B. L. Psikha

Influence of ligand lipophilicity on the NO-donating ability of the binuclear tetranitrosyl iron complexes in an erythrocyte suspension

The kinetics of nitric oxide release by representatives of a new class of synthetic nitric oxide donors, binuclear tetranitrosyl iron complexes (B-TNIC) with a structure of [Fe2(SR)2(NO)4], where R is pyrimidin-2-yl, 1-methylimidazol-2-yl, benzothiazol-2-yl, cysteamine, and penicillamine, was studied under the conditions simulating the blood vessel internal environment, when an erythrocyte suspension was used as a natural trap of nitric oxide. The apparent firstorder rate constants for intraerythrocyte methemoglobin formation were determined on the basis of the kinetic data. The NO-donating ability of B-TNIC was estimated from the values of these rate constants. It is assumed that the rate for the hydrolytic decomposition of B-TNIC decreases in the presence of erythrocytes due to binding of the complexes with the cell surface, leading to the restriction of their contact with an aqueous medium. Nitric oxide donating by the complex bearing penicillamine as a ligand is stoichiometric and independent of the presence of erythrocytes. This can be due to high hydrophilicity of this complex determined from the criterion of partition coefficient in an octanol-water system. This evidently provides a high level of solvation of the complex in an aqueous medium that prevents its binding with the erythrocyte surface.

Assessment of the competitiveness of the radical and nonradical mechanisms in the acid-catalyzed oxidation of styrene epoxide

The oxidizability value of styrene epoxide as the ratio k2/√k6, where k2 and k6 are the propagation and termination rate constants of the epoxide oxidation chain, respectively, was calculated from experimental data on the oxidation of neat styrene epoxide with oxygen in the presence of the initiator dicumyl peroxide in a quartz reactor of a manometric unit. Alternative calculations of the oxidizability value with the use of two procedures and two experimental setups yielded close values of k2/√k6 = 0.333exp(−22.5 kJ mol−1/RT) and k2/√k6 = 0.18exp(−22.1 kJ mol−1/RT)(l/(mol s))1/2 (393–413 K), which characterize a quite high stability of styrene epoxide toward oxidation by peroxide radicals. With allowance for the new data, it was concluded that the radical mechanism is unfeasible under the conditions of the earlier-studied mild acid-catalyzed oxidation of styrene epoxide (333–348 K).

Effect of polar solvents on the conversion of styrene epoxide in the presence of p-toluenesulfonic acid

The addition of tert-butyl alcohol to a styrene epoxide and catalyst (p-toluenesulfonic acid) solution in acetonitrile accelerates the acid-catalyzed total conversion of epoxide and simultaneous oxygen absorption by this binary system (BS). The introduction of water into the alcohol solution of BS increases the oxidation rate to an even greater extent. Experiments with the indicator m-nitroaniline showed that the addition of alcohol and water decreased the deprotonating ability, i.e., the acidity of the solution. The attempt was made to rationalize the paradoxical situation, at which the decline in acidity is accompanied by enhancement of the acidcatalyzed reaction of the epoxide conversion and oxidation, in terms of the participation of the protonated alcohol in the intermediate complex with epoxide.

Nitric oxide donation by the binuclear tetranitrosyl iron complexes in the presence of erythrocytes

The kinetic regularities of nitric oxide donation in the presence of erythrocytes by representatives of a new class of synthetic nitric oxide donors, binuclear tetranitrosyl iron complexes (B-TNIC) [Fe2(SR)2(NO)4] with thiol-containing ligands, where R is pyrimidin-2-yl, 1-methylimidazol-2-yl, benzothiazol-2-yl, and penicillamine residue, were established. The NO-donating ability of B-TNIC was estimated from the apparent rate constants of the first order for the formation of intraerythrocyte methemoglobin with the variation of the initial concentration of the complex. During the standard experimental time (15—17 min), three of the four complexes released in the solution no more than a quarter of available NO groups. Their NO-donating ability turned out to be variable increasing with an increase in the initial concentration of the complex. At the same time, the complex bearing penicillamine residues as ligands donated almost all NO groups within approximately 1 min. Features of NO donation by the B-TNIC in the presence of erythrocytes are related to the formation in the system of an additional equilibrium pool of the membrane-associated complex, which is characterized by a reduced rate of hydrolytic dissociation with NO releasing to the solution. The NO-donating ability of the B-TNIC in the presence of erythrocytes is determined by the ratio of volumes of the free and membrane-associated pools of the complex.

Kinetic regularities and reaction products of adenosine triphosphoric acid with the thiosulfate-nitrosyl iron complex

The kinetic characteristics of the reaction of adenosine triphosphoric acid (ATP) with the thiosulfate-nitrosyl iron complex (TNCI, donor of nitrogen monoxide) were obtained by the spectrofluorimetric method. The shape of the kinetic curves indicate a complicated mechanism of interaction between ATP and TNCI. The kinetic model for this process was proposed that satisfactorily describes the experimental data. The compositions and structures of the reaction products were determined. Iron in the reaction products is presented by two forms: FeII and FeIII. The structures for the [ATP-Fe2+S] and [ATP-Fe3+S] complexes were proposed.

Revealing of the cation-binding sites on the surface of hemoglobin in its reaction with the NO donor, the nitrosyl iron complex {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O

Deoxyhemoglobin (Hb) stabilizes the cationic nitrosyl iron complex with cysteamine {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O (CysAm), by slowing down its hydrolysis. In the absence of Hb, the electrochemical detection of NO release in the course of the hydrolysis using a sensor electrode gave the rate constant of (5.2±0.2)·10−5 s−1. The release of NO is a reversible process, and the amount of released NO is 1.4% of the CysAm concentration. In the presence of Hb, NO is released much more slowly, and the reaction is more intense than that in the absence of Hb. The adsorption of CysAm by an Hb molecule results in NO release from the CysAm-Hb complex with a rate constant of 1·10−8 s−1. The analysis of the Hb surface revealed the possible location of the cation-binding sites, which reversibly bind the cationic CysAm complex. The kinetic parameters of NO release from CysAm in the absence and in the presence of Hb were studied by the kinetic modeling.

Effect of hydrogenation on the oxidation resistance of decene oligomers

The kinetics of autoxidation and initiated oxidation of PAO-2, PAO-4, PAO-6, and PAO-10+ decene oligomer fractions at 140°C after a hydrogenation process was studied. Key reactions in the mechanism of oxidation were identified; the numerical values of the kinetic parameters were determined, and mathematical models, which quantitatively described the experimental data on the consumption of oxygen and the buildup of hydroperoxides during the course of the oxidation of the test samples, were obtained. The oxidation resistances of hydrogenated fractions were compared with each other. It was found that the kinetic parameters of the test samples were similar, and the oxidation of all of the hydrogenated oligomers occurred by approximately the same processes. Using the PAO-2 fraction at 120, 130, and 140°C as an example, the oxidizability of hydrogenated and unhydrogenated samples was compared. It was found that hydrogenation improved all of the kinetic characteristics and, consequently, increased the oxidation resistance of oligomers. However, the oligomers remained readily oxidizable substances, particularly, at elevated temperatures.

Two decomposition mechanisms of nitrosyl iron complexes [Fe 2 (μ-SR)(NO) 4 ]

Two decomposition mechanisms of nitrosyl iron complexes (NICs) [Fe2(μ-SR)(NO)4] in aqueous medium are known. One mechanism (for instance, in the case of complex [Fe2(μ-SC4H3N2)2(NO)4]) involves irreversible and rapid hydrolysis of NIC with the NO release accompanied with the formation of the products of further NO transformations. In the other mechanism (for instance, in the case of complexes [Fe2(μ-S(CH2)2NH3)2(NO)4]SO4• •2.5H2O and [Fe2(μ-SC5H11NO2)2(NO)4]SO4•5H2O), no hydrolysis occurs but NICs reversibly dissociate to release both NO and thiolate ligand into the medium. In the present work, the difference in the mechanisms of the NIC decomposition is explained by the difference in the NIC redox potentials. The experimental evidences of this fact are given.

Investigation of the reaction between phosphoenolpyruvic acid and thiosulfate-nitrosyl iron complex

Reaction between the nitric oxide donor, tetranitrosyl iron complex with thiosulfate ligand (TNIC), and phosphoenolpyruvic acid (PEP) was studied. Formation of products of the reaction between PEP and TNIC was confirmed by UV-, IR-spectroscopy, and mass spectrometry methods. Also, ions with mass numbers m/z 318 and m/z 256 were identi-fied among the reaction products, belonging to the compounds [O3S2—Fe—PEP]2– and [S—Fe—PEP]2–, respectively.

Kinetics and mechanism of the oxidation of the products of decene-1 oligomerization

The kinetic characteristics of the autooxidation and initiated oxidation of decene oligomers PAO-2, PAO-4, PAO-6, and PAO-10+ at 120°C were studied. The mechanism of the oxidation of PAO-2 fraction is examined in detail using a mathematical model. The key reactions in the mechanism of PAO-2 oxidation at 120, 130, and 140°C are identified and the kinetic parameters determined. It is shown that the high oxidizability of an oligomer is determined not only by the presence of double bonds and hydroperoxides in samples, but also by high rates of chain initiation and decomposition of hydroperoxides with formation of free radicals. A comparative study of the mechanism of oxidation of a PAO-2 sample prepared using an alternative technology was carried out. It is shown that both the qualitative and quantitative parameters of the oxidation stability of the produced oligomer depend on the production technology. With the aim of enhancing oxidation stability of the initial nonhydrogenated products of oligomerization, the process of oxidation of PAO-2 sample was studied in the presence of antioxidants based on sterically hindered phenols and aromatic amines. It is demonstrated that such inhibitors can be used for anti-oxidation protection of the primary oligomerization products during their storage.