Mechanistic studies of nitric oxide reactions with water soluble iron(II), cobalt(II), and iron(III) porphyrin complexes in aqueous solutions: implications for biological activity.

The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298-318 K and the hydrostatic pressure range 0.1-250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetra-meso-(sulfonatomesityl)porphinate]. Large and positive DeltaS(double dagger) and DeltaV(double dagger) values were observed for NO binding to and release from iron(III) complexes Fe(III)(TPPS) and Fe(III)(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for Delta and Delta for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k(on) = 1.5 x 10(9) and 1.9 x 10(9) M(-1) s(-1) for Fe(II)(TPPS) and Co(II)(TPPS), respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the Fe(II) complexes (k(on) = 3.6 x 10(7) M(-1) s(-1) for Fe(II)(TPPS)) displays negative Delta and Delta values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from Fe(II)(TPPS)(NO) and Co(II)(TPPS)(NO) by trapping free NO gave k(off) values of 6.3 x 10(-4) s(-1) and 1.5 x 10(-4) s(-1). The respective M(II)(TPPS)(NO) formation constants calculated from k(on)/k(off) ratios were 2.4 x 10(12) and 1.3 x 10(13) M(-1), many orders of magnitude larger than that (1.1 x 10(3) M(-1)) for the reaction of Fe(III)(TPPS) with NO.     

HNO trapping and assisted decomposition of nitroxyl donors by ferric hemes

The role of nitric oxide (NO) as a signalling molecule in biological systems has been thoroughly studied in the last decades. More recently, there has been an increasing interest in the one-electron reduction product of NO, namely nitroxyl (HNO/NO⁻). Some studies suggest that nitroxyl can be produced by nitric oxide synthases under certain conditions, and that distinct pharmacological effects are observed for NO and nitroxyl donors. HNO is capable of react with heme proteins, thiols, molecular oxygen, NO and HNO itself. However, only recently the different reactivity patterns are being thoroughly understood. Heme model compounds offer the opportunity to study the reaction kinetics without the complexity arising from ligand interactions with the protein matrix. In this study we analyzed the reaction between the commonly used nitroxyl donors sodium trioxodinitrate and toluene sulfohydroxamic acid, with the ferric model compounds microperoxidase-11 (MP11) and the cationic metalloporphyrin [Fe^IIITEPyP]⁵⁺ (Tetrakis N-ethylpyridinium-2yl porphyne). Our results show that there are two alternative modes of reactivity for nitroxyl donors towards heme in aqueous solutions. The first one comprises the heme assisted decomposition of the donor, enhancing its decomposition rate more than 100-fold. In the second, the donor produces HNO which subsequently reacts with the porphyrin. The observed rate constants (of about 10⁵ M⁻¹ s⁻¹) are consistent with the estimated data for the HNO reaction with heme proteins, and may be controlled by the leaving water ligand. This rate constant probably represents an upper limit for the bimolecular rate constant of HNO towards these proteins.     

Water increases rates of epoxidation by Mn(III)porphyrins/imidazole/IO4(-) in CH2Cl2. Analogy with peroxidase and chlorite dismutase

Manganese(III)-meso-tetraphenylporphyrin [Mn(TPP)] and manganese(III)-meso-tetrakis(pentafluorophenyl)porphyrin [Mn(TPFPP)] catalyse the epoxidation of cyclooctene by IO(4)(-) in the presence of excess imidazoles, in both dry CH(2)Cl(2) and CH(2)Cl(2) saturated with H(2)O. The reaction rates of the electron deficient Mn(TPFPP) are a factor 24 less than those of Mn(TPP); however, the former increases 15-30 times in the presence of water, while those of Mn(TPP) do so by a factor of 2-3. The most striking catalytic enhancement caused by the addition of water was observed with 2-methylimidazole and Mn(TPFPP). As deprotonation of imidazoles may play a significant role in the presence of water, we found that manganese(III)-meso-tetrakis(phenyl-4-sulfonato)porphyrin [Mn(TPPS)] decreases the NH proton pK(a) of axially coordinated imidazole from 14.2 to 9.5. We conclude that the imidazole ligand is partially deprotonated in the presence of water. The latter enables the solvation of imidazolium ions that are formed simultaneously. The imidazolate form of the co-catalyst is a much stronger donor than the imidazole itself, providing electron density to Mn(III) and thus promoting oxygen transfer. The failure of N-methylimidazole to increase the reaction rates upon addition of water supports this hypothesis. A functionally related deprotonation has been shown to occur in horseradish peroxidase (J. S. de Ropp, V. Thanabal, G. N. La Mar, J. Am. Chem. Soc. 1985, 107, 8270-8272) and in chlorite dismutase (B. R. Goblirsch, B. R. Streit, J. L. Dubois, C. M. Wilmot, J. Biol. Inorg. Chem. 2010, 15, 879-888). Mn(III)porphyrins in combination with imidazoles and water constitute a functional biomimetic model of peroxidases.     

Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(iii) porphyrin: formation of the nitrosylated Mn(ii) porphyrin as an intermediate

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn(III)TTEG-2-PyP(5+), a synthetic water soluble Mn(iii) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of Mn(III)TTEG-2-PyP(5+), which is followed by a ligand-to-metal electron transfer to get Mn(II)TTEG-2-PyP(4+) and the free radical derived from HU (HU˙). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of Mn(II)TTEG-2-PyP(4+), is the reaction of Mn(II)TTEG-2-PyP(4+) with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO(2)(-) and the initial Mn(III)TTEG-2-PyP(5+). The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.     

Anion binding to a ferric porphyrin complexed with per-O-methylated beta-cyclodextrin in aqueous solution

5,10,15,20-Tetrakis(4-sulfonatophenyl)porphinato iron(III) (Fe(III)TPPS) forms a very stable 1:2 complex with heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMe-beta-CD), whose iron(III) center is located at a hydrophobic cleft formed by two face-to-face TMe-beta-CD molecules. Various inorganic anions (X(-)) such as F(-), Cl(-), Br(-), I(-), N(3)(-), and SCN(-) coordinate to Fe(III)TPPS(TMe-beta-CD)(2) to form five-coordinate high-spin Fe(III)TPPS(X)(TMe-beta-CD)(2), while no coordination occurs with ClO(4)(-), H(2)PO(4)(-), NO(3)(-), and HSO(4)(-). Except for F(-), none of the anions investigated coordinate to Fe(III)TPPS in the absence of TMe-beta-CD due to extensive hydration to the anions as well as to Fe(III)TPPS. The present system shows a high selectivity toward the N(3)(-) anion. The thermodynamics suggests that Lewis basicity, hydrophilicity, and shape of an X(-) anion are the main factors to determine the stability of the Fe(III)TPPS(X)(TMe-beta-CD)(2) complex.     

Nanowells on silica particles in water containing long-distance porphyrin heterodimers

Smooth and nonswelling spherical silica particles with a diameter of 100 nm and an aminopropyl coating are soluble in water at pH 11, coagulate quickly at pH 3, and redissolve at pH 9. Electron microscopy as well as visible spectra of covalently attached porphyrins indicate the aggregation state of the particles. Long-chain alpha,omega-dicarboxylic acids with a terminal oligoethyleneglycol (=OEG)-amide group were attached in a second self-assembly step to the remaining amine groups around the porphyrins. Form-stable 2-nm wells were thus obtained and were characterized by fluorescence quenching experiments using the bottom porphyrin as a target. The one-dimensional diffusion of fitting quencher molecules along the 2-nm pathway took several minutes. Porphyrins with a diameter above 2 nm could not enter the form-stable gaps at all. Added tyrosine stuck irreversibly to the walls of the nanowells and prevented the entrance of quencher molecules, the OEG-headgroups fixated 2,6-diaminoanthraquinone. A ring of methylammonium groups was then fixed at the walls of the wells at a distance of 5 or 10 A with respect to the bottom porphyrin. 2,6-Disulfonatoanthraquinone was attached only loosely to this ring, but the exactly fitting manganese(III) meso-(tetraphenyl-4-sulfonato)porphyrinate (Mn(III) TPPS) was tightly bound. Transient fluorescence experiments showed a fast decay time of 0.2 ns for the bottom porphyrin, when the Mn(III) TPPS was fixated at a distance of 5 A. Two different dyes have thus been immobilized at a defined subnanometer distance in an aqueous medium.     

A "push-pull" mechanism for heterolytic o-o bond cleavage in hydroperoxo manganese porphyrins

A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese(III) (Mn(III)TDMImP) is shown to react with H(2)O(2) to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H(2)O(2). The visible spectrum of a compound 0 type intermediate, assigned as Mn(III)(OH)(OOH)TDMImP, can be directly observed under saturating H(2)O(2) conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O-O heterolysis step was found to have a very small activation enthalpy (ΔH(≠) = 4.2 ± 0.2 kcal mol(-1)) and a large, negative activation entropy (ΔS(≠) = -36 ± 1 cal mol(-1) K(-1)). The O-O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 ± 12 s(-1). These observations indicate that the O-O bond in Mn(III)(OH)(OOH)TDMImP is cleaved via a concerted "push-pull" mechanism. In the transition state, the axial (proximal) (-)OH is partially deprotonated ("push"), while the terminal oxygen in (-)OOH is partially protonated ("pull") as a water molecule is released to the medium. This mechanism is reminiscent of O-O bond cleavage in heme enzymes, such as peroxidases and cytochrome P450, and similar to the fast, reversible O-Br bond breaking and forming reaction mediated by similar manganese porphyrins. The small enthalpy of activation suggests that this O-O bond cleavage could also be made reversible.     

Nitrite catalyzes reductive nitrosylation of the water-soluble Ferri-Heme model FeIII(TPPS) to FeII(TPPS)(NO)

Quantitative investigation of the reaction of the ferri-heme model compound Fe(III)(TPPS)(H(2)O)(2) (1) to give Fe(II)(TPPS)(NO) (2) (TPPS = tetra(4-sulfonato-phenyl)porphinato) in buffered aqueous solution demonstrates a slow pH-independent reductive nitrosylation pathway in the pH range 4-6. The rate of this reaction is subject to modest general base catalysis. In the course of this study, a surprising catalytic pathway whereby nitrite ion (NO(2)(-)) strongly catalyzes the reduction of 1 to 2 under reductive nitrosylation conditions was demonstrated.     

Studies on the effects of metal ions and counter anions on the aggregate behaviors of meso-tetrakis(p-sulfonatophenyl)porphyrin by absorption and fluorescence spectroscopy

The aggregation behaviors of meso-tetrakis(p-sulfonatophenyl)porphyrin (TPPS) in the function of metal ions and their counter anions (Cl(-), SO(4)(2-), and NO(3)(-)) were investigated by absorption, fluorescence spectroscopy and resonance scattering spectrum. It was shown that the TPPS J-aggregates could be effectively promoted by metal ions under lower ionic strength. Moreover, the prominent effects of counter ions (Cl(-), SO(4)(2-), and NO(3)(-)) on TPPS J- and/or H-aggregate formation at higher ionic strength were observed. These results suggested that the counter anions play a significant role in the formation of TPPS J- and/or H-aggregates and their conversion each other. Very interestingly, the absorption spectrum of metal ions investigated except for Co(2+) leaves a WINDOW from ca. 450 to 550nm centered at 490nm in which the absorption of Cu(2+) or Ni(2+) ions per se was very weak. The spectrum window might be really significant in avoiding possible spectrum interferences when porphyrins are chosen as spectrometric reagents for the determination of metal ions based on J-aggregation.     

Bridging nitrate groups in [Mn(4)O(3)(NO(3))(O(2)CMe)(3)(R(2)dbm)(3)] (R = H, Et) and [Mn(4)O(2)(NO(3))(O(2)CEt)(6)(bpy)(2)](ClO(4)): acidolysis routes to tetranuclear manganese carboxylate complexes

New synthesis procedures are described to tetranuclear manganese carboxylate complexes containing the [Mn(4)O(2)](8+) or [Mn(4)O(3)X](6+) (X(-) = MeCO(2)(-), F(-), Cl(-), Br(-), NO(3)(-)) core. These involve acidolysis reactions of [Mn(4)O(3)(O(2)CMe)(4)(dbm)(3)] (1; dbm is the anion of dibenzoylmethane) or [Mn(4)O(2)(O(2)CEt)(6)(dbm)(2)] (8) with HX (X(-) = F(-), Cl(-), Br(-), NO(3)(-)); high-yield routes to 1 and 8 are also described. The X(-) = NO(3)(-) complexes [Mn(4)O(3)(NO(3))(O(2)CR)(3)(R'(2)dbm)(3)] (R = Me, R' = H (6); R = Me, R' = Et (7); R = Et, R' = H (12)) represent the first synthesis of the [Mn(4)O(3)(NO(3))](6+) core, which contains an unusual eta(1):mu(3)-NO(3)(-) group. Treatment of known [Mn(4)O(2)(O(2)CEt)(7)(bpy)(2)](ClO(4)) with HNO(3) gives [Mn(4)O(2)(NO(3))(O(2)CEt)(6)(bpy)(2)](ClO(4)) (15) containing a eta(1):eta(1):mu-NO(3)(-) group bridging the two body Mn(III) ions of the [Mn(4)O(2)](8+) butterfly core. Complex 7 x 4CH(2)Cl(2) crystallizes in space group P2(1)2(1)2(1) with (at -168 degrees C) a = 21.110(3) A, b = 22.183(3) A, c = 15.958(2) A, Z = 4, and V = 7472.4(3) A(3). Complex 15 x (3)/(2)CH(2)Cl(2) crystallizes in space group P2(1)/c with (at -165 degrees C) a = 26.025(4) A, b = 13.488(2) A, c = 32.102(6) A, beta = 97.27(1) degrees, Z = 8, and V = 11178(5) A(3). Complex 7 contains a [Mn(4)(mu(3)-O)(3)(mu(3)-NO(3))](6+) core (3Mn(III), Mn(IV)) as seen for previous [Mn(4)O(3)X](6+) complexes. Complex 15 contains a butterfly [Mn(4)(mu(3)-O)(2)](8+) core. (1)H NMR spectra have been recorded for all complexes reported in this work and the various resonances assigned. All complexes retain their structural integrity on dissolution in chloroform and dichloromethane. Magnetic susceptibility (chi(M)) data were collected on 12 in the 5-300 K range in a 10.0 kG (1 T) field. Fitting of the data to the theoretical chi(M) vs T expression appropriate for a [Mn(4)O(3)X](6+) complex of C(3)(v)() symmetry gave J(34) = -23.9 cm(-)(1), J(33) = 4.9 cm(-)(1), and g = 1.98, where J(34) and J(33) refer to the Mn(III)Mn(IV) and Mn(III)Mn(III) pairwise exchange interactions, respectively. The ground state of the molecule is S = 9/2, as found previously for other [Mn(4)O(3)X](6+) complexes. This was confirmed by magnetization data collected at various fields and temperatures. Fitting of the data gave S = 9/2, D = -0.45 cm(-1), and g = 1.96, where D is the axial zero-field splitting parameter.     

Oxidation of alcohols with sodium periodate catalyzed by supported Mn(III) porphyrins

The mild and efficient oxidation of alcohols with sodium periodate catalyzed by manganese(III) tetrakis(p-sulfonatophenylporphyrinato) acetate, [Mn(TPPS)], supported on polyvinylpyridine, [Mn(TPPS)-PVP], and Amberlite IRA-400, [Mn (TPPS)-Ad IRA-400], at room temperature is reported. The catalysts used in this study showed high activity not only in the oxidation of benzylic and linear alcohols but also in the oxidation of secondary alcohols at room temperature. These catalysts can be reused several times without significant loss of their activity.     

Electroprecipitation of Ag(II)/Ag(III) tetraphenylsulfonate porphyrin and electrocatalytic behavior of the films

The electrochemical precipitation on glassy carbon and gold electrodes of Ag(II) tetraphenylsulfonate porphyrin (Ag(II)TPPS) from aqueous HClO₄ solutions, is reported. Electrochemical quartz crystal microbalance (EQCM) results indicate the possible formation of an Ag(II)–Ag(III) porphyrin dimer species. This species is oxidized and reduced in two consecutive steps: oxidation at +0.31 and +0.36 V (vs. SCE) and reduction at +0.11 and +0.07 V. The films show catalytic behavior toward O₂ reduction in 10⁻² M HClO₄ at relatively low potentials (E<−0.1 V) but catalyze NO reduction at relatively high-reduction potentials (E<0.4 V). The electrochemical results seem to indicate that the catalytic cycle in the case of NO involves formation of Ag(II)TPPS–Ag(II)TPPS(NO)⁺ and its electroreduction to regenerate Ag(II)TPPS–Ag(III)TPPS and NO-reduction products.     

Comparative study on the inclusion behavior between meso-tetrakis(4-N-ethylpyridiniurmyl)porphyrin and beta-cyclodextrin derivatives

5,10,15,20-Tetrakis(4-N-ethylpyridiniurmyl)porphyrin (TEPyP) formed 1:1 stoichiometry inclusion complexes with beta-cyclodextrin (beta-CD) and its derivatives including hydroxypropyl-beta-cyclodextrin (HP-beta-CD), sulfobutylether-beta-cyclodextrin (SBE-beta-CD) in basic aqueous solution. The supramolecular system was investigated by the methods of fluorescence, UV-vis absorption spectroscopy, nuclear magnetic resonance (NMR) technique. The inclusion ability of cyclodextrins exhibited remarkable difference for beta-CD, HP-beta-CD and SBE-beta-CD. Association constants as high as K=1.1 x 10(4) M(-1) in the case of HP-beta-CD/TEPyP and 2.0 x 10(5) M(-1) in the case of SBE-beta-CD/TEPyP complexes were determined, whereas a lower value (K=550 M(-1)) was given in the case of beta-CD/TEPyP. The results showed that hydrogen bonding and charge attraction play important roles in the processes of host-guest interaction. The interaction mechanism of inclusion processes could be explained by the analysis of NMR spectroscopy. The supramolecular assembly was formed. beta-CD and HP-beta-CD approached from the primary face of cavities of CDs.     

Oxygen atom transfer from nitrite mediated by Fe(III) porphyrins in aqueous solution

Aqueous solutions of the iron(III) porphyrin complex FeIII(TPPS) (1, TPPS = tetra(4-sulfonatophenyl)-porphyrinato) and nitrite ion react with various substrates S to generate the ferrous nitrosyl complex FeII(TPPS)(NO) (2) plus oxidized substrate. When S is a water-soluble sulfonated phosphine, the product is the resulting monoxide. When air is introduced to the product solutions, 2 is rapidly reoxidized to 1; however, even in the absence of air, there is a slow regeneration of the ferric species with concomitant production of nitrous oxide. Thus, in an anaerobic aqueous environment, FeIII(TPPS) catalyzes oxygen atom transfer from nitrite ion to substrates with the eventual formation of N2O.     

Laser flash photolysis generation and kinetic studies of porphyrin-manganese-oxo intermediates. Rate constants for oxidations effected by porphyrin-Mn(V)-oxo species and apparent disproportionation equilibrium constants for porphyrin-Mn(IV)-oxo species

Porphyrin-manganese(V)-oxo and porphyrin-manganese(IV)-oxo species were produced in organic solvents by laser flash photolysis (LFP) of the corresponding porphyrin-manganese(III) perchlorate and chlorate complexes, respectively, permitting direct kinetic studies. The porphyrin systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP), and 5,10,15,20-tetrakis(4-methylpyridinium)porphyrin (TMPyP). The order of reactivity for (porphyrin)Mn(V)(O) derivatives in self-decay reactions in acetonitrile and in oxidations of substrates was (TPFPP) > (TMPyP) > (TPP). Representative rate constants for reaction of (TPFPP)Mn(V)(O) in acetonitrile are k = 6.1 x 10(5) M(-1) s(-1) for cis-stilbene and k = 1.4 x 10(5) M(-1) s(-1) for diphenylmethane, and the kinetic isotope effect in oxidation of ethylbenzene and ethylbenzene-d(10) is k(H)/k(D) = 2.3. Competitive oxidation reactions conducted under catalytic conditions display approximately the same relative rate constants as were found in the LFP studies of (porphyrin)Mn(V)(O) derivatives. The apparent rate constants for reactions of (porphyrin)Mn(IV)(O) species show inverted reactivity order with (TPFPP) < (TMPyP) < (TPP) in reactions with cis-stilbene, triphenylamine, and triphenylphosphine. The inverted reactivity results because (porphyrin)Mn(IV)(O) disproportionates to (porphyrin)Mn(III)X and (porphyrin)Mn(V)(O), which is the primary oxidant, and the equilibrium constants for disproportionation of (porphyrin)Mn(IV)(O) are in the order (TPFPP) < (TMPyP) < (TPP). The fast comproportionation reaction of (TPFPP)Mn(V)(O) with (TPFPP)Mn(III)Cl to give (TPFPP)Mn(IV)(O) (k = 5 x 10(8) M(-1) s(-1)) and disproportionation reaction of (TPP)Mn(IV)(O) to give (TPP)Mn(V)(O) and (TPP)Mn(III)X (k approximately 2.5 x 10(9) M(-1) s(-1)) were observed. The relative populations of (porphyrin)Mn(V)(O) and (porphyrin)Mn(IV)(O) were determined from the ratios of observed rate constants for self-decay reactions in acetonitrile and oxidation reactions of cis-stilbene by the two oxo derivatives, and apparent disproportionation equilibrium constants for the three systems in acetonitrile were estimated. A model for oxidations under catalytic conditions is presented.     

Solid phase extraction and spectrophotometric determination of trace amounts of thiocyanate in real samples

A simple, selective and rapid method for solid phase extraction and spectrophotometric determination of thiocyanate using a manganese (III) tetrakis (p-sulfonatophenyl) porphyrin, [Mn (TPPS) OAC] bound to Amberlite IR-400 has been developed. The influence of pH, amount of solid phase, sample matrix, type and amount of eluting agent and flow rates i.e. variables affecting the efficiency of the extraction system were evaluated and conditions of the sample, eluting solution and active phase were optimized. The maximal capacity was found to be as 1.16 microg mL(-1) for 1200 mL. Thiocyanate ions can be eluted quantitatively with 8 mL 0.3 M ferric chloride. The enrichment factor was 150. The linear range of the determination is between 0.4-2.0 microg mL(-1) for preconcentration method with a limit of detection of 2.8 ng mL(-1). The method has been successfully applied for determination of trace amounts of thiocyanate in tap water, saliva sample and a synthetic mixture.     

Fast nitroxyl trapping by ferric porphyrins

Iron(II) porphyrin nitrosyl complexes are obtained in high yields from the reaction of iron(III) porphyrins with the nitroxyl donors sodium trioxodinitrate and toluensulfohydroxamic acid. The reaction was found to proceed both in organic solvents and in aqueous media from iron(III) (meso-tetraphenyl) porphyrinate ([FeIII(TPP)]+) and iron(III) meso-tetrakis (4-sulfonatophenyl) porphyrinate ([FeIII(TPPS)]3-) or iron(III) protoporphyrin IX, respectively. The kinetic rate constant for the reaction of ([FeIII(TPPS)]3-) with sodium trioxodinitrate (kon) was estimated to be 1.00 +/- 0.04 x 107 M-1 s-1. As well as resulting in a versatile method for obtaining ferrous nitrosyl porphyrins, the reaction points at ferric porphyrins as efficient nitroxyl traps and provides a tool to model nitroxyl reactivity toward hemeproteins.     

First Synthesis of a Binuclear [Mn(II)(bipy)-Fe(III)(porphyrin)] Complex: Spectroscopic Characterization and First Evidence of Reversible Formation of Manganese(III) as Manganese Peroxidase

A [(P)Fe(III)-Mn(II)] bimetallic complex, mimicking the active site of manganese peroxidase, has been synthesized. A modified highly fluorinated porphyrin, 5,10,15-tris(pentafluorophenyl)-20-(o-aminophenyl)porphyrin, has been used to introduce, through a short spacer linked to the amino function, a manganese auxiliary ligand, 6-aminomethyl-2,2'-bipyridine. Two successive metalations by FeCl(2) and MnCl(2) afforded the [(P)Fe(III)-Mn(II)] bimetallic complex that has been characterized by elemental analysis and FAB(+) mass spectrometry. X-band EPR spectroscopy and magnetic susceptibility measurements were in agreement with two high spin Fe(III) and Mn(II) centers without magnetic exchange interaction. Moreover, there is no higher intermolecular association through &mgr;-chloro bridging as observed by EPR with a simpler chloromanganese complex, Mn(bipy)(2)Cl(2), at high concentration. Addition of pentafluoroiodosobenzene in methanol at 0 degrees C led to the progressive and complete disappearance of the EPR Mn(II) signals, that were recovered after addition of a phenol. This result is consistent with Mn(III) formation. This production of Mn(III) requires the presence of the iron porphyrin and is proposed to occur through the intermediate formation of a Fe(IV) dimethoxide species which can be related to the oxidation of Mn(II) catalyzed by manganese peroxidase compound II.     

Raman and infrared spectral study of meso-sulfonatophenyl substituted porphyrins (TPPSn,n = 1, 2A, 2O, 3, 4)

Raman and IR spectra of the free base p-sulfonatophenyl and phenyl meso-substituted porphyrins [5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4); 5,10,15-tris(4-sulfonatophenyl)-20-phenyl-porphyrin (TPPS3); 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS2A); 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS2O); and 5-(4-sulfonatophenyl)-10, 15,20-trisphenylporphyrin (TPPS1)] and their N-diprotonated derivatives (porphyrin diacids) were studied. The Raman spectra of the deuterated analogues of these porphyrins, in which the central hydrogen atoms were substituted with deuterium, were also measured. The observed vibrational bands were assigned on the basis of the deuteration shifts and compared with the structural analogues of these compounds. In IR spectra of the free-base porphyrins, the p-sulfonation of phenyl groups results in evident alteration for the phenyl modes and the porphyrin skeleton modes that are strongly coupled with phenyl vibrations. While the p-sulfonation of phenyl groups causes only slight changes for the high-frequency Raman bands (> 850 cm(-1)), dramatic shifts and band splitting were observed in the low-frequency region (< 500 cm(-1)) of Raman spectra. The observed differences of low-frequency Raman spectra were attributed to the alteration of the structure of the porphyrin ring, especially the CalphaCmCalpha bond-angles, by different meso-sulfonatophenyl substitutions. In addition, different packing style of TPPSn molecules in the aggregates is also responsible for the alteration of the vibrational spectra of the aggregated TPPSn.     

Monomeric and Trimeric Manganese(III) Complexes of 2-Hydroxy-5,10,15,20-tetraphenylporphyrin. Synthesis and Characterization

The hydrolysis of the monomeric five-coordinate (2-BzO-TPP)Mn(III)Cl complex has been investigated.(1) Evidence for the formation of the cyclic trimeric complex [(2-O-TPP)Mn(III)](3) is presented. The (1)H NMR spectroscopic evidence indicates that the trimeric manganese(III) complex has a head-to-tail cyclic trimeric structure with the pyrrolic alkoxide groups forming bridges from one macrocycle to the manganese(III) ion in the adjacent macrocycle PMn-O-PMn-O-PMn-O. The three manganese(III) porphyrin subunits are not equivalent. The characteristic upfield shift of the 3-H pyrrole resonance (-111.5 ppm at 291 K) was determined and considered as the diagnostic feature for the high-spin d(4) manganese(III)-pyrrole alkoxide coordination. The strong upfield shift of the 3-H resonance has been accounted for by the donation of the electron density from the filled orbital of the 2-O atom on the half-occupied d(z)()()2 orbital of the external manganese(III) ion. The other pyrrole resonances produce the complex multiplet at the typical -5 to -40 ppm region. The (1)H NMR spectra of the series of monomeric 2-substituted manganese(III) 5,10,15,20-tetraphenylporphyrin complexes (2-X-TPP)Mn(III)Cl have been obtained and analyzed. The pattern of the assigned seven pyrrole resonances reflects the asymmetry imposed by 2-substitution and has been used as a (1)H NMR spectroscopic probe to map the spin density distribution. The electronic effect is strongly localized at the beta-substituted pyrrole. The upfield shift of the 3-H resonance increases in the order (2-NO(2)-TPP)Mn(III)Cl < (2-BzO-TPP)Mn(III)Cl < (2-OCH(3)-TPP)Mn(III)Cl < (2-OH-TPP)Mn(III)Cl < (2-NH(2)-TPP)Mn(III)Cl < [(2-O-TPP)Mn(III)(OH)](-) following the increasing electron-donating properties of the beta-substituent.