Kinetics of the reactions of catechins with hypochlorite,peroxynitrite, and amino acid–derived peroxyl- radicals

Catechins, one of the class of flavonoids, are known as very efficient antioxidants. Here we investigated the kinetics of the reactions of three catechins, namely, catechin, epigallocatechin, and epigallocatechin gallate (EGCG) with some oxidants, which are formed in vivo under oxidative stress, hypochlorite, peroxynitrite, and amino acid peroxyl radicals. Stopped-flow spectrophotometry and pulse radiolysis technique with absorption detection were used to observe the formation of intermediate products of oxidized catechins. We found that catechins react with hypochlorite with the rate constant of the order of 10⁵–10⁶ M⁻¹ s⁻¹ at pH 7.4. Experimental kinetic traces of the reaction of EGCG with valine peroxyl radicals were fitted using chemical simulation, and the rate constant of this reaction was found to be 5 × 10⁵ M⁻¹ s⁻¹. The rate constants of the formation of unstable catechin quinones in the reaction with peroxynitrite were comparable to that of spontaneous peroxynitrite isomerization, which indicates that catechins are oxidized indirectly by peroxynitrite. Biological consequences of these reactions are discussed.     

Hydrolysis and Photolysis of Tris(tetraethylammonium) Pentacyanoperoxynitritocobaltate(III): Evidence for a Novel Complex,Pentacyanonitratocobaltate(III)

At pH 2, the rate constant of hydrolysis of tris(tetraethylammonium) pentacyanoperoxynitritocobaltate(III) in H₂O is 9.6×10⁻⁶ s⁻¹. In the absence of any light, ONOO⁻ is not replaced by H₂O and isomerizes within the coordination sphere to NO₃⁻. The novel complex [Co(CN)₅NO₃]³⁻ released NO₃⁻ slowly, as detected by ion chromatography. At pH 6, no hydrolysis is observed. Direct photolysis, both at pH 2 and pH 6, of tris(tetraethylammonium) pentacyanoperoxynitritocobaltate(III) by irradiation (YAG laser) at 355 nm destroys the coordinated ONOO⁻ and releases NO₂^., which hydrolyzes to NO₃⁻ and NO₂⁻. We also measured the ⁵⁹Co‐NMR spectra of [Co(CN)₅OONO]³⁻ and [Co(CN)₅H₂O]²⁻; the chemical shifts correspond very well to those predicted and are in agreement with the expected contribution to the ligand field by H₂O and ONOO⁻.     

Pulse radiolysis of butyl acrylate in aqueous solution

The pulse radiolysis of n-butyl acrylate (nBA) in aqueous solution was studied. The rate constant of the reaction of nBA with hydroxyl radicals was calculated as 1.5×10¹⁰ dm³ mol⁻¹ s⁻¹. The absorption spectrum of the OH^·–nBA adduct appeared to have a broad maximum at 300 nm. This spectrum was attributed to the α-carbon centred radicals. It decayed with the first-order rate constant k=1.5×10⁴ s⁻¹ (pH 10.8). The rate constant of the nBA reaction with hydrated electrons was determined as k=1.6×10¹⁰ dm³ mol⁻¹ s⁻¹. The spectrum of H^·–nBA adduct was similar to that recorded for OH^· adduct. It decayed with first-order kinetics at k=1.0×10⁴ s⁻¹. Spectra of the electron adduct were characterised by the band with a maximum at 285 nm (pH 10.0) or at 280 nm (pH 4.0) with ϵ=10 500 dm³ mol⁻¹ cm⁻¹. In acidic solution, radical anion formed upon addition of hydrated electrons to the nBA molecule, undergoes fast, reversible protonation. The decay of the reversibly protonated electron adduct was a second-order process at k=2.5×10⁹ dm³ mol⁻¹ s⁻¹. This reaction took place at the carbonyl oxygen. Slow, irreversible protonation of the electron adduct at high pH takes place at the β-carbon atom at k=2.9×10⁴ s⁻¹.     

Ag(I)‐Catalyzed Chlorination of Linezolid during Water Treatment: Kinetics and Mechanism

The kinetic and mechanistic study of Ag(I)‐catalyzed chlorination of linezolid (LNZ) by free available chlorine (FAC) was investigated at environmentally relevant pH 4.0–9.0. Apparent second‐order rate constants decreased with an increase in pH of the reaction mixture. The apparent second‐order rate constant for uncatalyzed reaction, e.g., k″_app = 8.15 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 0.076 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C and for Ag(I) catalyzed reaction total apparent second‐order rate constant, e.g., k″_app = 51.50 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 1.03 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C. The Ag(I) catalyst accelerates the reaction of LNZ with FAC by 10‐fold. A mechanism involving electrophilic halogenation has been proposed based on the kinetic data and LC/ESI/MS spectra. The influence of temperature on the rate of reaction was studied; the rate constants were found to increase with an increase in temperature. The thermodynamic activation parameters E_a, ΔH^#, ΔS^#, and ΔG^# were evaluated for the reaction and discussed. The influence of catalyst, initially added product, dielectric constant, and ionic strength on the rate of reaction was also investigated. The monochlorinated substituted product along with degraded one was formed by the reaction of LNZ with FAC.     

The kinetics of anion equilibration of human erythrocytes in phosphate-containing media of low chloride concentration

The pH-equilibration of human erythrocytes is accompanied by the transport of permeating anions. The kinetics of adaptation of erythrocytes to various outside solutions was investigated by continuous registration of extracellular pH and Cl⁻- and K⁺-concentration. In phosphate buffered solutions of low Cl⁻-content, the equilibration process consists of at least two components:

• •exchange of Cl⁻ for OH⁻ or HCO₃⁻;
• •exchange of Cl⁻ for phosphate

.The obtained time-dependence of extracellular pH and Cl⁻-concentration was fitted by means of a regression model which consists of two exponential terms assuming a fast and a slow transport component. By increasing of phosphate buffer concentration the fast time constant of Cl⁻- and pH-kinetics and the slow time constant of Cl⁻-kinetics were decreased whereas the slow pH-time constant was enhanced. CO₂-supply of the suspension tends to increase the slow time constant of pH- and Cl⁻-kinetics. These results can be explained by suggesting that the transport system is inhibited by phosphate or influenced by transmembrane potential.     

Selective,pH-Dependent Colorimetric and Fluorimetric Detection of Quadruplex DNA with 4-Dimethylamino(phenyl)-Substituted Berberine Derivatives

The 9- and 12-dimethylaminophenyl-substituted berberine derivatives 3 a and 3 b were readily synthesized by Suzuki-Miyaura reactions and shown to be useful fluorescent probes for the optical detection of quadruplex DNA (G4-DNA). Their association with the nucleic acids was investigated by spectrometric titrations, CD and LD spectroscopy, and with DNA-melting analysis. Both ligands bind to duplex DNA by intercalation and to G4-DNA by terminal π stacking. At neutral conditions, they bind with higher affinity (K_b=10⁵−10⁶ M⁻¹) to representative quadruplex forming oligonucleotides 22AG , c-myc , c-kit , and a2 , than to duplex calf thymus (ct) DNA (K_b=5-7×10⁴ M⁻¹). At pH 5, however, the affinity of 3 a towards G4-DNA 22AG is higher (K_b=1.2×10⁶ M⁻¹), whereas the binding constant towards ct DNA is lower (K_b=3.9×10³ M⁻¹) than under neutral conditions. Notably, the association of the ligand with DNA results in characteristic changes of the absorption and emission properties under specific conditions, which may be used for optical DNA detection. Other than the parent berberine, the ligands do not show a noticeable increase of their very low intrinsic emission intensity upon association with DNA at neutral conditions. In contrast, a fluorescence light-up effect was observed upon association to duplex (Φ_fl=0.01) and quadruplex DNA (Φ_fl=0.04) at pH 5. This fluorimetric response to G4-DNA association in combination with the distinct, red-shifted absorption under these conditions provides a simple and conclusive optical detection of G4-DNA at lower pH.     

A kinetic study of the decomposition of N-bromoserine

This article reports the kinetics of the decomposition of N-bromoserine formed rapidly by bromation of serine by BrO^?. The main decomposition products are glycolaldehyde, ammonia, carbon dioxide, and bromide ions at pH < 11.5, and β-hydroxypyruvic acid, ammonia, and bromide ions at pH > 11.5. The reaction is of order one with respect to N-bromoserine, and is independent of ionic strength and excess serine. The rate constant increases with increasing pH at pH > 11 and with decreasing pH at pH < 8, and over the range pH 8–11 has the constant value 1.67 × 10^?3 s^?1 at 298 K.     

Depurination of synthetic poly(inosinic acid) analogues

A poly(inosinic acid) analogue, poly{[1′-(β-hypoxanthine-9-yl)-5′-deoxy-D -erythro-pent-4′-enofuranose]-alt-[maleic acid]} (4), was synthesized by the alternating copolymerization of nucleoside derivative 1 with maleic anhydride and subsequent hydrolysis. N-Glycosidic bonds of the polymer were spontaneously hydrolyzed to liberate hypoxanthine from the polymer backbone in a buffer solution (pH 7.4) at room temperature. The depurination rate constant of the polymer at pH 7.4 and 37°C was measured to be 1.9 × 10⁻⁶ sec⁻¹, which was 10⁵-fold higher than that (3 × 10⁻¹¹ sec⁻¹) of the depurination of DNA that occurred in the biological systems. The increase in the depurination rate was attributable to the high potential energy of the polymer caused by the crowded environment around the bases, so that the polymer was more susceptible to the hydrolysis. Since natural nucleic acids often have compact structures with the crowded environment around the bases by the intricate chain folding, the depurination may also be accelerated in a similar manner in the biological system. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3361–3365, 1999     

Spectrokinetic study of SF5 and SF5O2 radicals and the reaction of SF5O2 with NO

UV spectra of SF₅ and SF₅O₂ radicals in the gas phase at 295 K have been quantified using a pulse radiolysis UV absorption technique. The absorption spectrum of SF₅ was quantified from 220 to 240 nm. The absorption cross section at 220 nm was (5.5 ± 1.7) × 10⁻¹⁹ cm². When SF₅ was produced in the presence of O₂ an equilibrium between SF₅, O₂, and SF₅O₂ was established. The rate constant for the reaction of SF₅ radicals with O₂ was (8 ± 2) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. The decomposition rate constant for SF₅O₂ was (1.0 ± 0.5) × 10⁵ s⁻¹, giving an equilibrium constant of K_eq = [SF₅O₂]/[SF₅][O₂] = (8.0 ± 4.5) × 10⁻¹⁸ cm³ molecule⁻¹. The SF₅ O₂ bond strength is (13.7 ± 2.0) kcal mol⁻¹. The SF₅O₂ spectrum was broad with no fine structure and similar to the UV spectra of alkyl peroxy radicals. The absorption cross section at 230 nm was found to (3.7 ± 0.9) × 10⁻¹⁸ cm². The rate constant of the reaction of SF₅O₂ with NO was measured to (1.1 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ by monitoring the kinetics of NO₂ formation at 400 nm. The rate constant for the reaction of F atoms with SF₄ was measured by two relative methods to be (1.3 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. © 1994 John Wiley & Sons, Inc.     

Re-evaluation of the rate constant for the H atom reaction with tert-butanol in aqueous solution

The rate constant (k) of the H+tert-butanol reaction at pH∼1–2 was measured by the time profile of the absorbance build-up of the tert-butanol radical and by competitive reactions for H atoms between maleic acid and tert-butanol. The determined rate constant, k=(1.15±0.2)×10⁶ mol⁻¹ dm³ s⁻¹, is nearly an order of magnitude higher than the previously published rate constants.     

Effect of pH on kinetics of the decay of the radicals formed during photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline in aqueous and micellar solutions

Kinetics of the decay of the transient radicals formed from 2,2,4,6-tetramethyl-1,2-dihydroquinoline (TMQ) in aqueous and micellar solutions of sodium dodecyl sulfate were studied by flash photolysis as a function of pH. In aqueous and micellar solutions of TMQ the mechanism of the decay of the transient species and the reaction products are different from those in homogeneous organic solutions. The decay of the transient radicals follows first-order kinetics in the entire range of pH under consideration in both aqueous and micellar solutions. In aqueous solutions at pH 9–12, the decay rate constant decreases from 25.3 to 3.7 s⁻¹. In micellar solutions at different pH, different types of micellar catalysis were observed. At pH 1, the rate constant in a micellar solution is slightly lower than that in an aqueous solution. At pH 3–11, the decay rate constant increases (positive micellar catalysis). The apparent rate constant depends linearly on the concentration of TMQ in micelles. The rate constant for the reaction of the transient radical cation with TMQ was determined (200 L mol⁻¹ s⁻¹). At pH>13, the decay rate constant in micelar solutions is lower than that in aqueous solutions (negative micellar catalysis). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 703–708, April, 1997.     

Kinetics of oxidation of SCN− by diperiodato cuprate(III) (DPC) in alkaline medium

The kinetics of oxidation of SCN⁻ by DPC has been investigated in alkaline medium. The reaction shows first-order dependence in [SCN⁻]. The pseudo-first-order rate constant (k_obs) changes differently under different [OH⁻]. At low [OH⁻], k_obs decreases when [OH⁻] increases, but when [OH⁻] increases to enough extent, k_obs increases with increase in [OH⁻]. Free radicals were observed in the process of reaction. A plausible mechanism involving Cu(HL)₂ and CuL as active substrates in the reaction has been proposed. The rate equations derived from the mechanism explain all the experimental phenomena satisfactorily. © 1996 John Wiley & Sons, Inc.     

Kinetics and equilibrium study of nickel(II) complexation by pterin

The kinetics of complexation of Ni(II) by pterin was studied in aqueous solutions with a stopped‐flow apparatus under conditions of pseudo‐first order in the temperature range 5–45°C, pH between 4.0 and 6.5, and ionic strength 0.4 M. The equilibrium constants, stoichiometry, and pK_a of the ligand and complex were also determined using a spectrophotometric technique. The results are consistent with the formation of a 1:1 complex between the metal ion and pterin. The first‐order experimental rate constant k_app is pH independent and shows the following dependence with the ion metal concentration at 25°C: k_app/s⁻¹ = 3.8 × 10⁻³ + 1.6 × 10⁻⁴ × [Ni(II)]⁻¹. A global activation energy of 57 ± 2 kJ/mol associated with the formation of a 1:1 chelate was measured. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 231–237, 2000     

Ocimum basilicum seed mucilage reinforced with montmorillonite for preparation of bionanocomposite film for food packaging applications

Use of nanocomposites is a well-established approach in enhancing the mechanical and barrier properties of bionanocomposite film for food packaging applications. The seed mucilage of Ocimum basilicum was employed for the preparation of bionanocomposite films with montmorillonite (MMT) as nanofiller. The films were prepared by solvent-casting method at varied solution pH (1, 3, 5 and 9) and MMT loading (1%, 3%, 5%, 10%, 15% and 20%). The films were characterized for physical, mechanical and barrier properties in addition to microstructure and X-ray diffraction pattern. XRD analysis revealed the exfoliated dispersion of MMT at pH 9, confirming its effective interaction with the bionanocomposite film. Maximum film tensile strength was achieved at a lower MMT load of 5%. Water vapour permeability reduced with increase in MMT loading up to 5%, followed by an increase at higher MMT loadings. Film formed at pH 9 showed tensile strength of 17.3 ± 0.33 MPa and reduced water vapour permeability (WVP) of 0.21 g mm.m⁻².hr⁻¹.kPa⁻¹.     

Study on the Interaction of Vitamin B12 with DNA by Spectroscopy and Electrochemical Methods

The Interaction between vitamin B₁₂ (VB₁₂) and fish sperm DNA was investigated in physiological buffer (pH 7.4) using the methylene blue (MB) dye as a spectral probe by spetcrophotometery, viscosity measurements and cyclic voltammetry. The apparent binding constant of vitamin B₁₂ with DNA was found to be 3.2×10⁵ mol⁻¹·L. The voltammetric behavior of vitamin B₁₂ has been investigated at glassy carbon electrode using cyclic voltammetry. Thermodynamic parameters including ΔH⁰, ΔS⁰ and ΔG⁰ for the interaction between VB₁₂ and DNA have determined as −2.3×10⁴, 27.54 and −3.1×10⁴J·mol⁻¹·K⁻¹ respectively. One indication of DNA binding mode with VB₁₂ was the change in viscosity when a small molecule associates with DNA. The diffusion coefficients of VB₁₂ in the absence (D₀)_f and presence of DNA (D₀)_b was calculated as 5.04×10⁻⁶ and 1.13×10⁻⁶ cm²·s⁻¹ respectively. The results indicated that vitamin B₁₂ can bind to DNA and the major binding mode was intercalative binding.     

Depyrimidination of synthetic poly(uridylic acid) analogue

A poly(uridylic acid) analogue, poly{[1′‐(β‐uracil‐1‐yl)‐5′‐deoxy‐D‐erythro‐pent‐4′‐enofuranose]‐alt‐[maleic acid]} (3), was synthesized by the alternating copolymerization of nucleoside derivative 1 and maleic anhydride and subsequent hydrolysis. N‐glycosidic bonds of the polymer were hydrolyzed spontaneously to liberate uracil from the polymer backbone in a buffer solution (pH 7.4) at room temperature. The depyrimidination rate constant of the polymer at pH 7.4 at 80 °C was 8.2 × 10⁻⁵ s⁻¹, which was 10⁴ times higher than that of the depyrimidination of DNA (1.2 × 10⁻⁹ s⁻¹) under the same condition. The activation energy for the depyrimidination was 16 kcal/mol, which was about half of that for the relevant nucleoside reactions. The increase in the depyrimidination rate was attributable to the high potential energy of the polymer caused by the crowded environment around the bases, so that the polymer was more susceptible to the hydrolysis. Because natural nucleic acids often have compact structures with a crowded environment around the bases by an intricate chain folding, the pyrimidination also may have been accelerated in a similar manner in the biological system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 423–429, 2000     

Radiation-induced degradation of aqueous 2,3-dihydroxynaphthalene

The degradation of aqueous 2,3-dihydroxynaphthaline (2,3-DHN) under the influence of γ-ray was investigated under various experimental conditions. Using 2.5×10⁻⁵ mol L⁻¹ 2,3-DHN in aerated media (pH=6–6.8) an initial degradation yield, G_i-(2,3-DHN)=0.32 was obtained, whereas in solutions saturated with N₂O the yield come to G_i-(2,3-DHN)=0.50.In airfree media the substrate decomposition was negligible. Possible reaction mechanisms are presented.Further, the rate constant, k(OH+2,3-DHN)=2.14×10¹⁰ L mol⁻¹ s⁻¹ was determined by competition reactions with PNDA.     

Preparation and evaluation of amphiphilic silica‐based monolithic column having surface‐bound octanoyl‐aminopropyl moieties for capillary electrochromatography

A novel amphiphilic silica‐based monolithic column having surface‐bound octanoyl‐aminopropyl moieties was successfully prepared by a one‐step in situ derivatization process. As expected, the amphiphilic monolithic column exhibited RP chromatographic behavior toward non‐polar solutes (e.g., alkyl benzenes) with high column performance. As the pH of the buffer inside the column increases, the EOF changed from −2.65×10⁻⁸m² V⁻¹s⁻¹ at pH 3.0 to 1.20×10⁻⁸ m² V⁻¹s⁻¹ at pH 8.0 with the reversion of EOF at about pH 6.4. Using acidic mobile phase, five aromatic acids can be efficiently separated in less than 6 min under co‐EOF conditions. For basic compounds, symmetrical peaks were obtained due to the existence of hydrophilic acyl amide group, which can effectively minimize the adsorption of the positively charged basic analyte to the silica‐based surface of the capillary column.     

Measurement System for Very Small Photoelastic Constant of Polymer Films

We produced a new system for measuring the small photoelastic constant of a polymer thin film with a small birefringence. Using our mesurement system, we evaluated the photoelastic constant of a polymer film in real time by quantitative analysis. Photoelastic constants of 11.30 × 10⁻¹² Pa⁻¹ for a cellulose triacetate film and 78.38 × 10⁻¹² Pa⁻¹ for a polycarbonate film were obtained. Furthermore, we obtained a small photoelastic constant of 0.12 × 10⁻¹² Pa⁻¹ for a cycloolefin film for liquid crystal displays, using our new measurement system. This value is very small. We emphasize that, if a small change in retardation and stress cannot be detected simultaneously using our system, then we cannot obtain such a small photoelastic constant.     

Rate constant for the gas-phase reaction of hydroxyl radical with the natural hydrocarbon bornyl acetate

The gas-phase reaction of bornyl acetate (bicyclo[2,2,1]-heptan-2-ol-1,7,7-trimethyl-acetate) with hydroxyl radical has been studied. A relative method was used to determine the rate constant for this reaction, with n-octane as reference compound. Methyl nitrite photolysis experiments were carried out in an environmental smog chamber at atmospheric pressure and (294±2) K. The rate constant determined for bornyl acetate is k=(13.9±2.2)×10⁻¹² cm³ molecule⁻¹ s⁻¹. The experimental rate constant has been compared with the rate constants calculated with the structure-activity relationship (SAR) and with the evolution trend of the acetate rate constants. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 497–502, 1998