consistent with two‐electron reduction to hydrogen peroxide and a lower limit to the rate constant (k) for the reaction of dioxygen is 4.03×104 M−l s−l.

Mechanism of the Palladium-Catalyzed Homocoupling of Arylboronic Acids: Key Involvement of a Palladium Peroxo Complex

The mechanism of the palladium-catalyzed homocoupling of arylboronic acids ArB(OH)(2) (Ar = 4-Z-C(6)H(4) with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been fully elucidated. The peroxo complex (eta(2)-O(2))PdL(2) (L = PPh(3)), generated in the reaction of dioxygen with the Pd(0) catalyst, was found to play a crucial role. Indeed, it reacts with the arylboronic acid to generate an adduct (coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid) characterized by (31)P NMR spectroscopy and ab initio calculations. This adduct reacts with a second molecule of arylboronic acid to generate trans-ArPd(OH)L(2) complexes. A transmetalation by the arylboronic acid gives trans-ArPdArL(2) complexes. The biaryl is then released in a reductive elimination. This reaction is at the origin of the formation of biaryls as byproducts in palladium-catalyzed Suzuki-Miyaura reactions when they are not conducted under oxygen-free atmosphere.