The Mechanism of NO Formation from the Decomposition of Dialkylamino Diazeniumdiolates: Density Functional Theory and CBS-QB3 Predictions

Mechanisms of nitric oxide generation in living systems

In modern chemical and biochemical studies, special attention is paid to molecular systems capable of generating nitric oxide (NO), which is one of the most important signalling molecules in the body and can trigger a whole cascade of reactions. Despite the importance of this molecule, the mechanisms of its formation in living organisms remain a subject of debate. This review combines the most important methods of releasing NO from endogenous and exogenous sources. The history of endogenous NO donors dates back more than 150 years, since the synthesis of nitroglycerin, which remains the standard vasodilator today, even though it is known that it and many other similar compounds lead to the development of a nitrate tolerance. Particular awareness is devoted to the mechanisms of NO formation without the participation of enzymes, since these methods are most important for creating exogenous sources of NO as drugs. The study of NO formation methods is centred on both the creation of new NO donors and understanding the mechanisms of tolerance to them.

Examining the effect of common nitrosating agents on chitosan using a glucosamine oligosaccharide model system

Chitosan has received substantial attention as a biomaterial due to its unique properties. It has become increasingly common to derivatize chitosan to produce nitric oxide (NO)-releasing materials that exert various therapeutic effects through the action of NO. It is generally the case that these NO-releasing polymers are prepared by exposure to high-pressure NO or nitrosating agents like nitrous acid (HNO₂) or alkyl nitrites (RONO). In our study, mass spectrometry and spectroscopic methods demonstrate that both monomeric and oligomeric glucosamine experience chemical alteration after exposure to HNO₂-based nitrosating conditions from the literature. In polymeric chitosan, HNO₂-based nitrosating conditions were found to induce degradation through the formation of 2,5-anhydro-d-mannose and oligosaccharides. In contrast, the RONO tert-butyl nitrite and high-pressure NO were not found to significantly degrade or otherwise alter the structure of glucosamine or its oligomers, supporting the suitability of these approaches.

Light-Induced Acid Generation on a Gatekeeper for Smart Nitric Oxide Delivery

We report herein the design of a light-responsive gatekeeper for smart nitric oxide (NO) delivery. The gatekeeper is composed of a pH-jump reagent as an intermediary of stimulus and a calcium phosphate (CaP) coating as a shielding layer for NO release. The light irradiation and subsequent acid generation are used as triggers for uncapping the gatekeeper and releasing NO. The acids generated from a light-activated pH-jump agent loaded in the mesoporous nanoparticles accelerated the degradation of the CaP-coating layers on the nanoparticles, facilitating the light-responsive NO release from diazeniumdiolate by exposing a NO donor to physiological conditions. Using the combination of the pH-jump reagent and CaP coating, we successfully developed a light-responsive gatekeeper system for spatiotemporal-controlled NO delivery.

Analysis of the HNO and NO donating properties of alicyclic amine diazeniumdiolates

Nitroxyl (HNO) donors have been shown to elicit a variety of pharmacological responses, ranging from tumoricidal effects to treatment of heart failure. Isopropylamine-based diazeniumdiolates have been shown to produce HNO on decomposition under physiological conditions. Herein, we report the synthesis and HNO release profiles of primary alicyclic amine-based diazeniumdiolates. These compounds extend the range of known diazeniumdiolate-based HNO donors. Acetoxymethyl ester-protected diazeniumdiolates were also synthesized to improve purification and cellular uptake. The acetoxymethyl derivative of cyclopentylamine diazeniumdiolate not only showed higher cytotoxicity toward cancer cells as compared to the parent anion but was also effective in combination with tamoxifen for targeting estrogen receptor α-negative breast cancer cells.

Decoding nitric oxide release rates of amine-based diazeniumdiolates

Amine-based diazeniumdiolates (NONOates) have garnered widespread use as nitric oxide (NO) donors, and their potential for nitroxyl (HNO) release has more recently been realized. While NO release rates can vary significantly with the type of amine, half-lives of seconds to days under physiological conditions, there is as yet no way to determine a priori the NO or HNO production rates of a given species, and no discernible trends have manifested other than that secondary amines produce only NO (i.e., no HNO). As a step to understanding these complex systems, here we describe a procedure for modeling amine-based NONOates in water solvent that provides an excellent correlation (R(2) = 0.94) between experimentally measured dissociation rates of seven secondary amine species and their computed NO release activation energies. The significant difference in behavior of NONOates in the gas and solvent phases is also rigorously demonstrated via explicit additions of quantum mechanical water molecules. The presented results suggest that the as-yet unsynthesized simplest amine-based NONOate, the diazeniumdiolated ammonia anion [H2N-N(O)═NO(-)], could serve as an unperturbed HNO donor. These results provide a step forward toward the accurate modeling of general NO and/or HNO donors as well as for the identification of tailored prodrug candidates.

The diversity of microbial responses to nitric oxide and agents of nitrosative stress close cousins but not identical twins

Nitric oxide and related nitrogen species (reactive nitrogen species) now occupy a central position in contemporary medicine, physiology, biochemistry, and microbiology. In particular, NO plays important antimicrobial defenses in innate immunity but microbes have evolved intricate NO-sensing and defense mechanisms that are the subjects of a vast literature. Unfortunately, the burgeoning NO literature has not always been accompanied by an understanding of the intricacies and complexities of this radical and other reactive nitrogen species so that there exists confusion and vagueness about which one or more species exert the reported biological effects. The biological chemistry of NO and derived/related molecules is complex, due to multiple species that can be generated from NO in biological milieu and numerous possible reaction targets. Moreover, the fate and disposition of NO is always a function of its biological environment, which can vary significantly even within a single cell. In this review, we consider newer aspects of the literature but, most importantly, consider the underlying chemistry and draw attention to the distinctiveness of NO and its chemical cousins, nitrosonium (NO(+)), nitroxyl (NO(-), HNO), peroxynitrite (ONOO(-)), nitrite (NO(2)(-)), and nitrogen dioxide (NO(2)). All these species are reported to be generated in biological systems from initial formation of NO (from nitrite, NO synthases, or other sources) or its provision in biological experiments (typically from NO gas, S-nitrosothiols, or NO donor compounds). The major targets of NO and nitrosative damage (metal centers, thiols, and others) are reviewed and emphasis is given to newer "-omic" methods of unraveling the complex repercussions of NO and nitrogen oxide assaults. Microbial defense mechanisms, many of which are critical for pathogenicity, include the activities of hemoglobins that enzymically detoxify NO (to nitrate) and NO reductases and repair mechanisms (e.g., those that reverse S-nitrosothiol formation). Microbial resistance to these stresses is generally inducible and many diverse transcriptional regulators are involved-some that are secondary sensors (such as Fnr) and those that are "dedicated" (such as NorR, NsrR, NssR) in that their physiological function appears to be detecting primarily NO and then regulating expression of genes that encode enzymes with NO as a substrate. Although generally harmful, evidence is accumulating that NO may have beneficial effects, as in the case of the squid-Vibrio light-organ symbiosis, where NO serves as a signal, antioxidant, and specificity determinant. Progress in this area will require a thorough understanding not only of the biology but also of the underlying chemical principles.

HNO and NO release from a primary amine-based diazeniumdiolate as a function of pH

The growing evidence that nitroxyl (HNO) has a rich pharmacological potential that differs from that of nitric oxide (NO) has intensified interest in HNO donors. Recently, the diazeniumdiolate (NONOate) based on isopropylamine (IPA/NO; Na[(CH(3))(2)CHNH(N(O)NO)]) was demonstrated to function under physiological conditions as an organic analogue to the commonly used HNO donor Angeli's salt (Na(2)N(2)O(3)). The decomposition mechanism of Angeli's salt is dependent on pH, with transition from an HNO to an NO donor occurring abruptly near pH 3. Here, pH is shown to also affect product formation from IPA/NO. Chemical analysis of HNO and NO production led to refinement of an earlier, quantum mechanically based prediction of the pH-dependent decomposition mechanisms of primary amine NONOates such as IPA/NO. Under basic conditions, the amine proton of IPA/NO is able to initiate decomposition to HNO by tautomerization to the nitroso nitrogen (N(2)). At lower pH, protonation activates a competing pathway to NO production. At pH 8, the donor properties of IPA/NO and Angeli's salt are demonstrated to be comparable, suggesting that at or above this pH, IPA/NO is primarily an HNO donor. Below pH 5, NO is the major product, while IPA/NO functions as a dual donor of HNO and NO at intermediate pH. This pH-dependent variability in product formation may prove useful in examination of the chemistry of NO and HNO. Furthermore, primary amine NONOates may serve as a tunable class of nitrogen oxide donor.

Localization of hydrophobic N-diazeniumdiolates in aqueous micellar solution

The interaction of phenyl-substituted zwitterionic N-diazeniumdiolates PhCH(2)N[N(O)NO](-)(CH(2))(2)NH(3)(+) (1) and PhCH(2)N[N(O)NO](-)(CH(2))(2)NH(2)(+)CH(2)Ph (2) with aqueous micellar solutions of prototypal surfactants was investigated by means of UV/vis and (1)H NMR spectroscopy in order to establish the localization of hydrophobic N-diazeniumdiolates in micelles as a model for the binding of the NO donors in biological membranes. In the presence of sodium dodecyl sulfate (SDS), significant shifts of the apparent pK(a) values of 1 and 2 were observed, suggesting strong electrostatic interaction between the diazeniumdiolates and the negatively charged SDS micelles. No effect on both pK(a) and rate of NO release was found in the presence of Triton X-100. The solubilization site of micellar bound N-diazeniumdiolates was established by (1)H NMR spectroscopic studies, taking advantage of the spectroscopic effects induced by CH-pi interactions. The spectra indicate that in alkaline solutions of SDS 1 resides preferably at the micellar surface within the interfacial region, whereas the more hydrophobic NO donor 2 penetrates into the apolar region of the micelle. This suggests hydrophobic interaction as the main driving force for micellar binding of 2 in alkaline solution. Similar studies in presence of Triton X-100 indicate that 1 and 2 are adsorbed within the poly(oxyethylene) layer of the micellar surface rather than penetrating the palisade layer of the micelles. In alkaline solutions of hexadecyltrimethylammonium bromide (CTAB), 1 and 2 bind to the cationic micellar aggregates, whereby the solubilization site strongly depends on the hydrophobicity of the substrate. Up to a moderate pH of 8, the hydrophobic NO donor 2 penetrates the hydrocarbon region of the micelles. As a result, the rate of NO release from 2 is noticeably inhibited by the micellar aggregates due to the higher local concentration of hydroxide ions along the micelle-water interface. From solubilization studies, guidelines for the development and application of future NONOates can be derived. The rate of NO release from micellar bound diazeniumdiolates is determined by the surface charge of the micelles. This ability to tune stability is significant for the design and selection of potential NO delivery systems (drug formulations).

Kinetic feasibility of nitroxyl reduction by physiological reductants and biological implications

Nitroxyl (HNO), the one-electron reduced and protonated congener of nitric oxide (NO), is a chemically unique species with potentially important biological activity. Although HNO-based pharmaceuticals are currently being considered for the treatment of chronic heart failure or stroke/transplant-derived ischemia, the chemical events leading to therapeutic responses are not established. The interaction of HNO with oxidants results in the well-documented conversion to NO, but HNO is expected to be readily reduced as well. Recent thermodynamic calculations predict that reduction of HNO is biologically accessible. Herein, kinetic analysis suggests that the reactions of HNO with several mechanistically distinct reductants are also biologically feasible. Product analysis verified that the reductants had in fact been oxidized and that in several instances HNO had been converted to hydroxylamine. Moreover, a theoretical analysis suggests that in the reaction of HNO with thiol reductants, the pathway producing sulfinamide is significantly more favorable than that leading to disulfide. Additionally, simultaneous production of HNO and NO yielded a biphasic oxidative capacity.

Disproportionation pathways of aqueous hyponitrite radicals (HN2O2(*)/N2O2(*-))

Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN2O2(*)/N2O2(*-)) by one-electron oxidation of the hyponitrite in aqueous solution. Although the radical decay conforms to simple second-order kinetics, its mechanism is complex, comprising a short chain of NO release-consumption steps. In the first, rate-determining step, two N2O2(*-) radicals disproportionate with the rate constant 2k = (8.2 +/- 0.5) x 10(7) M(-1) s(-1) (at zero ionic strength) effectively in a redox reaction regenerating N2O2(2-) and releasing two NO. This occurs either by electron transfer or, more likely, through radical recombination-dissociation. Each NO so-produced rapidly adds to another N2O2(*-), yielding the N3O3(-) ion, which slowly decomposes at 300 s(-1) to the final N2O + NO2(-) products. The N2O2(*-) radical protonates with pKa = 5.6 +/- 0.3. The neutral HN2O2(*) radical decays by an analogous mechanism but much more rapidly with the apparent second-order rate constant 2k = (1.1 +/- 0.1) x 10(9) M(-1) s(-1). The N2O2(*-) radical shows surprisingly low reactivity toward O2 and O2(*-), with the corresponding rate constants below 1 x 10(6) and 5 x 10(7) M(-1) s(-1). The previously reported rapid dissociation of N2O2(*-) into N2O and O(*-) does not occur. The thermochemistry of HN2O2(*)/N2O2(*-) is discussed in the context of these new kinetic and mechanistic results.

Diazeniumdiolate reactivity in model membrane systems

The effect of small unilamellar phospholipid vesicles on the acid-catalyzed dissociation of nitric oxide from diazeniumdiolate ions, R(1)R(2)N[N(O)NO](-), [1: R(1)=H(2)N(CH(2))(3)-, R(2)=H(2)N(CH(2))(3)NH(CH(2))(4)-; 2: R(1)=R(2)=H(2)N(CH(2))(3)-; 3: R(1)=n-butyl-, R(2)=n-butyl-NH2+(CH(2))(6)-; 4: R(1)=R(2)=nPr-] has been examined at pH 7.4 and 37 degrees C. NO release was catalyzed by anionic liposomes (DPPG, DOPG, DMPS, POPS and DOPA) and by mixed phosphatidylglycerol/phosphatidylcholine (DPPG/DPPC and DOPG/DPPC) covesicles, while cationic liposomes derived from 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the zwitterionic liposome DMPC did not significantly affect the dissociation rates of the substrates examined. Enhancement of the dissociation rate constant in DPPG liposome media (0.010M phosphate buffer, pH 7.4, 37 degrees C) at 10mM phosphoglycerol levels, ranged from 37 for 1 to 1.2 for the anionic diazeniumdiolate 4, while DOPA effected the greatest rate enhancement, achieving 49-fold rate increases with 1 under similar conditions. The observed catalysis decreases with increase in the bulk concentration of electrolytes in the reaction media. Quantitative analysis of catalytic effects has been obtained through the application of pseudo-phase kinetic models and equilibrium binding constants at different liposome interfaces are compared. The stoichiometry of nitric oxide release from 1 and 2 in DPPG/DPPC liposome media has been obtained through oxyhemoglobin assay. DPPG=1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DOPG=1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DMPS=1,2-dimyristoyl-sn-glycero-3-[phospho-l-serine], POPS=1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine], DOPA=1,2-dioleoyl-sn-glycero-3-phosphate; DPPC=1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DMPC=1,2-dimyristoyl-sn-glycero-3-phosphocholine, DOTAP=1,2-dioleoyl-3-trimethylammonium-propane.

Photoinduced Release of Nitroxyl and Nitric Oxide from Diazeniumdiolates†

Aqueous photochemistry of diazen-1-ium-1,2,2-triolate (Angeli's anion) and (Z)-1[N-(3-aminopropyl)-N-(3-aminopropyl)amino]diazen-1-ium-1,2-diolate (DPTA NONOate) has been investigated by laser kinetic spectroscopy. In neutral aqueous solutions, 266 nm photolysis of these diazeniumdiolates generates a unique spectrum of primary products including the ground-state triplet (3NO-) and singlet (1HNO) nitroxyl species and nitric oxide (NO*). Formation of these spectrophotometrically invisible products is revealed and quantitatively assayed by analyzing a complex set of their cross-reactions leading to the formation of colored intermediates, the N2O2*- radical and N3O3- anion. The experimental design employed takes advantage of the extremely slow spin-forbidden protic equilibration between 3NO- and 1HNO and the vast difference in their reactivity toward NO*. To account for the kinetic data, a novel combination reaction, 3NO-+1HNO, is introduced, and its rate constant of 6.6x10(9) M-1 s-1 is measured by competition with the reduction of methyl viologen by 3NO-. The latter reaction occurring with 2.1x10(9) M-1 s-1 rate constant and leading to the stable, colored methyl viologen radical cation is useful for detection of 3NO-. The distributions of the primary photolysis products (Angeli's anion: 22% 3NO-, 58% 1HNO, and 20% NO*; DPTA NONOate: 3% 3NO-, 12% 1HNO, and 85% NO*) show that neither diazeniumdiolate is a highly selective photochemical generator of nitroxyl species or nitric oxide, although the selectivity of DPTA NONOate for NO* generation is clearly greater.

Mechanism of pH-dependent decomposition of monoalkylamine diazeniumdiolates to form HNO and NO, deduced from the model compound methylamine diazeniumdiolate, density functional theory, and CBS-QB3 calculations

Isopropylamine diazeniumdiolate, IPA/NO, the product of the reaction of isopropylamine and nitric oxide, NO, decomposes in a pH-dependent manner to afford nitroxyl, HNO, in the pH range of 13 to above 5, and NO below pH 7. Theoretical studies using B3LYP/6-311+G(d) density functional theory, the polarizable continuum and conductor-like polarizable continuum solvation models, and the high-accuracy CBS-QB3 method on the simplified model compound methylamine diazeniumdiolate predict a mechanism involving HNO production via decomposition of the unstable tautomer MeNN+(O-)NHO-. The production of NO at lower pH is predicted to result from fragmentation of the amide/NO adduct upon protonation of the amine nitrogen.

Dendrimers as a Scaffold for Nitric Oxide Release

The preparation and characterization of nitric oxide (NO)-releasing dendrimer conjugates are reported. Generation 3 and 5 polypropylenimine dendrimers (DAB-Am-16 and DAB-Am-64) were modified at the exterior to impart different amine functionalities. The ability to store NO on a dendritic scaffold using N-diazeniumdiolate NO donors was examined via the reaction of primary amine, secondary amine, and amide functionalities with high pressures of NO (5 atm). The secondary amine dendrimer conjugates exhibited a high storage capacity for NO (up to 5.6 micromol NO/mg), greatly increasing the "payload" of released NO over existing macromolecular NO donors. The mechanism of diazeniumdiolate decomposition was proton initiated, generating NO spontaneously under physiological conditions (pH 7.4, 37 degrees C). The NO release durations (>16 h) observed for the secondary amine dendrimers were significantly longer compared to small molecule alkyl secondary amine diazeniumdiolates, thus illustrating a dendritic effect on NO release kinetics. The multivalent exterior of dendrimers allows for the future combination of NO donors and other functionalities on a single molecular scaffold, enabling diverse utility as NO storage/delivery systems.

Comparison of the NO and HNO donating properties of diazeniumdiolates: primary amine adducts release HNO in Vivo

Diazeniumdiolates, more commonly referred to as NONOates, have been extremely useful in the investigation of the biological effects of nitric oxide (NO) and related nitrogen oxides. The NONOate Angeli's salt (Na(2)N(2)O(3)) releases nitroxyl (HNO) under physiological conditions and exhibits unique cardiovascular features (i.e., positive inotropy/lusitropy) that may have relevance for pharmacological treatment of heart failure. In the search for new, organic-based compounds that release HNO, we examined isopropylamine NONOate (IPA/NO; Na[(CH(3))(2)CHNH(N(O)NO]), which is an adduct of NO and a primary amine. The chemical and pharmacological properties of IPA/NO were compared to those of Angeli's salt and a NO-producing NONOate, DEA/NO (Na[Et(2)NN(O)NO]), which is a secondary amine adduct. Under physiological conditions IPA/NO exhibited all the markers of HNO production (e.g., reductive nitrosylation, thiol reactivity, positive inotropy). These data suggest that primary amine NONOates may be useful as HNO donors in complement to the existing series of secondary amine NONOates, which are well-characterized NO donors.

Synthesis of Diazeniumdiolates from the Reactions of Nitric Oxide with Enolates

[reaction: see text] Reactions of nitric oxide with enolates derived from aliphatic methyl ketones containing alpha-methylene or alpha-methine groups and with enolates derived from alpha,alpha'-dimethylene or alpha,alpha'-dimethine ketones yield mono- or bis(diazeniumdiolate) products. Diazeniumdiolation occurs in the following order: alpha-methine > alpha-methylene > alpha-methyl. The amount of the base used alters the extent of diazeniumdiolation and the course of the reaction. Mono- and bis(diazeniumdiolate)-substituted methyl ketones are cleaved in the presence of excess base before and after the subsequent diazeniumdiolation of the alpha-methyl group. Similar to the trihalogenated methyl groups in the base-assisted halogenation reactions of methyl ketones, the bis(diazeniumdiolate)-substituted alpha-methylene and alpha-methyl groups act as leaving groups in the presence of excess base. The reaction of nitric oxide with a (approximately 20:80, cis/trans) mixture of 2,6-cyclohexananone yields the cis and trans isomers of 2,6-dimethylcyclohexanone-2,6-bis(diazeniumdiolate) in 12.9% and 57.6% yield. Single-crystal X-ray diffraction data determined for potassium cis-2,6-dimethylcyclohexanone-2,6-bis(diazeniumdiolate), cis-14b, reveal that the N(2)O(2-) substituent is planar with considerable delocalization of a double bond over the anionic four-atom group. Except for one of the diazeniumdiolate products, namely, potassium propanoate 2,2-bis(diazeniumdiolate), 8b, all are stable in neutral and basic aqueous media. Compound 8b slowly decomposes in neutral aqueous solution releasing nitrous oxide and nitric oxide gases but is stable in basic aqueous media. Differential scanning calorimetry data measured for the diazeniumdiolate products indicate that they decompose exothermally with most of them undergoing explosive decomposition at moderately high temperatures (181-274 degrees C).

E/Z conformation and the vibrational spectroscopy of Me2NN(O)=NOMe

The simple neutral diazenium diolate, O2-methyl-1-(N, N-dimethylamino)diazen-1-ium-1,2-diolate, [Me2NN(O)=NOMe], was experimentally examined by vibrational spectroscopy and the results compared to the theoretically calculated values in an effort to detect both Z and E conformers which result from the stereochemistry of the N=N multiple bond. Room-temperature Raman and infrared spectra were measured and the results compared with the values calculated theoretically with MP2 and density functional techniques (B3LYP). An analysis of the observed frequencies suggests that, down to a detection limit of about 1/1000, only a small quantity of trans (E) diazeniumdiolate, <0.05%, may be present at room temperature.

Effect of hydrophobic structure on the catalysis of nitric oxide release from zwitterionic diazeniumdiolates in surfactant and liposome media

The effect of phospholipid liposomes and surfactant micelles on the rate of nitric oxide release from zwitterionic diazeniumdiolates, R1R2N[N(O)NO]-, with significant hydrophobic structure, has been explored. The acid-catalyzed dissociation of NO has been examined in phosphate-buffered solutions of sodium dodecylsulfate (SDS) micelles and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-[phospho-(1-glycerol)] sodium salt (DPPG) phospholipid liposomes. The reaction behavior of dibenzylamine-, monobenzylamine-, and dibutylamine-derived substrates [1]: R1 = C6H5CH2, R2 = C6H5CH2 NH2+(CH2)2, 2: R1 = C6H5CH2, R2 = NH3+(CH2)2, and 3: R1 = n-butyl, R2 = n-butyl-NH2+(CH2)6] has been compared with that of SPER/NO, 4: R1 = H2N(CH2)3, R2 = H2N(CH2) 3NH2+(CH2)4]. Catalysis of NO release is observed in both micellar and liposome media. Hydrophobic interactions contribute to micellar binding for 1-3 and appear to be the main factor facilitating catalysis by charge neutral DPPC liposomes. Binding constants for the association of 1 and 3 with SDS micelles were 3-fold larger than those previously obtained with comparable zwitterionic substrates lacking their hydrophobic structure. Anionic DPPG liposomes were much more effective in catalyzing NO release than either DPPC liposomes or SDS micelles. DPPG liposomes (at 10 mM total lipid) induced a 30-fold increase in the NO dissociation rate of SPER/NO compared to 12- and 14-fold increases in that of 1 and 3.

Chemistry of the Diazeniumdiolates: Z ⇌ E Isomerism

Here, we explore the chemistry of the previously undocumented E form of diazeniumdiolates having the structure R(1)R(2)NN(O)=NOR(3). Reported crystallographic studies have uniformly revealed the Z configuration, and our attempts to observe a Z --> E conversion through thermal equilibration or photochemical means have, until now, consistently failed to reveal a significant amount of a second conformer. As a typical example, the NMR spectrum of trimethyl derivative Me(2)NN(O)=NOMe revealed no evidence for a second configuration. Electronic structure calculations attribute this finding to a prohibitively high interconversion barrier of approximately 40 kcal/mol. A similar result was obtained when we considered the case of R(1) = Me = R(3) and R(2) = H at the same levels of theory. However, when MeHNN(O)=NOMe was ionized by dissociating the N-H bond, the barrier was calculated to be lower by approximately 20 kcal/mol, with the E form of the anion being favored over Z. This circumstance suggested that an E isomer might be isolable if a Z anion were formed and given sufficient time to assume the E configuration, then quenched by reaction with an electrophile to trap and neutralize the E form and restore the putatively high interconversion barrier. Consistent with this prediction, basifying iPrHNN(O)=NOCH(2)CH(2)Br rapidly led to a six-membered heterocycle that was crystallographically characterized as containing the -N(O)=NO- functional group in the E configuration. The results suggest an approach for generating pairs of Z and E diazeniumdiolates for systematic comparison of the rates at which the individual isomers release bioactive NO and of other physicochemical determinants of their biomedical utility.

Quantum Mechanical Determinations of Reaction Mechanisms, Acid Base, and Redox Properties of Nitrogen Oxides and Their Donors

This chapter reviews computational methods based on quantum mechanics and commonly used commercial programs for the exploration of chemical phenomena, particularly in the field of nitrogen oxides. Examples from the literature are then used to demonstrate the application of these methods to the chemistry and biochemistry of various nitrogen oxides. These examples include determining reaction mechanisms using computed reaction energies, predicting rates of reactions using transition state theory, and determining chemical properties such as hydration equilibria, pKa's, and reduction potentials.

Controlled photochemical release of nitric oxide from O2-substituted diazeniumdiolates

Diazeniumdiolates are a well-established class of nitric oxide (NO) donors that have been employed in a wide variety of biochemical and pharmacological investigations. To provide a means of targeting NO release, photosensitive precursors to diazeniumdiolates have been developed and are reviewed here. After a brief description of diazeniumdiolate chemistry and the potential uses of photosensitive precursors to NO, three different classes of phototriggered diazeniumdiolates are discussed: 2-nitrobenzyl derivatives, meta-substituted benzyl derivatives, and naphthylmethyl and naphthylallyl derivatives. In addition, the photochemistry of diazeniumdiolate salts themselves is covered.

Mechanisms of HNO and NO production from Angeli's salt: density functional and CBS-QB3 theory predictions

The mechanism of decomposition of Angeli's salt, Na(2)N(2)O(3), was explored with B3LYP and CBS-QB3 computational methods. Angeli's salt produces both nitroxyl (HNO) and nitric oxide (NO), depending upon the pH of the solution. These calculations show that protonation on N(2), while less favorable than O protonation, leads spontaneously to HNO production, while diprotonation at O(3) leads to NO generation. K(a) values for protonation at different centers and rate constants have been found which reproduce experimental data satisfactorily.