Mechanistic aspects of the reactions of nitric oxide with transition-metal complexes

pH Controls the Rate and Mechanism of Nitrosylation of Water-Soluble FeIII Porphyrin Complexes

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)Fe(III)(OH) species formed by deprotonation of coordinated water in diaqua-ligated (P)Fe(III)(H(2)O)(2). The observed significant decrease in the rate of NO binding to (P)Fe(OH) as compared to that of (P)Fe(H(2)O)(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 Fe(III) 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)Fe(III)(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)Fe(III).

Dinitrosyl iron complexes (DNICs) [L(2)Fe(NO)2]- (L = thiolate): interconversion among {Fe(NO)2}9 DNICs, {Fe(NO)2}10 DNICs, and [2Fe-2S] clusters, and the critical role of the thiolate ligands in regulating NO release of DNICs

Dinitrosyl iron complex [(-SC(7)H(4)SN)(2)Fe(NO)(2)](-) (1) was prepared by reaction of [S(5)Fe(NO)(2)](-) and bis(2-benzothiozolyl) disulfide. In synthesis of the analogous dinitrosyl iron compounds (DNICs), the stronger electron-donating thiolates [RS](-) (R = C(6)H(4)-o-NHCOCH(3), C(4)H(3)S, C(6)H(4)NH(2), Ph), compared to [-SC(7)H(4)SN](-) of complex 1, trigger thiolate-ligand substitution to yield [(-SC(6)H(4)-o-NHCOCH(3))(2)Fe(NO)(2)](-) (2), [(-SC(4)H(3)S)(2)Fe(NO)(2)](-) (3), and [(SPh)(2)Fe(NO)(2)](-) (4), respectively. At 298 K, complexes 2 and 3 exhibit a well-resolved five-line EPR signal at g = 2.038 and 2.027, respectively, the characteristic g value of DNICs. The magnetic susceptibility fit indicates that the resonance hybrid of {Fe(+)((*)NO)(2)}(9) and {Fe(-)((+)NO)(2)}(9) in 2 is dynamic by temperature. The IR nu(NO) stretching frequencies (ranging from (1766, 1716) to (1737, 1693) cm(-)(1) (THF)) of complexes 1-4 signal the entire window of possible electronic configurations for such stable and isolable {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-). The NO-releasing ability of {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) is finely tuned by the coordinated thiolate ligands. The less electron-donating thiolate ligands coordinated to {Fe(NO)(2)}(9) motif act as better NO-donor DNICs in the presence of NO-trapping agent [Fe(S,S-C(6)H(4))(2)](2)(2-). Interconversion between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(10) [(Ph(3)P)(2)Fe(NO)(2)] was verified in the reaction of (a) [(RS)(2)Fe(NO)(2)](-), 10 equiv of PPh(3) and sodium-biphenyl, and (b) 2 equiv of thiol, [RS](-), and [(Ph(3)P)(2)Fe(NO)(2)], respectively. The biomimetic reaction cycle, transformation between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(9) [(R'S)(2)Fe(NO)(2)](-), reversible interconversion of {Fe(NO)(2)}(9) and {Fe(NO)(2)}(10) DNICs, and degradation/reassembly of [2Fe-2S] clusters may decipher and predict the biological cycle of interconversion of {Fe(NO)(2)}(9) DNICs, {Fe(NO)(2)}(10) DNICs, and the [Fe-S] clusters in proteins.

Nitrosylated Iron−Thiolate−Sulfinate Complexes with {Fe(NO)}6/7 Electronic Cores: Relevance to the Transformation between the Active and Inactive NO-Bound Forms of Iron-Containing Nitrile Hydratases

The five-coordinated iron-thiolate nitrosyl complexes [(NO)Fe(S,S-C6H3R)2]- (R = H (1), m-CH3 (2)), [(NO)Fe(S,S-C6H2-3,6-Cl2)2]- (3), [(NO)Fe(S,S-C6H3R)2]2- (R = H (10), m-CH3 (11)), and [(NO)Fe(S,S-C6H2-3,6-Cl2)2]2- (12) have been isolated and structurally characterized. Sulfur oxygenation of iron-thiolate nitrosyl complexes 1-3 containing the {Fe(NO)}6 core was triggered by O2 to yield the S-bonded monosulfinate iron species [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]- (R = H (4), m-CH3 (5)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2(2-) (6), respectively. In contrast, attack of O2 on the {Fe(NO)}7 complex 10 led to the formation of complex 1 accompanied by the minor products, [Fe(S,S-C6H4)2]2(2-) and [NO3]- (yield 9%). Reduction of complexes 4-6 by [EtS]- in CH3CN-THF yielded [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]2- (R = H (7), m-CH3 (8)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2- (9) along with (EtS)2 identified by 1H NMR. Compared to complex 10, complexes 7-9 with the less electron-donating sulfinate ligand coordinated to the {Fe(NO)}7 core were oxidized by O2 to yield complexes 4-6. Obviously, the electronic perturbation of the {Fe(NO)}7 core caused by the coordinated sulfinate in complexes 7-9 may serve to regulate the reactivity of complexes 7-9 toward O2. The iron-sulfinate nitrosyl species with the {Fe(NO)}6/7 core exhibit the photolabilization of sulfur-bound [O] moiety. Complexes 1-4-7-10 (or 2-5-8-11 and 3-6-9-12) are interconvertible under sulfur oxygenation, redox processes, and photolysis, respectively.

Thiolate coordination to Fe(II)–porphyrin NO centers

The interaction of the Fe(II)-porphyrin NO model complex [Fe(TPP)(NO)] (1, TPP=tetraphenylporphyrin) with thiophenolate ligands and tetrahydrothiophene is explored both computationally and experimentally. Complex 1 is reacted with substituted thiophenolates and the obtained six-coordinate adducts of type [Fe(TPP)(SR)(NO)](-) are investigated in solution using electron paramagnetic resonance (EPR) spectroscopy. From the obtained g values and (14)N hyperfine pattern of the NO ligand it is concluded that the interaction of the thiophenolates with the Fe(II) center is weak in comparison to the corresponding 1-methylimidazole adduct. The strength of the Fe-S bond is increased when alkylthiolates are used as evidenced by comparison with the published EPR spectra of ferrous NO adducts in cytochromes P450 and P450nor, which have an axial cysteinate ligand. These results are further evaluated by density functional (DFT) calculations. The six-coordinate model complex [Fe(P)(SMe)(NO)](-) (1-SMe; P=porphine ligand used for the calculations) has an interesting electronic structure where NO acts as a medium strong sigma donor and pi acceptor ligand. Compared to the N-donor adducts with 1-methylimidazole (1-MeIm), etc., donation from the pi(h)( *) orbital of NO to Fe(II) is reduced due to the stronger trans effect of the alkylthiolate ligand. This is reflected by the predicted longer Fe-NO bond length and smaller Fe-NO force constant for 1-SMe compared to the 1-MeIm adduct. Therefore, the Fe(II)-porphyrin NO adducts with trans alkylthiolate coordination have to be described as Fe(II)-NO(radical) systems. The N-O stretching frequency of these complexes is predicted below 1600cm(-1) in agreement with the available experimental data. In addition, 1-SMe has a unique spin density distribution where Fe has a negative spin density of -0.26 from the calculations. The implications of this unusual electronic structure for the reactivity of the Fe(II)-NO alkylthiolate adducts as they occur in cytochrome P450nor are discussed.

Mechanistic Studies on the Binding of Nitric Oxide to a Synthetic Heme−Thiolate Complex Relevant to Cytochrome P450

The synthetic heme-thiolate complex (SR) in methanol binds nitric oxide (k(on) = (2.7 +/- 0.2) x10(6) M(-)(1) s(-)(1) at 25 degrees C) to form SR(NO). The binding of NO to the SR complex in a noncoordinating solvent, such as toluene, was found to be almost 3 orders of magnitude faster than that in methanol. The activation parameters DeltaH(), DeltaS(), and DeltaV() for the formation of SR(NO) in methanol are consistent with the operation of a limiting dissociative mechanism, dominated by dissociation of methanol in SR(MeOH). In the presence of an excess of NO, the formation of SR(NO) is followed by subsequent slower reactions. The substantially negative activation entropy and activation volume values found for the second observed reaction step support an associative mechanism which involves attack of a second NO molecule on the thiolate ligand in the initially formed SR(NO) complex. The following slower reactions are strongly accelerated by a large excess of NO or by the presence of NO(2)(-) in the SR/NO reaction mixture. They can be accounted for in terms of dynamic equilibria between higher nitrogen oxides (NO(x)()) and reactive SR species, which lead to the formation of a nitrosyl-nitrite complex of SR(Fe(II)) as the final product. This finding is clearly supported by laser flash photolysis studies on the SR/NO reaction mixture, which do not reveal simple NO photolabilization from SR(Fe(III))(NO), but rather involve the generation of at least three photoinduced intermediates decaying with different rate constants to the starting material. The species formed along the proposed reaction pathways were characterized by FTIR and EPR spectroscopy. The results are discussed in terms of their relevance for the biological function of cytochrome P450 enzymes and in context of results for the reaction of NO with imidazole- and thiolate-ligated iron(III) hemoproteins.

Spin Density Distribution in Five- and Six-Coordinate Iron(II)−Porphyrin NO Complexes Evidenced by Magnetic Circular Dichroism Spectroscopy

Using magnetic circular dichroism (MCD) spectroscopy together with DFT calculations, the spin density distributions in five-coordinate [Fe(TPP)(NO)] (I) and six-coordinate [Fe(TPP)(MI)(NO)] (II, MI = 1-methylimidazole) are defined. In the five-coordinate complex, a strong Fe-NO sigma bond between pi(*)(h) and d(z)(2) is present that leads to a large transfer of spin density from the NO ligand to Fe(II) corresponding to an electronic structure with noticeable Fe(I)-NO(+) character. Consequently, the MCD spectrum is dominated by paramagnetic C-term contributions. On coordination of the sixth ligand, the spin density is pushed back from the iron toward the NO ligand, resulting in an Fe(II)-NO(radical) type of electronic structure. This is reflected by the fact that the MCD spectrum is dominated by diamagnetic contributions.

IrII(ethene): Metal or Carbon Radical?

One-electron oxidation of [(Me(n)tpa)Ir(I)(ethene)]+ complexes (Me(3)tpa = N,N,N-tri(6-methyl-2-pyridylmethyl)amine; Me(2)tpa = N-(2-pyridylmethyl)-N,N,-di[(6-methyl-2-pyridyl)methyl]-amine) results in relatively stable, five-coordinate Ir(II)-olefin species [(Me(n)tpa)Ir(II)(ethene)](2+) (1(2+): n = 3; 2(2+): n = 2). These contain a "vacant site" at iridium and a "non-innocent" ethene fragment, allowing radical type addition reactions at both the metal and the ethene ligand. The balance between metal- and ligand-centered radical behavior is influenced by the donor capacity of the solvent. In weakly coordinating solvents, 1(2+) and 2(2+) behave as moderately reactive metallo-radicals. Radical coupling of 1(2+) with NO in acetone occurs at the metal, resulting in dissociation of ethene and formation of the stable nitrosyl complex [(Me(3)tpa)Ir(NO)](2+) (6(2+)). In the coordinating solvent MeCN, 1(2+) generates more reactive radicals; [(Me(3)tpa)Ir(MeCN)(ethene)](2+) (9(2+)) by MeCN coordination, and [(Me(3)tpa)Ir(II)(MeCN)](2+) (10(2+)) by substitution of MeCN for ethene. Complex 10(2+) is a metallo-radical, like 1(2+) but more reactive. DFT calculations indicate that 9(2+) is intermediate between the slipped-olefin Ir(II)(CH(2)=CH(2)) and ethyl radical Ir(III)-CH(2)-CH(2). resonance structures, of which the latter prevails. The ethyl radical character of 9(2+) allows radical type addition reactions at the ethene ligand. Complex 2(2+) behaves similarly in MeCN. In the absence of further reagents, 1(2+) and 2(2+) convert to the ethylene bridged species [(Me(n)tpa)(MeCN)Ir(III)(mu(2)-C(2)H(4))Ir(III)(MeCN)(Me(3)tpa)](4+) (n = 3: 3(4+); n = 2: 4(4+)) in MeCN. In the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxo), formation of 3(4+) from 1(2+) in MeCN is completely suppressed and only [(Me(3)tpa)Ir(III)(TEMPO(-))(MeCN)](2+) (7(2+)) is formed. This is thought to proceed via radical coupling of TEMPO at the metal center of 10(2+). In the presence of water, hydrolysis of the coordinated acetonitrile fragment of 7(2+) results in the acetamido complex [(Me(3)tpa)Ir(III)(NHC(O)CH(3)))(TEMPOH)](2+) (8(2+)).

Photochemical release of nitric oxide from a regenerable, sol-gel encapsulated Ru–salen–nitrosyl complex

Light activation leads to release of NO from a silicate sol-gel material SG-RuNO prepared from the ruthenium complex, [Ru(salen)(OH2)(NO)]+ (salen = N,N'-bis-(salicylidene)ethyl-enediaminato); after photochemical NO photolabilization, SG-RuNO can be regenerated from the spent material via the subsequent reaction with aqueous nitrite.

Nitric Oxide Is a Signaling Molecule that Regulates Gene Expression

Nitric oxide (NO) is a dynamic and bioreactive molecule that can both participate in and inhibit the genesis of disease. Its ability to have an impact on a wide range of physiological events stems from its capacity to reversibly alter the expression of specific genes and the activities of a wide range of proteins and signaling pathways. Yet, NO* remains an enigmatic molecule. Recently developed technologies, including gene-chips, two-dimensional electrophoresis, RNA interference, matrix-assisted laser desorption ionization (MALDI)-TOF (time-of-flight) mass spectrometry, and protein arrays will allow us to better understand how NO* and associated reactive nitrogen species (RNS) regulate both physiology and disease states, toward the development of treatments using NO* synthase inhibitors or NO* donors.

Femtomolar sensitivity of a NO sensor from Clostridium botulinum

Nitric oxide (NO) is extremely toxic to Clostridium botulinum, but its molecular targets are unknown. Here, we identify a heme protein sensor (SONO) that displays femtomolar affinity for NO. The crystal structure of the SONO heme domain reveals a previously undescribed fold and a strategically placed tyrosine residue that modulates heme-nitrosyl coordination. Furthermore, the domain architecture of a SONO ortholog cloned from Chlamydomonas reinhardtii indicates that NO signaling through cyclic guanosine monophosphate arose before the origin of multicellular eukaryotes. Our findings have broad implications for understanding bacterial responses to NO, as well as for the activation of mammalian NO-sensitive guanylyl cyclase.

Iron catalyzed conversion of NO into nitrosonium (NO+) and nitroxyl (HNO/NO−) species

The conversion of NO into its congeners, nitrosonium (NO+) and nitroxyl (HNO/NO-) species, has important consequences in NO metabolism. Dinitrosyl iron complex (DNIC) combined with thiol ligands was shown to catalyze the conversion of NO into NO+, resulting in the synthesis of S-nitrosothiols (RSNO) both in vitro and in vivo. The formation mechanism of DNIC was proposed to involve the intermediate release of nitroxyl. Since the detection of hydroxylamine (as the product of a rapid reaction of HNO/NO- with thiols) is taken as the evidence for nitroxyl generation, we examined the formation of hydroxylamine, RSNO, and nitrite (the product of a rapid reaction of NO+ with water) in neutral solutions containing iron ions and thiols exposed to NO under anaerobic conditions. Hydroxylamine was detected in NO treated solutions of iron ions in the presence of cysteine, but not glutathione (GSH). The addition of urate, a major "free" iron-binding agent in humans, to solutions of GSH and iron ions, and the subsequent treatment of these solutions with NO increased the synthesis of GSNO and resulted in the formation of hydroxylamine. This caused a loss of urate and yielded a novel nitrosative/nitration product. GSH attenuated the urate decomposition to such a degree that it could be reflected as the function of GSH:urate. Results described here contribute to the understanding of the role of iron ions in catalyzing the conversion of NO into HNO/NO- and point to the role of uric acid not previously described.

The chemistry of nitrosative stress induced by nitric oxide and reactive nitrogen oxide species. Putting perspective on stressful biological situations

This review addresses many of the chemical aspects of nitrosative stress mediated by N2O3. From a cellular perspective, N2O3 and the resulting reactive nitrogen oxide species target specific motifs such as thiols, lysine active sites, and zinc fingers and is dependant upon both the rates of production as well as consumption of NO and must be taken into account in order to access the nitrosative environment. Since production and consumption are integral parts of N2O3 generation, we predict that nitrosative stress occurs under specific conditions, such as chronic inflammation. In contrast to conditions of stress, nitrosative chemistry may also provide cellular protection through the regulation of critical signaling pathways. Therefore, a careful evaluation of the chemistry of nitrosation based upon specific experimental conditions may provide a better understanding of how the subtle balance between oxidative and nitrosative stress may be involved in the etiology and control of various disease processes.

Structural, spectroscopic, and computational study of an octahedral, non-heme [Fe-NO](6-8) Series: [Fe(NO)(cyclam-ac)]2+/+/0

The reaction of low-spin [FeIICl(cyclam-ac)] with NO in CH3CN yields the octahedral, non-heme (nitrosyl)iron complex [Fe(NO)(cyclam-ac)](PF6) (2), S = 1/2, which can be one-electron oxidized with (tris(4-bromophenyl)ammoniumyl)hexachloroantimonate to the complex [Fe(NO)(cyclam-ac)](PF6)2 (1'), S = 0, or complex [Fe(NO)(cyclam-ac)]Cl(ClO4).H2O (1); (cyclam-ac)- represents the pentadentate monoanion of 1,4,8,11-tetraazacyclotetradecane-1-acetic acid. Similarly, in CH3CN solution 2 can be electrochemically or chemically reduced by one-electron to the neutral complex [Fe(NO)(cyclam-ac)]0 (3), S = 0. 1-3 represent members of the [FeNO]6-8 series, respectively. The electronic structure of these three species have been spectroscopically elucidated by EPR, UV-vis, Mössbauer, and IR spectroscopy. The crystal structures of 1 and 2 have been determined by X-ray crystallography. In addition, detailed density functional calculations have been performed for all three species taking into account the possibility that 3 is actually a protonated HNO or NOH species. For all complexes structures, energetics, IR parameters, and Mössbauer parameters as well as EPR parameters (g-values and hyperfine couplings) have been calculated using state-of-the art DFT methods which are compared to experiment. The results establish unequivocally that 3 is indeed the elusive [FeNO]8 species. Furthermore a detailed picture of the bonding in the low-spin non-heme iron nitrosyl series [FeNO]6, [FeNO]7, and [FeNO]8 has been developed. This allows a description of 1 as a low-spin ferrous complex containing an N-coordinated NO+, whereas 2 is a low-spin ferrous species with a NO* ligand and 3 is a low-spin ferrous complex with a coordinated NO-. On the basis of this description, all spectroscopic and geometric observables find a satisfactory interpretation.

Role of myoglobin as a scavenger of cellular NO in myocardium

Recent studies have detected a (1)H nuclear magnetic resonance (NMR) reporter signal of metmyoglobin (metMb) during bradykinin stimulation of an isolated mouse heart. The observation has led to the hypothesis that Mb reacts with cellular nitric oxide (NO). However, the hypothesis depends on an unequivocal detection of metMb signals in vivo. In solution, nitrite oxidization of Mb produces a characteristic set of paramagnetically shifted (1)H NMR signals. In the upfield spectral region, MbO(2) and MbCO exhibit the gammaCH(3) Val E11 signals at -2.8 and -2.4 ppm, respectively. In the same spectral region, nitrite oxidation of Mb produces a set of signals at -3.7 and -4.7 ppm at 35 degrees C. Previous studies have confirmed the visibility of metMb signals in perfused rat myocardium. With bradykinin infusion, perfusion pressure and rate-pressure product decrease, consistent with endogenous NO formation. However, neither myocardial O(2) consumption nor high-energy phosphate levels, as reflected in the (31)P NMR signals, show any significant change. Bradykinin still triggers a similar physiological response even in the presence of CO that is sufficient to inhibit 86% Mb. In all cases, the (1)H NMR spectra from perfused rat myocardium reveal no metMb signals. The results suggest that bradykinin-induced NO does not interact significantly with cellular Mb to produce an NMR-detectable quantity of metMb in the perfused rat myocardium. As a consequence, the experiments cannot confirm the intriguing proposal that Mb acts as a cellular NO scavenger.

Iron nitrosyls of a pentadentate ligand containing a single carboxamide group: syntheses, structures, electronic properties, and photolability of NO

Three iron complexes of a pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy(3)H, H is the dissociable amide proton) have been synthesized. All three species, namely, two nitrosyls [(PaPy(3))Fe(NO)](ClO(4))(2) (2) and [(PaPy(3))Fe(NO)](ClO(4)) (3) and one nitro complex [(PaPy(3))Fe(NO(2))](ClO(4)) (4), have been structurally characterized. These complexes provide the opportunity to compare the structural and spectral properties of a set of isostructural [Fe-NO](6,7) complexes (2 and 3, respectively) and an analogous genuine Fe(III) complex with an "innocent" sixth ligand ([(PaPy(3))Fe(NO(2))](ClO(4)), 4). The most striking difference in the structural features of 2 and 3 is the Fe-N-O angle (Fe-N-O = 173.1(2) degrees in the case of 2 and 141.29(15) degrees in the case of 3). The clean (1)H NMR spectrum of 2 in CD(3)CN reveals its S = 0 ground state and confirms its [Fe-NO](6) configuration. The binding of NO at the non-heme iron center in 2 is completely reversible and the bound NO is photolabile. Mössbauer data, electron paramagnetic resonance signal at g approximately 2.00, and variable temperature magnetic susceptibility measurements indicate the S = (1)/(2) spin state of the [Fe-NO](7) complex 3. Analysis of the spectroscopic data suggests Fe(II)-NO(+) and Fe(II)-NO(*) formulations for 2 and 3, respectively. The bound NO in 3 does not show any photolability. However, in MeCN solution, it reacts rapidly with dioxygen to afford the nitro complex 4, which has also been synthesized independently from [(PaPy(3))Fe(MeCN)](2+) and NO(2)(-). Nucleophilic attack of hydroxide ion to the N atom of the NO ligand in 2 in MeCN in the dark gives rise to 4 in high yield.

A conserved Val to Ile switch near the heme pocket of animal and bacterial nitric-oxide synthases helps determine their distinct catalytic profiles

Nitric oxide (NO) release from nitric oxide synthases (NOSs) is largely dependent on the dissociation of an enzyme ferric heme-NO product complex (Fe(III)NO). Although the NOS-like protein from Bacillus subtilis (bsNOS) generates Fe(III)NO from the reaction intermediate N-hydroxy-l-arginine (NOHA), its NO dissociation is about 20-fold slower than in mammalian NOSs. Crystal structures suggest that a conserved Val to Ile switch near the heme pocket of bsNOS might determine its kinetic profile. To test this we generated complementary mutations in the mouse inducible NOS oxygenase domain (iNOSoxy, V346I) and in bsNOS (I224V) and characterized the kinetics and extent of their NO synthesis from NOHA and their NO-binding kinetics. The mutations did not greatly alter binding of Arg, (6R)-tetrahydrobiopterin, or alter the electronic properties of the heme or various heme-ligand complexes. Stopped-flow spectroscopy was used to study heme transitions during single turnover NOHA reactions. I224V bsNOS displayed three heme transitions involving four species as typically occurs in wild-type NOS, the beginning ferrous enzyme, a ferrous-dioxy (Fe(II)O(2)) intermediate, Fe(III)NO, and an ending ferric enzyme. The rate of each transition was increased relative to wild-type bsNOS, with Fe(III)NO dissociation being 3.6 times faster. In V346I iNOSoxy we consecutively observed the beginning ferrous, Fe(II)O(2), a mixture of Fe(III)NO and ferric heme species, and ending ferric enzyme. The rate of each transition was decreased relative to wild-type iNOSoxy, with the Fe(III)NO dissociation being 3 times slower. An independent measure of NO binding kinetics confirmed that V346I iNOSoxy has slower NO binding and dissociation than wild-type. Citrulline production by both mutants was only slightly lower than wild-type enzymes, indicating good coupling. Our data suggest that a greater shielding of the heme pocket caused by the Val/Ile switch slows down NO synthesis and NO release in NOS, and thus identifies a structural basis for regulating these kinetic variables.

Synthesis, characterization and molecular structures of six-coordinate manganese nitrosyl porphyrins

Manganese(II) porphyrins are isoelectronic with iron(III) porphyrins, and previously reported work suggests that manganese nitrosyl porphyrins are good structural models for their kinetically unstable and biologically relevant ferric-NO analogues. We have prepared a new set of six-coordinate manganese nitrosyl porphyrins of the general form (por)Mn(NO)(L)(por = TTP, T(p-OCH3)PP; L = piperidine, methanol, 1-methylimidazole) in moderate to high yields. The (por)Mn(NO)(pip) complexes were prepared from the reductive nitrosylation of the (por)MnCl compounds with NO in the presence of piperidine. The IR spectra of the (por)Mn(NO)(pip) compounds as KBr pellets show new strong bands at 1746 cm(-1)(for TTP) and 1748 cm(-1)(for (T(p-OCH3)PP) due to the NO ligands. Attempted crystallization of one of these compounds (por = TTP) from dichloromethane-methanol resulted in the generation of the methanol complex (TTP)Mn(NO)(CH3OH). Reaction of the (por)Mn(NO)(pip) compounds with excess 1-methylimidazole gave the (por)Mn(NO)(1-MeIm) derivatives in good yields. The IR spectra of these compounds show nu(NO) bands that are approximately 12 cm(-1) lower than those of the (por)Mn(NO)(pip) precursors, indicative of greater Mn-->NO pi-backdonation in the 1-MeIm derivatives. X-Ray crystal structures of three of these compounds, namely (TTP)Mn(NO)(CH3OH), (TTP)Mn(NO)(1-MeIm) and (T(p-OCH3)PP)Mn(NO)(1-MeIm) were obtained, and reveal that the NO ligands in these complexes are linear.

Orthogonal properties of the redox siblings nitroxyl and nitric oxide in the cardiovascular system: a novel redox paradigm

Endogenous formation of nitric oxide (NO) and related nitrogen oxides in the vascular system is critical to regulation of multiple physiological functions. An imbalance in the production or availability of these species can result in progression of disease. Nitrogen oxide research in the cardiovascular system has primarily focused on the effects of NO and higher oxidation products. However, nitroxyl (HNO), the one-electron-reduction product of NO, has recently been shown to have unique and potentially beneficial pharmacological properties. HNO and NO often induce discrete biological responses, providing an interesting redox system. This article discusses the emerging aspects of HNO chemistry and attempts to provide a framework for the distinct effects of NO and HNO in vivo.

Crystal structures of ferrous horse heart myoglobin complexed with nitric oxide and nitrosoethane

The interactions of nitric oxide (NO) and organic nitroso compounds with heme proteins are biologically important, and adduct formation between NO-containing compounds and myoglobin (Mb) have served as prototypical systems for studies of these interactions. We have prepared crystals of horse heart (hh) MbNO from nitrosylation of aqua-metMb crystals, and we have determined the crystal structure of hh MbNO at a resolution of 1.9 A. The Fe-N-O angle of 147 degrees in hh MbNO is larger than the corresponding 112 degrees angle previously determined from the crystal structure of sperm whale MbNO (Brucker et al., Proteins 1998;30:352-356) but is similar to the 150 degrees angle determined from a MS XAFS study of a frozen solution of hh MbNO (Rich et al., J Am Chem Soc 1998;120:10827-10836). The Fe-N(O) bond length of 2.0 A (this work) is longer than the 1.75 A distance determined from the XAFS study and suggests distal pocket influences on FeNO geometry. The nitrosyl N atom is located 3.0 A from the imidazole N(epsilon) atom of the distal His64 residue, suggesting electrostatic stabilization of the FeNO moiety by His64. The crystal structure of the nitrosoethane adduct of ferrous hh Mb was determined at a resolution of 1.7 A. The nitroso O atom of the EtNO ligand is located 2.7 A from the imidazole N(epsilon) atom of His64, suggesting a hydrogen bond interaction between these groups. To the best of our knowledge, the crystal structure of hh Mb(EtNO) is the first such determination of a nitrosoalkane adduct of a heme protein.

Spectroscopic and density functional studies of the dinitrosyl metalloporphyrin complexes Fe(P)(NO)2 and Ru(P)(NO)2

Experimental evidence including infrared spectra for the formation of the dinitrosyl metalloporphyrin complexes M(P)(NO)(2) (M = Ru or Fe, P = tetraphenylporphyrin (TPP), octaethylporphyrin (OEP), or tetra-m-tolylporphryin (TmTP)) is described. Although observation of a single NO stretching band in the IR spectrum of each M(P)(NO)(2) complex first suggested a centrosymmetric (D(4)(h)() or C(2)(h)()) structure, DFT geometry optimizations and frequency calculations of model complexes indicate that the trans-syn (C(2)(v)()) conformation should be more stable. The frequency calculations resolve the apparent ambiguity in the IR spectra in terms of the relative oscillator strengths of the predicted IR bands.

Synthesis, characterization, and molecular structures of nitrosyl nitrito complexes of osmium porphyrins: Disproportionation of nitric oxide in its reaction with Os(P)(CO) (P = porphyrinato dianion)

The Os(P)(NO)(ONO) compounds (P = TTP, TMP, OEP, TmTP; TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion, TMP = 5,10,15,20-tetramesitylporphyrinato dianion, OEP = octaethylporphyrinato dianion, TmTP = tetra(m-tolyl)porphyrinato dianion) have been prepared from the reaction of the precursor carbonyl complexes Os(P)(CO) with excess nitric oxide. Nitrous oxide was detected as a by-product of the reaction. The IR spectra of the Os(P)(NO)(ONO) compounds (as KBr pellets) reveal bands in the 1790–1804 cm–1 range that are assigned to υNO. The IR spectra also reveal two new bands for each complex in the 1495–1531 and 913–962 cm–1 ranges indicative of O-bound nitrito ligands. The linearity of the bound NO groups and the O-binding of the trans nitrito ligands in the Os(P)(NO)(ONO) complexes are evident in the single-crystal X-ray crystal structures of the TTP and TMP derivatives. The kinetics of the reaction were studied by stopped-flow mixing techniques. Spectroscopic analysis of rapidly mixed solutions of Os(P)CO and NO in toluene showed a biphasic approach to the Os(P)(NO)(ONO) and N2O products, owing to the starting material Os(P)CO scavenging CO formed during the reaction to give Os(P)(CO)2 (KCO = 106 M–1). The dicarbonyl was the only transient species observed. It is proposed that the rate-determining step of the reaction leading to Os(P)(NO)(ONO) is NO displacement of CO from Os(P)(CO) via initial formation of an unstable 19 electron Os(P)(NO)(CO) intermediate.Key words: osmium, nitric oxide, X-ray, nitrosyl, porphyrin, kinetics.

Heme proteins and nitric oxide (NO): the neglected, eloquent chemistry in NO redox signaling and regulation

The role of nitric oxide (NO) in cellular physiology and signaling has been an important aspect in biomedical science over the last decade. As NO is a small uncharged radical, the chemistry of NO within the redox environment of the cell dictates the majority of its biological effects. The mechanisms that have received the most attention from a biological perspective involve reactions with oxygen and superoxide, despite the rich literature of metal-NO chemistry. However, NO and its related species participate in important chemistry with metalloproteins. In addition to the well known direct interactions of NO with heme proteins such as soluble guanylate cyclase and oxyhemoglobin, there is much important, but often underappreciated, chemistry between other nitrogen oxides and heme/metal proteins. Here the basic chemistry of nitrosylation and the interactions of NO and other nitrogen oxides with metal-oxo species such as found in peroxidases and monoxygenases are discussed.

Structural and biochemical studies of p21Ras S-nitrosylation and nitric oxide-mediated guanine nucleotide exchange

Ras is a guanine nucleotide-binding protein that cycles between inactive GDP-bound and active GTP-bound states to regulate a diverse array of cellular processes, including cell growth, apoptosis, and differentiation. The guanine nucleotide-bound state of Ras is tightly maintained by regulatory factors to promote regulated growth control. A class of regulatory molecules that lead to Ras activation are guanine nucleotide exchange factors (GEFs). Ras GEFs bind to Ras and facilitate GDP release, followed by GTP incorporation and Ras activation. Nitric oxide (NO) has also been shown to promote guanine nucleotide exchange (GNE) on Ras and increase cellular Ras-GTP levels, but the process by which NO-mediated GNE occurs is not clear. We initiated NMR structural and biochemical studies to elucidate how nitrosylation of Ras might lead to enhanced GNE. Surprisingly, our studies show that stable S-nitrosylation of Ras at Cys-118, does not affect the structure of Ras, its association with the Ras-binding domain of Raf (a downstream effector of Ras), or GNE rates relative to non-nitrosylated Ras. We have found, however, that the actual chemical process of nitrosylation, rather than the end-product of Ras S-nitrosylation, accounts for the enhanced GNE that we have observed and that has been previously observed by others.

Molecular recognition of NO/NO+ via multicenter (charge-transfer) binding to bridged diarene donors. Effect of structure on the optical transitions and complexation thermodynamics

Bridged diarenes form very strong [1:1] complexes with nitrosonium/nitric oxide in which the NO moiety is optimally sandwiched in the cleft between a pair of cofacial aromatic rings which act as a molecular "Venus flytrap". The spectral features of these associates are generally similar to those for [1:1] and [2:1] nitrosonium complexes with mononuclear alkyl-substituted benzenes, and they are appropriately described within the LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of donor/acceptor electronic transitions. The thermodynamics study indicates that the efficient binding is determined by (i) the close matching of the donor/acceptor redox potentials and (ii) the ability of bridged diarenes for multicentered interactions with a single NO moiety. The best fit of the electronic and structural parameters is provided by a calixarene host that allows the interacting centers to be arranged in a manner similar to those extant in [2:1] nitrosonium complexes with analogous (nonbridged) aromatic donors; this results in its very strong noncovalent binding with nitrosonium/nitric oxide with the formation constant of K(B) approximately 10(8) M(-)(1) and free-energy change of -DeltaG degrees = 45 kJ mol(-)(1). Such strong, selective, and reversible bindings of nitrosonium/nitric oxide by (cofacial) aromatic centers thus provide the basis for the development of efficient NO sensors/absorbents and also suggest their potential relevance to biochemical systems.

Reactions of deoxy-, oxy-, and methemoglobin with nitrogen monoxide. Mechanistic studies of the S-nitrosothiol formation under different mixing conditions

The reaction between hemoglobin (Hb) and NO* has been investigated thoroughly in recent years, but its mechanism is still a matter of substantial controversy. We have carried out a systematic study of the influence of the following factors on the yield of S-nitrosohemoglobin (SNO-Hb) generated from the reaction of NO* with oxy-, deoxy-, and metHb: 1) the volumetric ratio of the protein and the NO* solutions; 2) the rate of addition of the NO* solution to the protein solution; 3) the amount of NO* added; and 4) the concentration of the phosphate buffer. Our results suggest that the highest SNO-Hb yields are mostly obtained by very slow addition of substoichiometric amounts of NO* from a diluted solution. Possible pathways of SNO-Hb formation from the reaction of NO* with oxy-, deoxy-, and metHb are described. Our data strongly suggest that, because of mixing artifacts, care should be taken to use results from in vitro experiments to draw conclusion on the mechanism of the reaction in vivo.

Density Functional Study of the Electronic Structure and Related Properties of Pt(NO)/Pt(NO2) Redox Couples

Density functional calculations at the B3LYP level of theory, using the SDD basis set, provide satisfactory description of geometric, energetic, electronic and spectroscopic properties of the Pt(NO)/Pt(NO2) redox couple. The neutral Pt(NO) species adopts a bent 2A' ground state, while the cationic [Pt(NO)]+ species adopts a linear 1Σ+ ground state. The B3LYP/SDD- predicted Pt-N bond lengths are 2.016 and 1.777 Å for Pt(NO) (2A') and [Pt(NO)]+ (1Σ+), respectively, while the ∠Pt-N-O bent angle for [Pt(NO)] (2A') is 119.6°. On the other hand, the anionic [Pt(NO)]- species adopts the bent 1A' ground state with a Pt-N bond length of 1.867 Å and a ∠Pt-N-O bent angle of 122.5°. The computed binding energies of the NO, NO+ and NO- ligands with Pt(0) were found to be 29.9 (32.8), 69.9 (78.4) and 127.4 (128.7) kcal/mol at the B3LYP/SDD and CCSD(T)/SDD (numbers in parentheses) levels of theory, respectively. Moreover, the structure of the [Pt(NO2)]+ component of the Pt(NO)/Pt(NO2) redox couple and its transformation to [Pt(NO)]+ upon reaction with CO was analysed in the framework of the DFT theory. The coordination of the CO ligand to [Pt(NO2)]+ affords the cationic mixed-ligand [Pt(CO)(NO2)]+ complex, which is stabilized by 66.6 (60.5) kcal/mol, with respect to the separated [Pt(NO2)]+ and CO in their ground states. The O-transfer reaction from the coordinated NO2 to the coordinated CO ligands in the presence of the [Pt(NO2)]+ species corresponds to an exothermic process; the heat of the reaction (∆RH) is -85.2 (-80.5) kcal/mol and the activation barrier amounts to 27.7 (33.0) kcal/mol. Finally, the equilibrium structures of selected stationary points related to the transformation of NO to NO2 ligand located on the potential energy surfaces of the [Pt(NO),O2], [Pt(NO)+,O2], and [Pt(NO)-,O2] systems were analysed in the framework of the DFT theory. The computed interaction energies of O2 with Pt(NO), [Pt(NO)]+ and [Pt(NO)]- species were found to be 106.9 (105.3), 49.2 (48.4) and 26.9 (26.5) kcal/mol, respectively. The O2 ligand is coordinated to the Pt central atom in an end-on mode for [Pt(NO),O2] and [Pt(NO)-,O2] systems and in a side-on mode for the [Pt(NO)+,O2] system. The transformation of NO to NO2 in [Pt(NO)]- species upon reaction with dioxygen corresponds to an exothermic process; the heat of the reaction (∆RH) is -60.6 (-55.8) kcal/mol, while the activation barrier amounts to 35.5 (30.2) kcal/mol. Calculated structures, relative stability and bonding properties of all stationary points are discussed with respect to computed electronic and spectroscopic properties, such as charge density distribution and harmonic vibrational frequencies.

Comparison of the reactivity of nitric oxide and nitroxyl with heme proteins. A chemical discussion of the differential biological effects of these redox related products of NOS

Investigations on the biological effects of nitric oxide (NO) derived from nitric oxide synthase (NOS) have led to an explosion in biomedical research over the last decade. The chemistry of this diatomic radical is key to its biological effects. Recently, nitroxyl (HNO/NO(-)) has been proposed to be another important constituent of NO biology. However, these redox siblings often exhibit orthogonal behavior in physiological and cellular responses. We therefore explored the chemistry of NO and HNO with heme proteins in different redox states and observed that HNO favors reaction with ferric heme while NO favors ferrous, consistent with previous reports. Further results show that HNO and NO were equally effective in inhibiting cytochrome P450 activity, which involves ferric and ferrous complexes. The differential chemical behavior of NO and HNO toward heme proteins provides insight into mechanisms of activity that not only helps explain some of the opposing effects observed in NOS-mediated events, but offers a unique control mechanism for the biological action of NO.

Kinetic, mechanistic, and DFT study of the electrophilic reactions of nitrosyl complexes with hydroxide

We present a kinetic study of OH(-) additions to several nitrosyl complexes containing mainly ruthenium and different coligands (polypyridines, amines, pyridines, cyanides). According to a first-order rate law in each reactant, we propose a fast ion pair formation equilibrium, followed by addition of OH(-) to the [MX(5)NO](n) moieties, with formation of the [MX(5)NO(2)H](n(-1)) intermediates. Additional attack by a second OH(-) gives the final products, [MX(5)NO(2)]((n-2)). A linear plot was found for ln k(4) (the addition rate constant) against the redox potential for nitrosyl reduction, E(NO+/-NO), showing a free-energy relationship with a slope close to 20 V(-1), consistent with an associative mechanism. Theoretical DFT calculated descriptors, as the charge density in the [MNO] moieties and the LUMO energies, qualitatively correlate with the rate constants. A linear to bent transformation was calculated for the nitrosyl complexes, as they evolve to the angular MNO(2)H and MNO(2) complexes. The geometries were optimized for the different complexes and adduct-intermediates, showing significant changes in the relevant distances and angles upon OH(-) addition. IR vibrations and electronic transitions were also calculated. The complete reaction profile was studied for the nitroprusside ion, including the description of the transition state structure. Experimental activation parameters revealed that both the activation enthalpies and entropies increase when going from the negatively charged to the positively charged complexes. As the rate constants increase in the same direction, we conclude that the reactions are entropically driven, compensating, this function, the increase in the activation enthalpies. The latter trend can be explained by the energies involved in angular reorganization after OH(-) coordination, which are larger as the positive charge in the nitrosyl moiety becomes larger. The use of E(NO+/-NO) as a predictive tool for electrophilic reactivity could be extended to similar reactions implying other nucleophiles, such as amines and thiolates.

Kinetics and mechanism of the formation of nitroprusside from aquapentacyanoferrate(III) and NO: complex formation controlled by outer-sphere electron transfer

The kinetics and mechanism of the reaction between nitric oxide and aquapentacyanoferrate(III) were studied in detail. Pentacyanonitrosylferrate (nitroprusside, NP) was produced quantitatively in a pseudo-first-order process. The complex-formation rate constant was found to be 0.252 +/- 0.004 M(-1) s(-1) at 25.5 degrees C, pH 3.0 (HClO(4)), and I = 0.1 M (NaClO(4)), for which the activation parameters are DeltaH++ = 52 +/- 1 kJ mol(-1), DeltaS++ = -82 +/- 4 J K(-1) mol(-1), and DeltaV++ = -13.9 + 0.5 cm(3) mol(-1). These data disagree with earlier studies on complex-formation reactions of aquapentacyanoferrate(III), for which a dissociative interchange (I(d)) mechanism was suggested. The aquapentacyanoferrate(II) ion was detected as a reactive intermediate in the reaction of aquapentacyanoferrate(III) with NO, by using pyrazine and thiocyanate as scavengers for this intermediate. In addition, the reactions of other [Fe(III)(CN)(5)L](n-) complexes (L = NCS(-), py, NO(2)(-), and CN(-)) with NO were studied. These experiments also pointed to the formation of Fe(II) species as intermediates. It is proposed that aquapentacyanoferrate(III) is reduced by NO to the corresponding Fe(II) complex through a rate-determining outer-sphere electron-transfer reaction controlling the overall processes. The Fe(II) complex rapidly reacts with nitrite producing [Fe(II)(CN)(5)NO(2)](4)(-), followed by the fast and irreversible conversion to NP.