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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.

Contradicts numerous reports in literature that NO binds to aquacobalamine (vitamin B12a) to produce a nitrosyl complex. Absence of reaction between aquacobalamin and NO; Presence of nitrite impurities in aqueous solutions of NO that react with aquacobalamin to produce the nitrite complex.