Interactions of organic nitroso compounds with metals

Synthesis and tautomerization study of pseudonitrosites to 1,2-nitroximes

4-R-Substituted 2-nitro-1-nitrosoethylbenzenes (R = H, CH3, OCH3, Cl, F) have been synthesized under solvent-free conditions and the mechanism of their tautomerization to 2-isonitroso-1-nitro-2-phenylethanes have been investigated by 1H NMR spectroscopy.Key words: tautomerization, pseudonitrosite, mechanism, 2-nitro-1-nitrosoethylbenzene, 1H NMR spectroscopy

New members of a class of dinitrosyliron complexes (DNICs): interconversion and spectroscopic discrimination of the anionic {Fe(NO)2}9 [(NO)2Fe(C3H3N2)2]- and [(NO)2Fe(C3H3N2)(SR)]- (C3H3N2 = deprotonated imidazole; R = tBu, Et, Ph)

The anionic {Fe(NO)2}(9) DNIC[(NO)2Fe(C3H3N2)2](-) (2) (C3H3N2 = deprotonated imidazole) containing the deprotonated imidazole-coordinated ligands and DNICs [(NO)2Fe(C3H3N2)(SR)](-) (R = (t)Bu(3), Et(4), Ph(5)) containing the mixed deprotonated imidazole-thiolate coordinated ligands, respectively, were synthesized by thiol protonation or thiolate(s) ligand-exchange reaction. The anionic {Fe(NO)2}(9) DNICs 2- 5 were characterized by IR, UV-vis, EPR, and single-crystal X-ray diffraction. The facile transformation among the anionic {Fe(NO)2}(9) DNICs 2- 5 and [(NO)2Fe(S(t)Bu)2](-)/[(NO)2Fe(SEt)2](-)/[(NO)2Fe(SPh)2](-) was demonstrated in this systematic study. Of importance, the distinct electron-donating ability of thiolates serve to regulate the stability of the anionic {Fe(NO)2}(9) DNICs and the ligand-substitution reactions of DNICs. At 298 K, DNIC 2 exhibits the nine-line EPR signal with g = 2.027 (aN(NO) = 2.20 and aN(Im-H) = 3.15 G; Im-H = deprotonated imidazole) and DNIC 3 displays the nine-line signals with g = 2.027 (aN(NO) = 2.35 and aN(Im-H) = 4.10 G). Interestingly, the EPR spectrum of complex 4 exhibits a well-resolved 11-line pattern with g = 2.027 (aN(NO) = 2.50, aN(Im-H) = 4.10 G, and aH = 1.55 G) at 298 K. The EPR spectra (the pattern of hyperfine splitting) in combination with IR nu NO spectra (DeltanuNO = the separation of NO stretching frequencies, DeltanuNO = approximately 62 cm (-1) for 2 vs approximately 50 cm(-1) for 3- 5 vs approximately 43 cm(-1) for [(NO)2Fe(S(t)Bu)2](-)/[(NO)2Fe(SEt)2](-)/[(NO)2Fe(SPh)2](-)) may serve as an efficient tool for the discrimination of the existence of the anionic {Fe(NO)2}(9) DNICs containing the different ligations [N,N]/[N,S]/[S,S].

Performing organic chemistry with inorganic compounds: electrophilic reactivity of selected nitrosyl complexes

The inorganic nitrosyl (NO(+)) complexes [Fe(CN) 5NO](2-), [Ru(bpy)2(NO)Cl](2+), and [IrCl 5(NO)](-) are useful reagents for the nitrosation of a variety of organic compounds, ranging from amines to the relatively inert alkenes. Regarding [IrCl 5(NO)](-), its high electrophilicity and inertness define it as a unique reagent and provide a powerful synthetic route for the isolation and stabilization of coordinated nitroso compounds that are unstable in free form, such as S-nitrosothiols and primary nitrosamines. Related to the high electrophilicity of [IrCl 5(NO)](-), an unusual behavior is described for its PPh 4(+) salt in the solid state, showing an electronic distribution represented by Ir(IV)-NO(*) instead of Ir (III)-NO(+) (as for the K(+) and Na(+) salts).

Assessment of long-range corrected functionals performance for n→π* transitions in organic dyes

The first n-->pi* transitions of 18 nitroso and 16 thiocarbonyl dyes have been computed by time-dependent density functional theory (TD-DFT) using pure as well as global and range-separated hybrid functionals. It turns out that the accuracy of all hybrids is relatively similar, i.e., the inclusion of a growing fraction of exact exchange does neither worsen nor improve significantly the raw TD-DFT estimations. However, after a simple linear regression, it appears that the range-separated hybrids provide a better accuracy than global hybrids.

Reactivity of aquacobalamin and reduced cobalamin toward S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine

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 approximately 1.2 x 10(6) 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.

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

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

Crystal structures of the nitrite and nitric oxide complexes of horse heart myoglobin

Nitrite is an important species in the global nitrogen cycle, and the nitrite reductase enzymes convert nitrite to nitric oxide (NO). Recently, it has been shown that hemoglobin and myoglobin catalyze the reduction of nitrite to NO under hypoxic conditions. We have determined the 1.20 A resolution crystal structure of the nitrite adduct of ferric horse heart myoglobin (hh Mb). The ligand is bound to iron in the nitrito form, and the complex is formulated as MbIII(ONO-). The Fe-ONO bond length is 1.94 A, and the O-N-O angle is 113 degrees . In addition, the nitrite ligand is stabilized by hydrogen bonding with the distal His64 residue. We have also determined the 1.30 A resolution crystal structures of hh MbIINO. When hh MbIINO is prepared from the reaction of metMbIII with nitrite/dithionite, the FeNO angle is 144 degrees with a Fe-NO bond length of 1.87 A. However, when prepared from the reaction of NO with reduced MbII, the FeNO angle is 120 degrees with a Fe-NO bond length of 2.13 A. This difference in FeNO conformations as a function of preparative method is reproducible, and suggests a role of the distal pocket in hh MbIINO in stabilizing local FeNO conformational minima.

Mechanism of NO transfer from NO-donors (SNAP and G-MNBS) to ferrous tetraphenylporphyrin in CH3OH

[reaction: see text] The mechanism of NO transfer from NO-donors (SNAP and G-MNBS) to ferrous tetraphenylporphyrin (TPPFe(II)) in CH(3)OH is discovered for the first time by using a laser flash technique. The results show that the NO transfer is completed by NO(+) transfer followed by electron transfer rather than direct NO transfer in one step.

Reductive nitrosylation of water-soluble iron porphyrins by S-nitroso-N-acetylpenicillamine: rate constants and EPR characterization

Reductive nitrosylation of the water-soluble iron derivatives of the cationic Fe(III)(TMPyP) and anionic Fe(III)(TPPS) porphyrins [where TMPyP=tetra-meso-(4-N-methylpyridiniumyl)porphinate and TPPS=tetra-meso-(4-sulfonatophenyl)porphinate] by the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) was studied using optical absorption spectroscopy and electron paramagnetic resonance. Nitrosylation rates were obtained, the reaction was found to be first order in the SNAP concentration and the stoichiometry of the reaction was one to one. The similarity between the obtained second-order rate constants for both porphyrins, k(TMPyP)=0.84 x 10(3)M(-1)s(-1) and k(TPPS)=0.97 x 10(3)M(-1)s(-1), suggested that the reaction mechanism is approximately independent of the nature of the porphyrin meso-substituents. A mechanism was proposed involving the hydrolysis of SNAP by an out of plane liganded H(2)O yielding the sulfenic acid of N-acetylpenicillamine and the transfer of NO(-) to Fe(III). The EPR (electron paramagnetic resonance) spectra of the SNAP- and gaseous NO-treated porphyrins were obtained and compared. The difference between the spectra of the cationic and anionic porphyrins indicates different local symmetry and Fe-N-O bond angle. SNAP-treatment produced much more resolved hyperfine structures than gaseous NO-treatment.

Visualization of nitric oxide in living cells by a copper-based fluorescent probe

Nitric oxide (NO) serves as a messenger for cellular signaling. To visualize NO in living cells, we synthesized a turn-on fluorescent probe for use in combination with microscopy. Unlike existing fluorescent sensors, the construct--a Cu(II) complex of a fluorescein modified with an appended metal-chelating ligand (FL)--directly and immediately images NO rather than a derivative reactive nitrogen species. Using spectroscopic and mass spectrometric methods, we established that the mechanism of the reaction responsible for the NO-induced fluorescence involves reduction of the complex to Cu(I) with release of the nitrosated ligand, which occurs irreversibly. We detected NO produced by both constitutive and inducible NO synthases (cNOS and iNOS, respectively) in live neurons and macrophages in a concentration- and time-dependent manner by using the Cu(II)-based imaging agent. Both the sensitivity to nanomolar concentrations of NO and the spatiotemporal information provided by this complex demonstrate its value for numerous biological applications.

Probing shapes of bichromophoric metal-organic complexes using ion mobility mass spectrometry

Ion mobility mass spectrometry (IM-MS) was used to probe the structures of several metal complexes carrying pendant chromophores. The three complexes investigated were the copper(II) complex Cu(DAC)2+ (DAC = 1,8-bis(9-methylanthracyl)cyclam, cyclam = 1,4,8,11-tetraazacyclotetradecane), the N-nitrosylated ligand DAC-NO, and the Roussin's red salt ester (mu-S,mu-S')-protoporphyrin-IX-bis(2-thioethyl ester)tetranitrosyldiiron (PPIX-RSE). From the IM-MS data coupled with theoretical calculations, it was found that [Cu(II)(DAC - H)]+ exists as a single conformer, with one anthracenyl group above the cyclam and the other below, similar to the crystal structure of Cu(II)(DAC)2+. The metal-free N-nitrosylated ligand (DAC-NO + H)+ has two conformations: one family of structures has one anthracenyl group above the cyclam and one below, while the other has both anthracenyl groups on the same side of the cyclam. These observations are consistent with 1H NMR data for the neutral DAC-NO complex that indicate the presence of two geometric isomers in solution. The third species, PPIX-RSE, has a porphyrin chromophore covalently linked to an Fe2S2(NO)4 cluster for use as a precursor for the photochemical delivery of nitric oxide in single- and two-photon excitation processes. Ion mobility indicates the presence of two (PPIX-RSE + H)+ conformations, consistent with the previous interpretation of the bimodal fluorescence lifetime decay seen for PPIX-RSE. DFT structures, in good agreement with the IM-MS cross sections, indicate two "bent" conformations with the planes of the porphyrin and Fe2S2 rings at different angles with respect to each other.

The solid-state molecular structure of the S-nitroso derivative of L-cysteine ethyl ester hydrochloride

Nitrosation of protein sulfhydryl groups to form thionitrites (S-nitrosothiols) has been reported to be important in the biochemistry of nitric oxide. Such S-nitrosation of protein thiol residues has been shown to alter the function of some proteins. In this brief communication, we report the X-ray crystal structure of S-nitroso-L-cysteine ethyl ester hydrochloride. Two rotamers with respect to the NCCS moiety are present in the crystal: the major rotamer is in the gauche+ conformation, and the minor rotamer is in the rare anti (trans, antiperiplanar) conformation for a cysteinyl compound. Importantly, the CSNO groups for both rotamers are in the syn (cis, synperiplanar) form. To the best of our knowledge, this is the first reported high-resolution solid-state structure of an S-nitroso derivative of a cysteine or cysteinyl-containing compound.

Dual function catalysts. Dehydrogenation and asymmetric intramolecular Diels-Alder cycloaddition of N-hydroxy formate esters and hydroxamic acids: evidence for a ruthenium-acylnitroso intermediate

The chiral ruthenium salen complex, 13b, functions as an efficient catalyst for the sequential oxidation and asymmetric Diels-Alder cycloaddition of hydroxamic acids and N-hydroxy formate esters. This result provides evidence for the formation of a ruthenium-nitroso formate (acyl nitroso) intermediate. The Diels-Alder precursors are prepared from simple building blocks, and the cycloadducts, bridged oxazinolactams, can serve as useful intermediates in organic synthesis.

Dinitroso and polynitroso compounds

The growing interest in the chemistry of C-nitroso compounds (RN=O; R = alkyl or aryl group) is due in part to the recognition of their participation in various metabolic processes of nitrogen-containing compounds. C-Nitroso compounds have a rich organic chemistry in their own right, displaying interesting intra- and intermolecular dimerization processes and addition reactions with unsaturated compounds. In addition, they have a fascinating coordination chemistry. While most of the attention has been directed towards C-nitroso compounds containing a single -NO moiety, there is an emerging area of research dealing with dinitroso and polynitroso compounds. In this critical review, we present and discuss the synthetic routes and properties of these relatively unexplored dinitroso and polynitroso compounds, and suggest areas of further development involving these compounds. (126 references.).

Nitrosoarene−Cu(I) Complexes Are Intermediates in Copper-Catalyzed Allylic Amination

Reactions of nitrosobenzene and N,N'-diethyl-4-nitrosoaniline with [Cu(CH3CN)4]PF6 produce novel homoleptic Cu(I)-nitrosoarene complexes, [Cu(ArNO)3]PF6, 1 (Ar = Ph) and 2 (Ar = 4-Et2NC6H4NO). The X-ray structure of 2 reveals that the copper is coordinated in a severely distorted trigonal planar geometry to the N-atom of the nitrosoarene ligand. Reactions of the PhNO complex 1 with olefins and an olefin/diene mixture provide evidence for its role as an intermediate and possibly the active nitrogen transfer agent in the Cu-catalyzed allylic amination of olefins by aryl hydroxylamines.

Rich chemistry of nitroso compounds

Nitrosobenzene or nitrosopyridine are found to be attractive electrophiles in catalytic enantioselective carbon-nitrogen and/or carbon-oxygen bond forming reactions. In the presence of designer Lewis or Brønsted acid catalysts, catalytic enantioselective O- and N-nitroso aldol reaction or nitroso Diels-Alder reaction proceed smoothly. The scope and limitation of new catalytic processes are described.

The first catalytic inverse-electron demand hetero-Diels–Alder reaction of nitroso alkenes using pyrrolidine as an organocatalyst

The first catalytic inverse-electron demand hetero-Diels-Alder reaction of nitroso alkenes has been developed. Nitroso alkenes were generated in situ from alpha-halooximes and underwent [4 + 2]-cycloadditions with enamines as dienophiles formed from aldehydes and pyrrolidine (10 mol%) as an organocatalyst. The presence of a suitable heterogeneous buffer system was found to be essential and best results were obtained with sodium acetate trihydrate. The resulting 5,6-dihydro-4H-oxazines were obtained in moderate to good yields under mild reaction conditions. A catalytic cycle has been proposed and evidence for the cycloaddition mechanism has been obtained. Moderate asymmetric induction (42% ee) was observed when a chiral secondary amine was used.

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.

Synthesis and solid-state molecular structures of bis- and mono-nitrosobenzene complexes of ruthenium porphyrins

Bis-nitrosobenzene complexes of the form (por)Ru(PhNO)2 (por = TPP, TTP; TPP = tetraphenylporphyrinato dianion, TTP = tetratolylporphyrinato dianion) have been prepared in good yields from the reaction of the (por)Ru(CO) precursor with excess PhNO in dichloromethane. The IR spectra of the complexes (as KBr pellets) displayed new bands at ~1348 cm–1, due to υNO. The solid-state molecular structure of (TPP)Ru(PhNO)2 was determined by single-crystal X-ray diffraction, and revealed that the PhNO ligands are bound to the Ru center via the N-binding mode. Reactions of the (por)Ru(PhNO)2 complexes with excess 1-methylimidazole gave the mono-nitrosobenzene complexes (por)Ru(PhNO)(1-MeIm). The IR spectra revealed a lowering of υNO in these mononitrosobenzene derivatives by ~27 cm–1, a feature consistent with the replacement of one π-acid PhNO ligand with the more basic 1-MeIm ligand. The solid-state molecular structure of (TPP)Ru(PhNO)(1-MeIm) reveals, in addition to the N-binding of the PhNO ligand, an essentially parallel arrangement of the C-N-O (of PhNO) and imidazole planes; this is in contrast with the (TPP)Ru(PhNO)2 complex, in which the C-N-O planes (of PhNO) are essentially perpendicular.Key words: nitroso, X-ray, ruthenium, porphyrin, imidazole.