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The kinetics of the binding and release of NO is studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The mechanistic interpretation is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature and pressure.

The reactivity pattern for a simple model water-soluble iron(III) porphyrin[meso-tetrakis(sulfonatomesityl)-porphinato]-iron(III), (TMPS)Fe(super III) was reported which shows that NO(nitric oxide) binding to hemes is not always controlled by the lability of the iron center, but can be determined by electronic and structural factors. The role of these factors should always be considered in addition to lability of the iron center in the interpretation of reactivity patterns observed in the interactions on NO with various heme systems.

A study is made to investigate the specific interactions, which would lead to the observed modification of the physiological activity of NO and selected No-donors. It is concluded that binding of NP to cobalamin should neither significantly affect the NO-donating ability of NP, nor exclude cobalamin as an effective scavenger of cyanide released from metabolized NP.

Kinetic and mechanistic information on the reaction of nitric oxide with reduced form of aquacobalamin is studied. This contributes to the biological interactions observed between vitamin B12 and NO.

The reports regarding vitamin B(sub12a) or aquacobalamin that binds to NO to produce a nitrosyl complex reaction, which plays an important role in various biological processes is presented. A careful study of the system shows that there is no binding and the presence of nitrite impurities in aqueous solutions of NO produce the nitrite complex reaction.

The reactions of aquacobalamin (Cbl(III)H2O, vitamin B12a) and reduced cobalamin (Cbl(II), vitamin B12r) with the nitrosothiols S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) were studied in aqueous solution at pH 7.4. UV-vis and NMR spectroscopic studies and semiquantitative kinetic investigations indicated complex reactivity patterns for the studied reactions. The detailed reaction routes depend on the oxidation state of the cobalt center in cobalamin, as well as on the structure of the nitrosothiol. Reactions of aquacobalamin with GSNO and SNAP involve initial formation of Cbl(III)-RSNO adducts followed by nitrosothiol decomposition via heterolytic S-NO bond cleavage. Formation of Cbl(III)(NO-) as the main cobalamin product indicates that the latter step leads to efficient transfer of the NO- group to the Co(III) center with concomitant oxidation of the nitrosothiol. Considerably faster reactions with Cbl(II) proceed through initial Cbl(II)-RSNO intermediates, which undergo subsequent electron-transfer processes leading to oxidation of the cobalt center and reduction of the nitrosothiol. In the case of GSNO, the overall reaction is fast (k ≈ 1.2 ⊗ 106 M-1 s-1) and leads to formation of glutathionylcobalamin (Cbl(III)SG) and nitrosylcobalamin (Cbl(III)(NO-)) as the final cobalamin products. A mechanism involving the reversible equilibrium Cbl(II) + RSNO ⇄ Cbl(III)SR + NO is suggested for the reaction on the basis of the obtained kinetic and mechanistic information. The corresponding reaction with SNAP is considerably slower and occurs in two distinct reaction steps, which result in the formation of Cbl(III)(NO-) as the ultimate cobalamin product. The significantly different kinetic and mechanistic features observed for the reaction of GSNO and SNAP illustrate the important influence of the nitrosothiol structure on its reactivity toward metal centers of biomolecules. The potential biological implications of the results are briefly discussed. [ABSTRACT FROM AUTHOR]

The water-soluble, non-μ-oxo dimer-forming porphyrin, [5,10,15,20.tetrakis-4′-t-butylphenyl-2′,6′-bis-(N-methylene(4″-t-butylpyridinium))porphyrinato]iron(III) octabromide, (P8+)FeIII, with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and ¹H NMR spectroscopy and 17O NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of (P8+)FeIII indicated a pKa1 value of 5.0 for coordinated water in (P8+)FeIII(H2O)2. The monohydroxo-ligated (P8+)FIII(OH)(H2O) formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, kex = 2.4 ⊗ 106 S-1, significantly faster than water exchange on (P8+)FeIII(H2O)2, viz. kex = 5.5 ⊗ 104 s-1 at 25 °C. The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, (P8+)FeII(NO+)(L) (L = H2O or OH-). The diaqua-ligated (P8+)FeIII(H2O)2 binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other (P)FeIII(H2O)2 complexes. Coordination of NO to (P8+)FeIII(OH)(H2O) at high pH follows an associative mode, as evidenced by negative ΔS‡on and ΔV‡on values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate (P8+)FeIII(H2O)2 (kon = 15.1 ⊗ 10³ M-1 s-1) to (P8+)FeIII(OH)(H2O) (kon = 1.56 ⊗ 10³ M-1 s-1 at 25 °C) is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these (P)FeIII complexes as a function of pH is discussed on the basis of available experimental data. [ABSTRACT FROM AUTHOR]

In-depth kinetic and mechanistic studies on the reversible binding of NO to water-soluble iron(III) porphyrins as a function of pH revealed unexpected reaction kinetics for monohydroxo-ligated (P)FeIII(OH) species formed by deprotonation of coordinated water in diaqua-ligated (P)FeIII(H2O)2. The observed significant decrease in the rate of NO binding to (P)Fe(OH) as compared to that of (P)Fe(H2O)2 does not conform with expectations based on previous mechanistic work on NO-heme interactions, which would point to a diffusion-limited reaction for the five-coordinate FeIII center in (P)Fe(OH). The decrease in rate and an associatively activated mode of NO binding observed at high pH is ascribed to an increase in the activation barrier related to spin state and structural changes accompanying NO coordination to the high-spin (P)FeIII(OH) complex. The existence of such a barrier has previously been observed in the reactions of five-coordinate iron(II) hemes with CO and is evidenced for the first time for the process involving coordination of NO to the iron heme complex. The observed reactivity pattern, relevant in the context of studies on NO interactions with synthetic and biologically important hemes (in particular, hemoproteins), is reported here for an example of a simple water-soluble iron(III) porphyrin [meso-tetrakis(sulfonatomesityl)-porphinato]-iron(III), (TMPS)FeIII. [ABSTRACT FROM AUTHOR]

Examines the reversible binding of nitric oxide to reduced cobalamin B[sub 12r]. Kinetics of the binding and dissociation reactions; Estimation of activation parameters; Formation of water-bound intermediates.

Kinetic and mechanistic studies on the formation of an oxoiron(IV) porphyrin cation radical bearing a thiolate group as proximal ligand are reported. The SR complex, a functional enzyme mimic of P450, was oxidized in peroxo-shunt reactions under different experimental conditions with variation of solvent, temperature, and identity and excess of oxidant in the presence of different organic substrates. Through the application of a low-temperature rapid-scan stopped-flow technique, the reactive intermediates in the SR catalytic cycle, such as the initially formed SR acylperoxoiron(III) complex and the SR high-valent iron(IV) porphyrin cation radical complex [(SR(*+))Fe(IV)=O], were successfully identified and kinetically characterized. The oxidation of the SR complex under catalytic conditions provided direct spectroscopic information on the reactivity of [(SR(.+))Fe(IV)=O] towards the oxidation of selected organic substrates. Because the catalytically active species is a synthetic oxoiron(IV) porphyrin cation radical bearing a thiolate proximal group, the effect of the strong electron donor ligand on the formation and reactivity/stability of the SR high-valent iron species is addressed and discussed in the light of the reactivity pattern observed in substrate oxygenation reactions catalyzed by native P450 enzyme systems.

The present study focuses on the oxidation of the water-soluble and water-insoluble iron(III)-porphyrin complexes [Fe(III)(TMPS)] and [Fe(III)(TMP)] (TMPS = meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphyrinato, TMP = meso-tetrakis(2,4,6-trimethylphenyl)porphyrinato), respectively, by meta-chloroperoxybenzoic acid (m-CPBA) in aqueous methanol and aqueous acetonitrile solutions of varying acidity. With the application of a low-temperature rapid-scan UV/Vis spectroscopic technique, the complete spectral changes that accompany the formation and decomposition of the primary product of O-O bond cleavage in the acylperoxoiron(III)-porphyrin intermediate [(P)Fe(III)-OOX] (P = porphyrin) were successfully recorded and characterized. The results clearly indicate that the O-O bond in m-CPBA is heterolytically cleaved by the studied iron(III)-porphyrin complexes independent of the acidity of the reaction medium. The existence of two different oxidation products under acidic and basic conditions is suggested not to be the result of a mechanistic changeover in the mode of O-O bond cleavage on going from low to high pH values, but rather the effect of environmental changes on the actual product of the O-O bond cleavage in [(P)Fe(III)-OOX]. The oxoiron(IV)-porphyrin cation radical formed as a primary oxidation product over the entire pH range can undergo a one- or two-electron reduction depending on the selected reaction conditions. The present study provides valuable information for the interpretation and improved understanding of results obtained in product-analysis experiments.

The mechanism of acetate-assisted transmetalation of tetrapyrroles was investigated in a model system consisting of chlorophyll a and copper(II) acetate in organic solvents by using a spectroscopic and kinetic approach. Surprisingly, acetate ions bind to the central Mg in chlorophyll much more strongly than do acetonitrile, methanol and even pyridine, one of the best ligands in chlorophyllic systems. This exceptionally strong non-symmetrical axial ligation of the central Mg by acetate causes its out-of-plane displacement and deformation of the tetrapyrrole ring, thus facilitating the interaction with an incoming CuII complex. This mechanism is controlled by a keto-enol tautomerism of the chlorophyll isocyclic ring. Additionally, depending on solvent, acetate activates the incoming metal ions. These new insights allow to suggest a mechanism for the acetate method of metal exchange in tetrapyrrolic macrocycles, which resembles biological insertion of metal ions into porphyrins. It also provides a guideline for the design of more efficient methods for the metalation of porphyrins and related macrocycles.

The reactions of a water-soluble iron(III)-porphyrin, [meso-tetrakis(sulfonatomesityl)porphyrinato]iron(III), [Fe(III)(tmps)] (1), with m-chloroperoxybenzoic acid (mCPBA), iodosylbenzene (PhIO), and H(2)O(2) at different pH values in aqueous methanol solutions at -35 degrees C have been studied by using stopped-flow UV/Vis spectroscopy. The nature of the porphyrin product resulting from the reactions with all three oxidants changed from the oxo-iron(IV)-porphyrin pi-cation radical [Fe(IV)(tmps(*+))(O)] (1(++)) at pH<5.5 to the oxo-iron(IV)-porphyrin [Fe(IV)(tmps)(O)] (1(+)) at pH>7.5, whereas a mixture of both species was formed in the intermediate pH range of 5.5-7.5. The observed reactivity pattern correlates with the E degrees' versus pH profile reported for 1, which reflects pH-dependent changes in the relative positions of E degrees'(Fe(IV)/Fe(III) ) and E degrees'(P(*+)/P) for metal- and porphyrin-centered oxidation, respectively. On this basis, the pH-dependent redox equilibria involving 1(++) and 1(+) are suggested to determine the nature of the final products that result from the oxidation of 1 at a given pH. The conclusions reached are extended to water-insoluble iron(III)-porphyrins on the basis of literature data concerning the electrochemical and catalytic properties of [Fe(III)(P)(X)] species in nonaqueous solvents. Implications for mechanistic studies on [Fe(P)]-catalyzed oxidation reactions are briefly addressed.

The water-soluble, non-mu-oxo dimer-forming porphyrin, [5,10,15,20-tetrakis-4'-t-butylphenyl-2',6'-bis-(N-methylene-(4''-t-butylpyridinium))porphyrinato]iron(III) octabromide, (P(8+))Fe(III), with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and 1H NMR spectroscopy and 17O NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of (P(8+))Fe(III) indicated a pKa1 value of 5.0 for coordinated water in (P(8+))Fe(III)(H2O)2. The monohydroxo-ligated (P(8+))Fe(III)(OH)(H2O) formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, k(ex) = 2.4 x 10(6) s(-1), significantly faster than water exchange on (P(8+))Fe(III)(H2O)2, viz. k(ex) = 5.5 x 10(4) s(-1) at 25 degrees C. The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, (P(8+))Fe(II)(NO+)(L) (L = H2O or OH-). The diaqua-ligated (P(8+))Fe(III)(H2O)2 binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other (P)Fe(III)(H2O)2 complexes. Coordination of NO to (P(8+))Fe(III)(OH)(H2O) at high pH follows an associative mode, as evidenced by negative deltaS(double dagger)(on) and deltaV(double dagger)(on) values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate (P(8+))Fe(III)(H2O)2 (k(on) = 15.1 x 10(3) M(-1) s(-1)) to (P(8+))Fe(III)(OH)(H2O) (k(on) = 1.56 x 10(3) M(-1) s(-1) at 25 degrees C) is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these (P)Fe(III) complexes as a function of pH is discussed on the basis of available experimental data.