REACTION MECHANISMS OF NITRIC OXIDE WITH BIOLOGICALLY RELEVANT METAL CENTERS. Solvent Exchange on Metal Ions. Elsevier; 2003. pp. 203-57. [DOI: 10.1016/s0898-8838(03)54004-1]

Reactivity of nitrogen species with inorganic and organic compounds in water

Many studies on the reactive nitrogen species (RNS, ^●NO₂, ^●NO and ^●NH₂) with pollutants in water have been performed to understand the abatement of inorganic and organic compounds by these species, and the mechanisms of the formation of oxidative transformation products, especially nitrogenous oxidized byproducts. In this review, approaches to generate RNS in aqueous solution is first presented, followed by a summary of their reactivity with a wide range of compounds. The second-order rate constants (k, M^-1 s^-1) for the reactivity of ^●NO₂ and ^●NO with a wide range of inorganic radical and nonradical species were correlated with thermodynamic one-electron oxidation potentials (E⁰). The positive correlation between log(k) versus E⁰ suggests one-electron transfer reactions. The Hammett-type correlations were developed for the reactions of ^●NO₂ and ^●NH₂ with organic compounds, using the unsubstituted benzene as a reference molecule (i.e., Σσ_o,p,m = 0) to calculate Σσ_o,p,m = σ_o + σ_p + σ_m for each organic molecule. Linear negative correlations of log(k) with Σσ_o,p,m were obtained for both ^●NO₂ and ^●NH₂, suggesting electrophilic substitution mechanism. The correlations presented herein may assist in eliminating organic micropollutants in water treatment and reuse processes.

Experimental and Computational Insight into the Mechanism of NO Binding to Ferric Microperoxidase. The Likely Role of Tautomerization to Account for the pH Dependence

According to the current paradigm, the metal-hydroxo bond in a six-coordinate porphyrin complex is believed to be significantly less reactive in ligand substitution than the analogous metal-aqua bond, due to a much higher strength of the former bond. Here, we report kinetic studies for nitric oxide (NO) binding to a heme-protein model, acetylated microperoxidase-11 (AcMP-11), that challenge this paradigm. In the studied pH range 7.4-12.6, ferric AcMP-11 exists in three acid-base forms, assigned in the literature as [(AcMP-11)Fe^III(H₂O)(HisH)] (1), [(AcMP-11)Fe^III(OH)(HisH)] (2), and [(AcMP-11)Fe^III(OH)(His^-)] (3). From the pH dependence of the second-order rate constant for NO binding (k_on), we determined individual rate constants characterizing forms 1-3, revealing only a ca. 10-fold decrease in the NO binding rate on going from 1 (k_on⁽¹⁾ = 3.8 × 10⁶ M^-1 s^-1) to 2 (k_on⁽²⁾ = 4.0 × 10⁵ M^-1 s^-1) and the inertness of 3. These findings lead to the abandonment of the dissociatively activated mechanism, in which the reaction rate can be directly correlated with the Fe-OH bond energy, as the mechanistic explanation for the process with regard to 2. The reactivity of 2 is accounted for through the existence of a tautomeric equilibrium between the major [(AcMP-11)Fe^III(OH)(HisH)] (2a) and minor [(AcMP-11)Fe^III(H₂O)(His^-)] (2b) species, of which the second one is assigned as the NO binding target due to its labile Fe-OH₂ bond. The proposed mechanism is further substantiated by quantum-chemical calculations, which confirmed both the significant labilization of the Fe-OH₂ bond in the [(AcMP-11)Fe^III(H₂O)(His^-)] tautomer and the feasibility of the tautomer formation, especially after introducing empirical corrections to the computed relative acidities of the H₂O and HisH ligands based on the experimental pK_a values. It is shown that the "effective lability" of the axial ligand (OH^-/H₂O) in 2 may be comparable to the lability of the H₂O ligand in 1.

Reactions of the unsaturated ditungsten complexes [W2Cp2(μ-PPh2)2(CO)x] (x = 1, 2) with nitric oxide: stereoselective carbonyl displacement and oxygen-transfer reactions of a nitrite ligand

The dicarbonyl complex trans-[W2Cp2(μ-PPh2)2(CO)2] (Cp = η(5)-C5H5) reacted rapidly with NO (5% in N2) at 273 K to give selectively cis-[W2Cp2(μ-PPh2)2(NO)2]. In contrast, the analogous reactions of monocarbonyl [W2Cp2(μ-PPh2)2(μ-CO)] yielded either trans-[W2Cp2(μ-PPh2)2(NO)2] or the nitrito complex [W2Cp2(μ-PPh2)2(ONO)(CO)(NO)] (W-W = 2.9797(4) Å), depending on experimental conditions, with the latter presumably arising from reaction with trace amounts of oxygen in the medium. The stereoselectivity of the above reactions can be rationalized by assuming the participation of 33-electron [W2Cp2(μ-PPh2)2(CO)(NO)] intermediates which rapidly add a second molecule of NO via η(2)-C5H5 intermediates to eventually yield the corresponding dinitrosyls with inversion of the stereochemistry at the dimetal center, as supported by density functional theory (DFT) calculations. The nitrito complex was thermally unstable and evolved through oxygen transfer either to the carbonyl ligand, to yield the above dinitrosyls with release of CO2, or to the phosphide ligand, to give the phosphinito derivative cis-[W2Cp2(μ-OPPh2)(μ-PPh2)(NO)2], depending on experimental conditions. According to DFT calculations, the first process would involve transient dissociation/recombination of the nitrite ligand followed by coupling to carbonyl to give an intermediate with a chelate W{C,N-C(O)ON(O)} ring. Indeed, the nitrite ligand could be easily removed upon reaction of the nitrito complex with Na(BAr'4), but immediate decomposition also took place to render the electron-precise dicarbonyl [W2Cp2(μ-PPh2)2(CO)2(NO)]BAr'4 (W-W = 2.9663(3) Å) as the unique product (Ar' = 3,5-C6H3(CF3)2). Attempts to decarbonylate the latter complex photochemically yielded instead the oxo derivatives cis- and trans-[W2Cp2(μ-PPh2)2(O)(NO)]BAr'4 as the only isolable products (W-W = 2.980(2) and 3.0077(3) Å, respectively).

Nitrite reduction mediated by heme models. Routes to NO and HNO?

The water-soluble ferriheme model Fe(III)(TPPS) mediates oxygen atom transfer from inorganic nitrite to a water-soluble phosphine (tppts), dimethyl sulfide, and the biological thiols cysteine (CysSH) and glutathione (GSH). The products with the latter reductant are the respective sulfenic acids CysS(O)H and GS(O)H, although these reactive intermediates are rapidly trapped by reaction with excess thiol. The nitrosyl complex Fe(II)(TPPS)(NO) is the dominant iron species while excess substrate is present. However, in slightly acidic media (pH ≈ 6), the system does not terminate at this very stable ferrous nitrosyl. Instead, it displays a matrix of redox transformations linking spontaneous regeneration of Fe(III)(TPPS) to the formation of both N2O and NO. Electrochemical sensor and trapping experiments demonstrate that HNO (nitroxyl) is formed, at least when tppts is the reductant. HNO is the likely predecessor of the N2O. A key pathway to NO formation is nitrite reduction by Fe(II)(TPPS), and the kinetics of this iron-mediated transformation are described. Given that inorganic nitrite has protective roles during ischemia/reperfusion (I/R) injury to organs, attributed in part to NO formation, and that HNO may also reduce net damage from I/R, the present studies are relevant to potential mechanisms of such nitrite protection.

Nitrosation, thiols, and hemoglobin: energetics and kinetics

Nitrosothiols are powerful vasodilators. Although the mechanism of their formation near neutral pH is an area of intense research, neither the energetics nor the kinetics of this reaction or of subsequent reactions have been addressed. The following considerations may help to guide experiments. (1) The standard Gibbs energy for the homolysis reaction RSNO → RS(•) + NO(•)(aq) is +110 ± 5 kJ mol(-1). (2) The electrode potential of the RSNO, H(+)/RSH, NO(•)(aq) couple is -0.20 ± 0.06 V at pH 7. (3) Thiol nitrosation by NO(2)(-) is favorable by 37 ± 5 kJ mol(-1) at pH 7. (4) N(2)O(3) is not involved in in vivo nitrosation mechanisms for thermodynamic--its formation from NO(2)(-) costs 59 kJ mol(-1)--or kinetic--the reaction being second-order in NO(2)(-)--reasons. (5) Hemoglobin (Hb) cannot catalyze formation of N(2)O(3), be it via the intermediacy of the reaction of Hb[FeNO(2)](2+) with NO(•) (+81 kJ mol(-1)) or reaction of Hb[FeNO](3+) with NO(2)(-) (+88 kJ mol(-1)). (6) Energetically and kinetically viable are nitrosations that involve HNO(2) or NO(•) in the presence of an electron acceptor with an electrode potential higher than -0.20 V. These considerations are derived from existing thermochemical and kinetics data.

Mechanistic studies on the reactions of cyanide with a water-soluble Fe(III) porphyrin and their effect on the binding of NO

The reaction of the water-soluble Fe(III)(TMPS) porphyrin with CN(-) in basic solution leads to the stepwise formation of Fe(III)(TMPS)(CN)(H(2)O) and Fe(III)(TMPS)(CN)(2). The kinetics of the reaction of CN(-) with Fe(III)(TMPS)(CN)(H(2)O) was studied as a function of temperature and pressure. The positive value of the activation volume for the formation of Fe(III)(TMPS)(CN)(2) is consistent with the operation of a dissociatively activated mechanism and confirms the six-coordinate nature of the monocyano complex. A good agreement between the rate constants at pH 8 and 9 for the formation of the dicyano complex implies the presence of water in the axial position trans to coordinated cyanide in the monocyano complex and eliminates the existence of Fe(III)(TMPS)(CN)(OH) under the selected reaction conditions. Both Fe(III)(TMPS)(CN)(H(2)O) and Fe(III)(TMPS)(CN)(2) bind nitric oxide (NO) to form the same nitrosyl complex, namely, Fe(II)(TMPS)(CN)(NO(+)). Kinetic studies indicate that nitrosylation of Fe(III)(TMPS)(CN)(2) follows a limiting dissociative mechanism that is supported by the independence of the observed rate constant on [NO] at an appropriately high excess of NO, and the positive values of both the activation parameters ΔS(‡) and ΔV(‡) found for the reaction under such conditions. The relatively small first-order rate constant for NO binding, namely, (1.54 ± 0.01) × 10(-2) s(-1), correlates with the rate constant for CN(-) release from the Fe(III)(TMPS)(CN)(2) complex, namely, (1.3 ± 0.2) × 10(-2) s(-1) at 20 °C, and supports the proposed nitrosylation mechanism.

Specific detection of gaseous NO and 15NO in the headspace from liquid-phase reactions involving NO-generating organic, inorganic, and biochemical samples using a mid-infrared laser

Nitric oxide (NO) is an important biological signaling agent. The specific detection of NO represents a continuing challenge in the field of NO research. Many methods are currently employed for the detection of NO. Here, we report a qualitative but specific detection method for gaseous NO liberated in and from solution taking advantage of its low solubility. Importantly, our mid-infrared laser absorption method does not depend on any chemical derivatization of NO, and is applicable over a wide range of concentrations for both protein work and in organic-inorganic modeling work. We also apply this method to the specific detection of 15NO.

Fiber-optic infrared reflectance spectroelectrochemical studies of osmium and ruthenium nitrosyl porphyrins containing alkoxide and thiolate ligands

We have examined the redox behavior of the osmium and ruthenium compounds (OEP)M(NO)(OEt) and (OEP)M(NO)(SEt) (OEP = octaethylporphyrinato dianion; M = Os, Ru) by cyclic voltammetry and infrared spectroelectrochemistry. The compound (OEP)Os(NO)(OEt) undergoes a single reversible oxidation process in dichloromethane. In contrast, the thiolate compound (OEP)Os(NO)(SEt) undergoes a net irreversible oxidation resulting in formal loss of the SEt ligand. Extended Hückel calculations on crystal structures of these two compounds provide insight into the nature of their HOMOs. In the case of the alkoxide compound, the HOMO is largely metal centered, with 70% of the charge located in the metal's orbital and approximately 25% on the porphyrin ring. However, the HOMO of the thiolate compound consists of a pi bonding interaction between the metal dxz orbital and the px orbital on the sulfur, and a pi antibonding interaction between the metal d orbital and a pi* orbital on NO. The redox behavior of the Ru analogues have been determined, and are compared with those of the Os compounds.