Catecholase biomimetic catalytic activity of copper(II) complexes with 2-methyl-3-amino-(3H)-quinazolin-4-one

The oxidation of catechol by molecular oxygen in the presence of a catalytic amount of copper(II) complex with 2-methyl-3-amino-(3H)quinazoline-4-one (MAQ) and various anions (Cl⁻, Br⁻, ClO ₄ ⁻ , SCN⁻, NO ₃ ⁻ and SO ₄ ^– ) was studied. The catecholase biomimetic catalytic activity of the copper(II) complexes has been determined spectrophotometrically by monitoring the oxidative transformation of catechol to the corresponding light absorbing o-quinone (Q). The rate of the catalytic oxidation reaction was investigated and correlated with the catalyst structure, time, concentration of catalyst and substrate and finally solvent effects. Addition of pyridine or Et₃N showed a dramatic effect on the rate of oxidation reaction. Kinetic investigations demonstrate that the rate of oxidation reaction has a first order dependence with respect to the catalyst and catechol concentration and obeying Michaelis–Menten Kinetics. It was shown that the catalytic activity depends on the coordination environment of the catalyst created by the nature of counter anions bound to copper(II) ion in the complex molecule and follows the order: Cl⁻ > NO ₃ ⁻ > Br⁻ > SO ₄ ^– > SCN⁻ > ClO ₄ ⁻ . To further elucidate the catalytic activity of the complexes, their electrochemical properties were investigated and the catecholase mimetic activity has been correlated with the redox potential of the Cu²⁺/Cu⁺ couple in the complexes.     

Kinetics and mechanism of base hydrolysis of chromium(III) complexes with oxalates and quinolinic acid

Base hydrolysis of [Cr(ox)₂(quin)]³⁻ (where quin²⁻ is N,O-bonded 2,3-pyridinedicarboxylic acid dianion) causes successive ligand dissociation and leads to a formation of a mixture of oligomeric chromium(III) species, known as chromates(III). The reaction proceeds through [Cr(ox)(quin)(OH)₂]³⁻ and [Cr(quin)(OH)₄]³⁻ formation. Dissociation of oxalato ligands is preceded by the opening of the Cr-quin chelate-ring at the Cr–N bond. The kinetics of the chelate-ring opening and the first oxalate dissociation were studied spectrophotometrically, within the lower energy d–d band region at 0.4–1.0 M NaOH. The pseudo-first-order rate constants (k _obs0 and k _obs1) were calculated using SPECFIT software for an A → B → C reaction pattern. Additionally, kinetics of base hydrolysis of [Cr(ox)(quin)(OH)₂]³⁻ and cis-[Cr(ox)₂(OH)₂]³⁻ were studied. The determined pseudo-first-order rate constants were independent of [OH⁻]. A mechanism is postulated that the reactive intermediate with the one-end bonded quin ligand, [Cr(ox)₂(O-quin)(OH)]⁴⁻, formed in the first reaction stage, subsequently undergoes oxalates substitution. Kinetic parameters for the chelate-ring opening and the first oxalate dissociation were determined.     

Reactivity of inorganic α-nucleophiles in the transfer of phosphoryl and phosphonyl groups

The kinetic behavior of inorganic α-nucleophiles (NH₂O⁻, HOO⁻, ClO⁻, BrO⁻, and F⁻ ions) in the transfer of phosphoryl and phosphonyl groups was studied. It was shown that ClO⁻ and BrO⁻ ions are unique α-nucleophiles, exhibiting not only supernucleophilic characteristics but also high oxidizing activity. The undoubted leaders among inorganic nucleophiles are the HOO⁻ ion and hydroxylamine. The latter cleaves substrates that are stable in water unusually quickly not only in alkaline but also in neutral and acidic media. L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, 70 R. Lyuksemburg ul., Donetsk 83114, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 4, pp. 226–232, July–August, 2000.     

Mixed-ligand chromium(III)-oxalate-pirydinedicarboxylate complexes: potential biochromium sources: kinetic studies in NaOH solutions and effect on 3T3 fibroblasts proliferation

Base hydrolysis of [Cr(ox)₂(pda)]³⁻ (where pda is N,O-bonded 2,4- and 2,5- pyridinedicarboxylic acid dianion) causes successive ligand dissociation and leads to formation of a mixture of oligomeric chromium(III) species, known as chromates(III). The main reaction path proceeds through [Cr(ox)(pda)(OH)₂]³⁻ and [Cr(pda)(OH)₄]³⁻ complexes. The kinetics of the first oxalate dissociation was studied spectrophotometrically, within the lower energy d–d band region, at 0.4–1.0 M NaOH. The character of spectroscopic changes was consistent with a consecutive reaction model, where the chelate-ring opening and the one-end bonded oxalato liberation are the first and the second reaction stages. The pseudo-first order rate constants (k _obs0 and k _obs1) were calculated using SPECFIT software for an A → B → C reaction pattern. Additionally, kinetics of base hydrolysis of [Cr(ox)₃]³⁻ were studied. The calculated rate constants were independent of [OH ⁻ ]. Kinetic parameters for the chelate-ring opening and the first oxalate dissociation were determined. Effect of the [Cr(ox)₂(pda)]³⁻ and [Cr(2,4-pda)₃]³⁻ complexes on 3T3 fibroblasts proliferation was studied. The results manifested low cytotoxicity of these complexes, which makes them promising candidates for dietary supplements.     

Non-alkane behavior of cyclopropane and its derivatives: characterization of unconventional hydrogen bond interactions

Eight cyclopropane derivatives (Δ − R) have been modeled, with R = −H, −CH₃, −NH₂, −C ≡ CH, −C ≡ CCH₃, −OH, −F and −C ≡ N. All geometries have been fully optimized at the MP2/ AUG-cc-pVTZ level of calculations. Natural bond orbital analyses reveal extra p character (sp^λ, λ > 3) in the C-C bonds of the cyclopropyl rings. The banana-like σ _CC bonds in the rings are described in detail. Alkene-like complexes between Δ − R molecules and hydrogen fluoride are identified. These weakly bonded complexes are formed through unconventional hydrogen bond interactions between the hydrogen atom in the HF molecule and the carbon–carbon bonds in the cyclopropane ring. A topological analysis of the electronic charge density and its Laplacian has been used to characterize the interactions. The possible relevance of such complexes in the modeling of substrate–receptor interactions in some anti-AIDS drugs is discussed. Contribution to the Serafin Fraga Memorial Issue.     

Intramolecular nucleophilicS N 2 substitution at the tetrahedral carbon atom: Anab initio study

Pentacoordination of carbon atom in bicyclic organic compounds of the pentalene type was studied by theab initio RHF/6-31G^** and MP2(full)/6-31G^** methods. It was shown that intramolecularS _N 2 reactions with energy barriers within the energy scale of NMR spectroscopy can occur in systems in which a linear orientation of the attacking and leaving groups is realized. The barrier to the intramolecular nucleophilic substitution reaction in 2,3-dihydro-3-formylmethylenefuran is 36.9 (RHF) and 27.7 kcal mol⁻¹ (MP2) and decreases to 16.4 and 19.4 kcal mol⁻¹, respectively, in the case of diprotonation at the O atoms in this system. For model pentalene type compounds containing electron-deficient B atoms in the ring, theab initio calculations predict a further decrease in the barrier height (down to less than 10 kcal mol⁻¹). Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1246–1256, July, 1999.     

Isomerization and decomposition of germiranes: a density functional study

The formation and decomposition pathways of germiranes (germacyclopropanes), i.e., products of reactions of the GeH₂ and GeMe₂ germylenes with ethylene, tetramethylethylene, buta-1,2,3-triene, and tetramethylbuta-1,2,3-triene, were studied using the density functional approach (PBE/TZ2P approximation). The thermodynamic stabilities of the structures under consideration were evaluated by calculating the Gibbs free energies under normal conditions (ΔG°₂₉₈). Addition of germylenes to the C=C bond can proceed as a single-step process without a barrier or involve the formation of a π-complex (the barrier to this process is lower than the sum of the energies of isolated reactants). Stability of the germiranes formed is determined by their stability to retrodecomposition into the initial germylene and olefin and to the three-membered ring opening followed by simultaneous 1,2-migration of the substituent at the Ge atom and formation of the secondary germylene. Alkyl substituents can efficiently block the opening of the three-membered ring and transformation of the cyclic structure into the secondary germylene, simultaneously decreasing the germirane stability to retrodecomposition. Decomposition into germylene and olefin under normal conditions is thermally favorable for hexamethylgermirane (ΔG°₂₉₈ = −5.7 kcal mol⁻¹), being thermally forbidden for the other germiranes studied in this work (Δ G°₂₉₈ > 0). The activation energy (E _a) for the germirane ring opening depends on the substituents at the germanium atom, namely, E _a ≤ 10 kcal mol⁻¹ for unsubstituted germiranes and E _a > 30 kcal mol⁻¹ for methyl-substituted germiranes. Taking the experimentally isolated germirane as an example, it was shown how the introduction of substituents and modification of the carbon skeleton make it possible to stabilize the germacyclopropane system. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1943–1951, September, 2005.     

Determination of the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in human hair by solid-phase extraction and gas chromatography-mass spectrometry

Summary A method for the determination of the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in human hair has been developed and validated. Hair samples (200 mg) were dissolved in NaOH (1 M) and PhIP was isolated by successive solid-phase extraction on a polystyrene-divinylbenzene column and on a silica-based mixed-mode column with C₈ and-SO₃ ⁻ functional groups. Quantification was performed by gas chromatography-electron-impact ionization high-resolution mass spectrometry in selected-ion-monitoring mode. The method was validated for determination of PhIP in the concentration range 0.5–25 ng g⁻¹ hair with [²H₃]PhIP as internal standard. The limit of quantification was 0.26 ng g⁻¹ hair. Within-day and between-day precision were in the ranges 1–27% and 2–15% relative standard deviation, respectively. The hair sample used for method validation was found to contain 0.26 ng PhIP g⁻¹ hair.     

New products and reaction pathways for the reaction of dimethylsulfoxide with peroxonitrite in aqueous solutions

The first observed basic products of the reaction of dimethylsulfoxide with peroxonitrite were methyl nitrite and methane with pH=7 and methyl nitrate at pH>10. The dependence of the yield of these and other reaction products on pH, concentrations of the substrate and peroxonitrite, and the additives NO ₂ ⁻ and CO₂ was studied. It is proposed that CH₄ and CH₃ONO are formed in the reaction between DMSO and the cis-form of⁻OONO, while DMSO₂ and CH₃ONO₂ are formed in reactions with the trans-form of⁻OONO or the dimer HOONO/OONO respectively. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 1, pp. 41–46, Jaunuary–February, 2000.     

Direct coating of quantum dots with silica shell

This paper describes a method for direct coating of fluorescent semiconductor nanoparticles with silica shell. The fluorescent semiconductor nanoparticles used were CdSe _x Te_1–x nanoparticles coated with ZnS and succeedingly surface-modified with carboxyl groups, or quantum dots (Q-dots). The Q-dots were silica-coated by performing sol–gel reaction of tetraethyl orthosilicate (TEOS) using NaOH as a catalyst in the presence of the Q-dots. Quasi-perfect Q-dots/silica core-shell particles were formed at 5.0 M H₂O and 4.0 × 10⁻⁴ M NaOH. Under these concentrations of H₂O and NaOH, the particle size of Q-dots/silica particles could be varied from 20.1 to 38.1 nm as the TEOS concentration increased from 2.5 × 10⁻⁴ to 50 × 10⁻⁴ M. The Q-dots/silica particles showed fluorescence as well as the uncoated Q-dots.     

Regularities of self-propagating high-temperature synthesis in the solid 8-hydroxyquinoline—chloramine B system

The regularities of chemical reactions in solid 8-hydroxyquinoline—chloramine B mixtures were studied under conditions of organic self-propagating high-temperature synthesis (SHS), isothermal reaction, and thermal explosion in the 20–220 °C temperature range. Comprehensive physicochemical analysis and microstructural study of the reaction products were carried out. The temperature of SHS initiation (58 °C), the heat of the reaction (129±9 kJ mol⁻¹), the stoichiometric coefficient (1), the maximum temperature (T _max=98–140 °C), and the velocity of SHS wave propagation (u=0.15–0.55 mm s⁻¹) were determined. Depending on the ratio of the reactants (n), a low-temperature non-degeerate stable gasless mode (n≤1,T _max=115 °C,E _a=42 kcal mol⁻¹) and a high-temperature mode (n>1,T _max=140 °C,E _a=0.4 kcal mol⁻¹) are possible for SHS. The SHS affords monohydroxy and monochloro derivatives of 8-hydroxyquinoline, benzenesulfonamide, NaCl, NaOH, and H₂O. The mechanism of the solid-phase reaction at temperatures below 58 °C includes surface, solid-phase, and gas-phase diffusion; that for SHS is capillary spreading of the hydroxyquinoline melt. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2271–2284, December, 1999.     

Amidocrownophanes as Anion Receptor

Crownophanes composed of 28-membered ring atoms having two hydroxy groups, two amide groups, and aromatic parts such as naphthalene rings and either pyridine or benzene ring, can bind anions with high affinity and selectivity. The anion-coordination ability of these species has been observed by ¹H NMR techniques. As anion guest molecules, we selected some halides, dihydrogenphosphate and acetate ions. It has been found that amidocrownophanes, 3 and 4, can recognize anions in the order;H₂PO ₄ ⁻ >F⁻>CH₃COO⁻>Cl⁻>>Br⁻ and I⁻, while not only 1, 2, and 5 having no hydroxy group but also 6 having 27-membered ring have no ability for anion recognition under the same conditions. In order to exhibit the recognition ability for anion receptor, plural amide groups, hydroxy groups, and m-phenylene or 1,6-pyridyl rigid part play an important role in this macrocyclic system.     

Kinetic and equilibrium study of the deprotonation of 4-nitrophenyl[bis(ethylsulphonyl)]methane by organic bases in acetonitrile in the presence of common cation BH<Superscript>+</Superscript> and tetrabutylammonium perchlorate

The results of kinetic and equilibrium experiments with the set of reaction of proton abstraction from 4-nitrophenyl[bis(ethylsulphonyl)]methane in acetonitrile are reported. Two strong organic bases are used: 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD). The rates of proton transfer reaction have been measured by T-jump method in the presence of perchlorate of the appropriate base as a common cation BH⁺ and supporting electrolyte-tetrabutylammonium perchlorate (TBAP) in the temperature range between 20–40°C are: k _H =1.32×10⁷−2.00×10⁷ and 2.82×10⁷−4.84×10⁷ dm ³mol⁻¹s⁻¹ for MTBD and TBD respectively. The enthalpies of activation ΔH _MTBD ^≠ =13.5 and ΔH _TBD ^≠ =18.1 kJmol⁻¹. The entropies of activation are negative: ΔS _MTBD ^≠ =−62.3 and ΔS _TBD ^≠ =−40.3 Jmol⁻¹K⁻¹. The change of the absorbance of the anion of 4-nitrophenyl[bis9ethylsulphonyl)]methane at the temperature 25°C in the presence of common cation BH⁺ gives the equilibrium constants K=705 and 906 M⁻¹ for MTBD and TBD respectively. Kinetic and equilibrium results are discussed. The possible mechanism of proton transfer reaction between 4-nitrophenyl[bis(ethylsulphonyl)]methane and cyclic organic bases: MTBD and TBD in acetonitrile is proposed.     

Thiocyanate-selective electrode based on N-salicylidene-benzylamineato copper(II) complex

Highly selective poly(vinyl chloride) (PVC) membrane electrode based on N-salicylidene-benzylamineato copper(II) complexes [Cu(SBA)₂] as new carriers towards thiocyanate-selective electrode was reported. The influence of membrane composition, pH and possible interfering anions were investigated on the response properties of the electrode. The resulting electrode exhibits anti-Hofmeister selectivity sequence: SCN⁻ > ClO₄⁻ > Sal⁻ > I⁻ > Br⁻ > NO₃⁻ > NO₂⁻ > SO₃²⁻ > H₂PO₄⁻ > Cl⁻ > SO₄²⁻, and a near-Nernstian potential linear range for thiocyanate from 1.0 × 10⁻¹ to 9.0 × 10⁻⁷ M with a detection limit of 7.0 10⁻⁷ M and a slope of , over a wide pH range of 3.0–9.0 in phosphate buffer solution at 20°C. The proposed electrode has a fast response time of about 5–10 s and can be used for at least 3 months without any considerable divergence in potential. The electrode was successfully applied to the determination of thiocyanate in waste water and human urine and saliva samples. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 3, pp. 293–299. The text was submitted by the authors in English.     

Determination of ultra-trace amounts of inorganic selenium species in natural water by ion chromatography-inductively coupled plasma-mass spectrometry coupled with nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> solid phase extraction

This work presents a nano-Al₂O₃ solid phase extraction technique for the determination of ultra-trace amounts of inorganic selenium species in aqueous systems using ion chromatography inductively coupled plasma-mass spectrometry (IC-ICP-MS). In this experiment, the inorganic selenium species were successfully extracted on a nano-Al₂O₃ solid phase column and then quantitative eluted with a 100 mmol L⁻¹ NaOH solution. Extraction conditions such as solvent identity, solvent concentration, solvent volume, solvent pH and salt addition were optimized. Under the optimum extraction conditions (elute solvent: 100 mmol L⁻¹ NaOH, solvent volume: 4 mL, pH: 7.0), low detection limits (Se (IV): 6 ng L⁻¹, Se (VI): 11 ng L⁻¹; RSD<5.0%) and good linear range (0.5–100 ng mL⁻¹, R² > 0.999) were obtained for all of the analytes. Good spiked recoveries over the range of 80–98% were obtained by applying the proposed method on real environmental water samples. These results indicated that this method is very sensitive and reliable when monitoring trace levels of inorganic selenium species in aqueous samples.      

Another Approach to the Modified Zhuravlev Equation on the Basis of the Oxidation of V2O4 and V6O13

It has been found that the modified Zhuravlev equation, [(1−α)^−1/3−1]²=ktⁿ, which describes the kinetics of oxidation of V₂O₄ and V₆O₁₃ in the temperature range 820–900 K and in the oxygen pressure range 1.0–20 kPa, can be derived via the assumption that the changes in the observed activation energy result from the changing contributions of the two diffusion processes controlling the reaction rate. The values of the observed activation energy are in the range 160–175 kJ mol⁻¹ for V₂O₄ and 188–201 kJ mol⁻¹ for V₆O₁₃ in the scope of the experimental oxygen pressures and temperatures and conversion degrees of 0.1–0.9. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ultra-trace level determination of nitrite in human saliva by spectrofluorimetry using 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacene

A rapid, highly sensitive and selective fluorogenic method for the determination of traces of nitrite is described. It is based on the reaction of weakly fluorescent 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacence (DAMBO) and nitrite in acidic aqueous solution to give 1,3,5,7-tetramethyl-8-(5-benzotriazolyl)-difluoroboradiaza-s-indacene (DAMBO-T), which is highly fluorescent. The optimum reaction conditions and other analytical parameters are investigated to enhance the sensitivity of the method. The fluorescence enhancement at 507 nm is linearly related to the concentration of nitrite in the range of 6.0 × 10⁻⁹–5.0 × 10⁻⁷ mol L⁻¹ with a correlation coefficient of R = 0.9995 (n = 10) and a detection limit of 1.0 × 10⁻¹⁰ mol L⁻¹. The R.S.D. is 1.12% (n = 10). The method is applied to the determination of nitrite in human saliva samples with the recoveries of 96. 24–105.30%. Correspondence: Ke-Jing Huang, Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China     

Mechanism of the reduction of molecular nitrogen to hydrazine by niobium (iii) hydroxide

The effect of the concentration of water on the rate of reduction of molecular nitrogen to hydrazine by niobium(iii) hydroxide in alkaline H₂O−MeOH and D₂O−MeOD mixtures was studied. In both cases, the reaction rate is maximum when [H₂O]=4 mol L⁻¹, and the inverse isotopic effect (K ^D/k ^H>1) is observed when [H₂O]<20 mol L⁻¹. Similar regularity was observed for the reaction of hydrogen elimination. It was found that HD is formed in the H₂O−MeOH system in the presence of D₂. The conclusion was made that the ratedetermining stage in hydrazine formation is the transfer of a hydride ion to the dinitrogen molecule coordinated to the binuclear Nb^III center. A kinetic scheme satisfactorily explaining the effect of the concentration of water ([H₂O]=1.5−49.0 mol L⁻¹) on the reaction rate constant was proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1600–1604, September, 1997.     

Kinetic studies on H<Superscript>+</Superscript>-catalysed aquation of chromium(III)-oxalato-asparaginato and chromium(III)-oxalato-histidinato complexes

Three chromium(III) complexes with asparagine (Asn) and histidine (His) of the [Cr(ox)₂(Aa)]²⁻ type, where Aa = N,O–Asn, N,O–His or N,N′–His, were obtained and characterized in solution. The complexes with N,O–Aa undergo acid-catalysed aquation to give a free amino acid and cis-[Cr(ox)₂(H₂O)₂]⁻, whereas the complex with N,N′–His undergoes parallel reaction paths: (1) isomerization to the N,O–His complex and (2) liberation of an oxalate ligand. Kinetics of the N,O–Aa complexes in HClO₄ media were studied spectrophotometrically under pseudo-first-order conditions. The absorbance changes were attributed to the chelate ring opening at the Cr–N bond. The linear dependence of rate constants on [H⁺] was established, and a mechanism for the chelate ring cleavage was postulated. The existence of a metastable intermediate with O-monodentate Aa ligand was proved experimentally. Effect of [Cr(ox)₂(Aa)]²⁻ on 3T3 fibroblasts proliferation was studied. The tests revealed low cytotoxicity of the complexes. Complexes with Ala, His and Cys are good candidates for biochromium sources.     

Calorimetric investigation of interactions in the LaNi4.75Al0.25−H2 and LaNi4.8Sn0.2−H2 systems

The interaction of hydrogen with intermetalic compounds LaNi_4.75Al_0.25 and LaNi_4.8Sn_0.2 has been studied in the temperature range 308–353 K by the calorimetry titration method. The mechanism of hydrogenation was investigated. It was shown that, as the temperature increases, the initial concentration of hydrogen in the metal lattice needed for β-hydride formation decreases. It was assumed that this effect is related to the concentration of H^δ+ atoms, which “oxidize” the metallic matrix according to the scheme H^δ++M⁰→H^δ−+M⁺. The enthalpy and entropy of hydrogenation for the LaNi_4.75Al_0.25−H₂ system were calculated from thep-C-T curves and the calorimetry results. The thermodynamic parameters of the LaNi_4.8Sn_0.2−H₂ system were obtained for the first time. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1841–1844, October, 1999.