Gas-phase fragmentation of long-lived cysteine radical cations formed via no loss from protonated S-nitrosocysteine

In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with S-nitrosoglutathione followed by transfer of the resulting S-nitrosocysteine into the gas phase by electrospray ionization mass spectrometry (ESI-MS). Independent of the way it was formed, protonated S-nitrosocysteine readily fragments via bond homolysis to form a long-lived radical cation of cysteine (Cys^•+), which fragments under collision-induced dissociation (CID) conditions via losses in the following relative abundance order: •COOH ≫ CH₂S > •CH₂SH-H₂S. Deuterium labeling experiments were performed to study the mechanisms leading to these pathways. DFT calculations were also used to probe aspects of the fragmentation of protonated S-nitrosocysteine and the radical cation of cysteine. NO loss is found to be the lowest energy channel for the former ion, while the initially formed distonic Cys^•+ with a sulfur radical site undergoes proton and/or H atom transfer reactions that precede the losses of CH₂S, •COOH, •CH₂SH, and H₂S.     

A new reaction pathway in the ester aminolysis catalyzed by glymes and crown ethers

Butylaminolysis of p-nitrophenyl acetate in chlorobenzene in the presence of different kinds of phase-transfer catalysts (crown ethers and glymes) supports the existence of a reaction pathway exhibiting a first-order dependence on the concentration of the phase transfer catalyst and a second-order dependence on the concentration of butylamine. This novel reaction pathway must be included in the mechanism traditionally accepted for the catalysis by phase-transfer agents of aminolysis reactions in aprotic solvents.     

Reaction of hydroxyl radicals with S-nitrosothiols: determination of rate constants and end product analysis

The reaction of the hydroxyl radical (.OH) with S-nitroso derivatives of cysteine, acetylcysteine and glutathione was studied at neutral and acidic pH. The second-order rate constants were determined by a competition kinetic method using a deoxyribose-thiobarbituric acid assay. The rate constants were diffusion controlled and were 2.27, 1.94 and 1.46 x 10(10) dm3 mol-1 s-1, for S-nitrosocysteine, S-nitrosoacetylcysteine and S-nitrosoglutathione respectively, at neutral pH. The major products of the degradation induced by .OH were found to be the corresponding disulfide (-S-S-) and nitrite (NO2-) at neutral pH as well as at pH 3. Simultaneous proton formation has also been observed. A plausible mechanism based on the formation of an intermediate thiol radical (RS.), as a result of electron transfer from the S-nitrosothiols (RSNOs) to .OH, is proposed for the formation of disulfide and nitrite at both pHs. The high rate constant values and the degradation of these compounds demonstrate the potential role of .OH in RSNO metabolism under physiological conditions.     

Reactivity of sulfur nucleophiles with N-methyl-N-nitroso-p-toluenesulfonamide

The transfer of the nitroso group from N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) to cysteine (CYS) and 2-aminoethanethiol (AET) has been studied in a pH range between pH = 7 and pH = 13. Kinetic results clearly indicate that both nucleophiles react through the corresponding thiolate to give the corresponding nitrosothiol. The existence of two (AET) or three (CYS) macroscopic acidity constants has been kinetically evidenced and the nitrosation rates of the corresponding bases have been identified. Nitrosation rate constants of the different species present in the reaction medium have been determined and a Bronsted-type plot has been established giving a beta(nuc) value approximately equal to 0.08 clearly different from the values of beta(nuc) approximately equal to 0.7 obtained in the nitrosation of primary and secondary amines by MNTS. The low beta(nuc) value has been attributed to the need for previous desolvation of the nucleophile.     

Nitroso group transfer from substituted N-methyl-N-nitrosobenzenesulfonamides to amines. Intrinsic and apparent reactivity.

We have studied the nitroso group transfer from substituted N-methyl-N-nitrosobenzenesulfonamides to primary and secondary amines, observing that the rate of the reaction increases as a consequence of the presence of electron withdrawing groups on the aromatic ring of the nitrosating agents. The rate constants determined for the nitroso group transfer, ktr, give good Bronsted-type relationships between log ktr (rate constant for nitroso group transfer) and pKaR2NH2+ and pKaleaving group. The study of the nitrosation processes of secondary amines catalyzed by ONSCN and denitrosation catalyzed by SCN-, in combination with the formation equilibrium of ONSCN, has enabled us to calculate the value of the equilibrium constant for the loss of the NO+ group from a protonated N-nitrosamine (pKNOR2N+HNO), which can be defined by analogy with pKaR2NH2+. The value of pKNOX-NO for the loss of the NO+ group from an N-methyl-N-nitrosobenzenesulfonamide was obtained in a similar way. By using values of delta pKNO = pKNOR2N+HNO - pKNOX-NO, we were able to calculate the equilibrium constant for the nitroso group transfer and characterize the transition state. On the basis of Bronsted-type correlations, we have obtained values of beta nuclnorm and alpha lgnorm approximately equal to 0.55, showing a perfectly balanced transition state. In terms of the Marcus theory, the calculation of the intrinsic barriers for the nitroso group transfer reaction shows that the presence of electron withdrawing groups on the aromatic ring of the N-methyl-N-nitrosobenzenesulfonamides does not cause these barriers to vary.     

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.     

S-nitrosocysteine and cystine from reaction of cysteine with nitrous acid. A kinetic investigation

The formation of the S-nitrosocysteine (CySNO) in aqueous solution starting from cysteine (CySH) and sodium nitrite is shown to strongly depend on the pH. Experiments conducted within the pH range 0.5-7.0 show that at pH below 3.5 the NO+ (or H2NO 2 +) is the main nitrosating species, while at higher pH (>3.5) the nitrosating species is most likely the N2O3. A kinetic study provided a general kinetic equation, V(CySNO) = k1[HNO2][CySH]eq [H+] + k2[HNO2]2. The first term of this equation is predominant at pH lower than 3.5, in agreement with the literature for the direct nitrosation of thiols with nitrous acid; the value for the third-order rate constant, k(1) = 7.9 x 10(2) L(2) mol(-2) min(-1), was calculated. For experiments at pH higher than 3.5, the second term becomes prevalent and the second-order rate constant k(2) = (3.3 +/- 0.1) x 10(3) L mol(-1) min(-1) was calculated. A competitive oxidation process leading to the direct formation of cystine (CySSCy) has been also found. Most likely also for this process two different mechanisms are involved, depending on the pH, and a general kinetic equation, V(CySSCy) = k3[CySH](eq)[HNO2][H+] + k3'[CySH]eq[HNO2], is proposed.     

[Fe2(μ-SC5H4N)2(NO)4] as a New Potential NO Donor: Synthesis,Structure, and Properties

A new potential donor of nitrogen monoxide, a binuclear iron sulfur nitroso complex, was prepared by exchange reaction of Na₂Fe₂(S₂O₃)₂(NO)₄ with pyridine-2-thiol in the presence of sodium thiosulfate at pH 12. The molecular and crystal structures of [Fe₂(-SC₅H₄N)₂(NO)₄] were studied by X-ray diffraction analysis. The type of iron coordination by pyridine-2-thiol in the presence of a coordinated NO molecule was determined. In vacuum, the structure of the complex is destroyed, which is accompanied by NO evolution, while exposure to UV radiation results in decomposition of the complex and in a release of N₂O.     

The kinetics of ozone decomposition in water,the influence of pH and temperature

The influence of pH on the kinetics of ozone decomposition in water was studied. Over the pH range 1–8, the kinetics was well described by a second-order reaction equation. Starting with approximately pH 3, the pH dependence of the logarithm of the constant at 19°C corresponded to the equation logk= −2.66 + (0.49 ± 0.03)pH. The activation energy of ozone decomposition was found to be 76.0 ± 8.3 kJ/mol.     

Reduction of nitrates,the no donors,by hemoglobin in the presence of cysteine

Hemoglobin (Hb) reduces 3,3-bis(nitroxymethyl)oxetane (NMO) only in the presence of cysteine (Cys) via intermediate cysteine thionitrate. The kinetics of the reaction of NMO with Cys and the kinetics and mechanism of formation of NO in the ternary system Hb-NMO-Cys were studied. The formation rate of Hb-NO in the ternary system is higher than that of Hb-NO in the reaction of Hb only with NO ₂ ⁻ generated in the binary system NMO-Cys. The second-order rate constants of the main reaction steps in the system Hb-NMO-Cys were estimated by simulating the kinetics of the reactions with a system of equations taking into account equilibria between all components of the reaction mixture. Hemoglobin reduces cysteine thionitrate, the intermediate in the reaction of NMO with Cys, to NO. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 725–731, April, 2007.     

15N CIDNP investigations of the peroxynitric acid nitration of L-tyrosine and of related compounds

Peroxynitric acid (O2NOOH) nitrates L-tyrosine and related compounds at pH 2-5. During reaction with O2(15)NOOH in the probe of a 15N NMR spectrometer, the NMR signals of the nitration products of L-tyrosine, N-acetyl-L-tyrosine, 4-fluorophenol and 4-methoxyphenylacetic acid appear in emission indicating a nitration via free radicals. Nuclear polarizations are built up in radical pairs [15NO2* , PhO*]F or [15NO2* , ArH*+]F formed by diffusive encounters of 15NO2 with phenoxyl-type radicals PhO or with aromatic radical cations ArH*+. Quantitative 15N CIDNP investigations with N-acetyl-L-tyrosine and 4-fluorophenol show that the radical-dependent nitration is the only reaction pathway. During the nitration reaction, the 15N NMR signal of 15NO3- also appears in emission. This is explained by singlet-triplet transitions in radical pairs [15NO2* , 15NO3*]S generated by electron transfer between O2(15)NOOH and H15NO2 formed as a reaction intermediate. During reaction of peroxynitric acid with ascorbic acid, 15N CIDNP is again observed in the 15N NMR signal of 15NO3- showing that ascorbic acid is oxidized by free radicals. In contrast to this, O2(15)NOOH reacts with glutathione and cysteine without the appearance of 15N CIDNP, indicating a direct oxidation without participation of free radicals.     

S-nitrosocaptopril formation in aqueous acid and basic medium. A vasodilator and angiotensin converting enzyme inhibitor

The reaction of S-nitrosocaptopril (NOcap) formation was studied in both aqueous acid and basic medium. Captopril (cap) reacts rapidly with nitrous acid in strong acid medium to give the stable--in the timescale of the experiments--NOcap. The kinetic study of the reaction involving the use of stopped-flow, shows that at low sodium nitrite (nit) concentration, the reaction is first-order in both [nit], [H(+)], and is strongly catalysed by Cl(-) or Br(-) (= X(-)): rate = (k(3) + k(4)[X(-)])[H(+)][nit][cap]. In aqueous buffered solution of acetic acid-acetate the reaction rate is much slower and the decomposition of NOcap was observed; however, the rate of NOcap decay is more than 30-fold slower than its formation. In aqueous basic medium of carbonate-hydrogen carbonate buffer, as well as in alkaline medium, the kinetics of the nitroso group (NO) transfer from tert-butyl nitrite (tBN) to cap was studied using either conventional or stopped flow methods. In mild basic medium, the NOcap decomposes. The NOcap formation is first-order in both tBN and cap concentrations, and the reaction rate increases with pH until to, approximately, pH 11.5, above which value it becomes pH independent or even invariable with the [OH(-)]. Kinetic results show that the thiolate ion of cap is the reactive species. In fact, the presence of anionic micelles of sodium dodecyl sulfate (SDS) inhibits the reaction due to the separation of the reagents; whereas, cationic micelles of tetradecyltrimethylammonium bromide (TTABr) catalyse the reaction at low surfactant concentration due to reagents concentration in the small volume of the micelle. The rate equation is: rate = k(f) K(SH)[cap][tBN]/(K(SH) + [H(+)]). The rate of NOcap decomposition in mild basic medium is first-order in both [cap] and [NOcap], and decreases on increasing pH; but, in alkaline medium the NOcap is stable within the timescale of the experiments. Based on the results, the NOcap decomposition yields the disulfide compound that is formed in the nucleophilic attack of the -SH group of cap to the sulfur electrophilic center of NOcap, -S-N=O. The resulting rate equation is: rate = k(d)[H(+)][cap][NOcap]/(K(SH) + [H(+)]).     

Catalytic effect of copper on the hexacyanoferrate(III)-cyanide redox reaction-I The uncatalysed and catalysed reactions

A kinetic study of the hexacyanoferrate(III)-cyanide redox reaction has been made in connection with development of a new catalytic method for copper. The reaction kinetics change with time from first- to second-order dependence with respect to hexacyanoferrate(III). The reaction is nearly inverse first-order with respect to hexacyanoferrate(II) and first-order with respect to cyanide. The reaction shows a strong positive primary salt effect, but a very small increase in the reaction rate with temperature is found. A parallel reaction proceeds with a first-order dependence with respect to hydroxide. A tentative mechanism is proposed for the first reaction, involving the formation of cyanogen radicals. The second reaction corresponds to the well-known decomposition of hexacyanoferrate(III) in alkaline medium. The catalysed reaction exhibits similar kinetics with respect to hexacyanoferrate(II) and (III) but is zero-order with respect to cyanide and hydroxide and first-order with respect to catalyst. The proposed mechanism involves two consecutive interactions of the hexacyanoferrate(III) with copper(I) and with copper(II) cyanide complexes respectively, followed by a 2-electron oxidation of a co-ordinatively bridging cyanide group.     

General base and general acid catalyzed intramolecular aminolysis of esters. Cyclization of esters of 2-aminomethylbenzoic acid to phthalimidine

Plots of log k(0) vs pH for the cyclization of trifluoroethyl and phenyl 2-aminomethylbenzoate to phthalimidine at 30 degrees C in H(2)O are linear with slopes of 1.0 at pH >3. The values of the second-order rate constants k(OH) for apparent OH(-) catalysis in the cyclization reactions are 1.7 x 10(5) and 5.7 x 10(7) M(-)(1) s(-)(1), respectively. These rate constants are 10(5)- and 10(7)-fold greater than for alkaline hydrolysis of trifluoroethyl and phenyl benzoate. The k(OH) for cyclization of the methyl ester is 7.2 x 10(3) M(-)(1) s(-)(1). Bimolecular general base catalysis occurs in the intramolecular nucleophilic reactions of the neutral species. The value of the Bronsted coefficient beta for the trifluoroethyl ester is 0.7. The rate-limiting step in the general base catalyzed reaction involves proton transfer in concert with leaving group departure. The mechanism involving rate-determining proton transfer exemplified by the methyl ester in this series (beta = 1.0) can then be considered a limiting case of the concerted mechanism. General acid catalysis of the neutral species reaction or a kinetic equivalent also occurs when the leaving group is good (pK(a) 相似文献   

Hydrophobic N‐Diazeniumdiolates and the Aqueous Interface of Sodium Dodecyl Sulfate (SDS) Micelles

Zwitterionic diazeniumdiolates of the form RN[N(O)NO^?](CH₂)₂NH₂⁺R, where R=CH₃ ( 1 ), (CH₂)₃CH₃ ( 2 ), (CH₂)₅CH₃ ( 3 ), and (CH₂)₇CH₃ ( 4 ) were synthesized by reaction of the corresponding diamines with nitric oxide. Spectrophotometrically determined pK_a(O) values, attributed to protonation at the terminal oxygen of the diazeniumdiolate group, show shifts to higher values in dependence of the chain lengths of R. The pH dependence of the decomposition of NO donors 1 – 3 was studied in buffered solution between pH 5 and 8 at 22 °C, from which pK_a(N) values for protonation at the amino nitrogen, leading to release of NO, were estimated. It is shown that the decomposition of these diazeniumdiolates is markedly catalyzed by anionic SDS micelles. First‐order rate constants for the decay of 1 – 4 were determined in phosphate buffer pH 7.4 at 22 °C as a function of SDS concentration. Micellar binding constants, K_SM, for the association of diazeniumdiolates 1 – 3 with the SDS micelles were also determined, again showing a significant increase with increasing length of the alkyl side chains. The decomposition of 1 – 3 in micellar solution is quantitatively described by using the pseudo‐phase ion‐exchange (PIE) model, in which the degree of micellar catalysis is taken into account through the ratio of the second‐order rate constants (k_2m/k_2w) for decay in the micelles and in the bulk aqueous phase. The decay kinetics of 1 – 3 were further studied in the presence of cosolvents and nonionic surfactants, but no effect on the rate of NO release was observed. The kinetic data are discussed in terms of association to the micelle–aqueous phase interface of the negatively charged micelles. The apparent interfacial pH value of SDS micelles was evaluated from comparison of the pH dependence of the first‐order decay rate constants of 2 and 3 in neat buffer and the rate data obtained for the surfactant‐mediated decay. For a bulk phase of pH 7.4, an interfacial pH of 5.7–5.8 was determined, consistent with the distribution of H⁺ in the vicinity of the negatively charged micelles. The data demonstrate the utility of 2 and 3 as probes for the determination of the apparent pH value in the Stern region of anionic micelles.     

Novel catalytic effects in ester aminolysis in chlorobenzene

The mechanism of glyme catalyzed ester aminolysis in chlorobenzene should be modified by including a new reaction pathway that shows a first-order dependence on the concentration of the phase transfer catalyst and a second-order dependence on butylamine.     

Photochemical Activities of N‐Nitroso Carboxamides and Sulfoximides and Their Application to DNA Cleavage

N‐Nitroso compounds containing benzene, fluorene or fluorenone rings were synthesized. Photolysis of these compounds with 312‐nm UV light provided the NO ^. species, the presence of which was corroborated by use of an EPR method and of 2‐phenyl‐4,4,5,5‐tetramethylimidazolin‐1‐oxyl 3‐oxide (PTIO) as a trapping agent. During irradiation of N‐methyl‐N‐nitroso‐9‐fluorenone carboxamide ( 14 c ) in the absence of PTIO, it underwent decomposition followed by recombination to give the heterocyclic nitric oxide radical 15 . Incorporation of intercalating moieties endowed the N‐nitroso compounds with DNA‐cleaving ability through single‐strand scission upon UV irradiation in a phosphate buffer (pH 5.0–8.0) under aerobic conditions.     

Removal of copper by oxygenated pyrolytic tire char: kinetics and mechanistic insights

The kinetics of copper ion (Cu(II)) removal from aqueous solution by pyrolytic tire char was modeled using five different conventional models. A modification to these models was also developed through a modified equation that accounts for precipitation. Conventional first- and second-order reaction models did not fit the copper sorption kinetics well, indicating a lack of simple rate-order dependency on solute concentration. Instead, a reversible first-order rate reaction showed the best fit to the data, indicating a dependence on surface functional groups. Due to the varying solution pH during the sorption process, modified external and internal mass transfer models were employed. Results showed that the sorption of copper onto oxygenated chars was limited by external mass transfer and internal resistance with and without the modification. However, the modification of the sorption process produced very different results for unoxygenated chars, which showed neither internal nor external limitation to sorption. Instead, its slow sorption rate indicates a lack of surface functional groups. The sorption of Cu(II) by oxygenated and unoxygenated chars was also found to occur via three and two distinct stages, respectively.     

Theoretical evidence that Cu(I) complexation promotes degradation of S-nitrosothiols

The degradation of S-nitrosothiols (RSNOs) to release NO is believed to be catalyzed by CuI ions, but the mechanism remains unclear. Kinetic experiments have shown that decomposition rates vary significantly with the chemical nature of the RSNO considered. On the basis of first-principles calculations, the catalytic role of CuI ion is investigated for the decomposition of S-nitrosocysteine and its N-acetylated and ethyl ester derivatives, and for S-nitrosohomocysteine. This preliminary study focuses on the CuI-RSNO intermediates involved in the decomposition pathway. The model chemistry has been validated by comparing calculated CuI-ligand binding energies and S-N bond homolysis energies with available experimental data. Calculations show that the formation of CuI-RSNO intermediates results in weakening of the S-N bond and strengthening of the N-O bond, which would promote S-N bond breaking and NO release from S-nitrosothiols.     

The outer-sphere oxidation of nitrosyliron(II)hemoglobin by peroxynitrite leads to the release of nitrogen monoxide

It has been suggested that nitrosyliron(II)hemoglobin may represent a form of stabilized NO. and may be responsible for NO. delivery in the peripheral circulation. In this work, we show that NO. can be released from nitrosyliron(II)hemoglobin through reaction with peroxynitrite. Outer-sphere oxidation of the iron center generates nitrosyliron(III)hemoglobin, from which NO. dissociates at a rate of ca. 1 s(-1). The second-order rate constant for the reaction of peroxynitrite with nitrosyliron(II)hemoglobin is (6.1 +/- 0.3) x 10(3) M(-1) s(-1) (at pH 7.2 and 20 degrees C). In the presence of 1.2 mM CO(2), the rather large value of the second-order rate constant, (5.3 +/- 0.2) x 10(4) M(-1) s(-1) (at pH 7.2 and 20 degrees C), indicates that this reaction may take place in vivo. The reactive nitrogen species generated from this reaction, N(2)O(3) and/or NO(2), may lead to protein modifications, such as nitration of tyrosine and/or tryptophan residues and nitrosation of cysteine residues.