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High Rate Constants (high + rate_constant)
Selected AbstractsScirpusin A, a hydroxystilbene dimer from Xinjiang wine grape, acts as an effective singlet oxygen quencher and DNA damage protectorJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 5 2010Qingjun Kong Abstract BACKGROUND: Grapes and red wines are rich sources of phenolic compounds such as anthocyanins, catechins, flavonols and stilbenes, most of which are potent antioxidants showing cardioprotective properties. We first isolated scirpusin A, a hydroxystilbene dimer, from a wine grape of Xinjiang, and studied its antioxidant activity. RESULTS: Reactive oxygen species scavenging effects and the protection against reactive singlet oxygen-induced DNA damage of scirpusin A have been investigated in our experiments. The concentration of scirpusin A required to inhibit 50% of 1O2 generation was 17 µmol L,1, while addition of scirpusin A at 140 µmol L,1 caused complete inhibition. Further kinetic study revealed that the reaction of Scirpusin A with singlet oxygen has an extremely high rate constant (ka = 4.68 × 109 L mol,1 s,1). Scirpusin A (140 µmol L,1) exhibited significant inhibition effects on pBR322 DNA breakage. However, scavenging effects of scirpusin A on superoxide anion O2,, and hydroxyl radical ·OH were not potent as the inhibitor rates at a concentration of 1400 µmol L,1 were 28.83% and 19.5%, respectively. CONCLUSION: The present study shows that scirpusin A is a selective quencher of singlet oxygen and a protector against reactive singlet oxygen-induced pBR322 DNA damage at very low concentrations. Copyright © 2010 Society of Chemical Industry [source] Successful removal of p -quinone with chitosan in an aqueous phase in relation to degree of deacetylationBIOFACTORS, Issue 1-4 2004Tomoki Takahashi Abstract Phenol oxidant is successfully removed by using chitosan particles in the aqueous phase. Removal of p -quinone by chitosan from crab shells was investigated kinetically from molecular weight (MW) of chitosan, deacetylation degree (DD) and reaction temperature. The rate constant assuming first-ordered reaction on removal of p -quinone in aqueous phase primarily depended on the MW of chitosan, not on the DD. Quantities of chitosan exceeding 5 × 105 MW are able to obtain a sufficiently high rate constant (10,3 s,1). At higher temperatures, higher rate constants were obtained in the entire experimental MW and DD. The activation energy obtained was 43.8 kJ·mol,1. [source] Charge and Mass Transfer Across the Metal/Solution InterfaceISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2008Eliezer Gileadi Electrode reactions are characterized by charge transfer across the interface. The charge can be carried by electrons or by ions. It is shown here that when both mass and charge cross the interface, the charge must be carried by the ionic species, not by the electrons, as a result of the very large difference in the time scale for electron and ion transfer. A prime example of charge transfer by ions is metal deposition. It is proposed that ion transfer occurs by migration of the ions across the interface, under the influence of the high electrostatic field in the double layer. The rate constants observed for metal deposition are comparable to those for outer-sphere charge transfer. These unexpectedly high rate constants for metal deposition are explained by a model in which removal of the solvation shell and reduction of the effective charge on the metal ion occur in many small steps, and a make-before-break mechanism exists, which lowers the total Gibbs energy of the system as it moves along the reaction coordinate from the initial to the final state. [source] Amide and lactam hydrolysis of N -(2-hydroxyacetyl)-2-pyrrolidone: effective catalysis,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2006Lisaedy García Borboa Abstract When N -(2-hydroxyacetyl)-2-pyrrolidone (open form) is dissolved in water at pH,>,8, irreversible cleavage of the exocyclic and endocyclic amide CN bond occurs. The latter rupture corresponds to the lactam opening yielding N -(4-hydroxyacetyl)butanoic acid (NBA). NBA is produced from the ester hydrolysis of the ester-amide macrocycle that is in equilibrium with the cyclol form of the open form. We have previously reported this latter equilibrium for N -(2-aminoacetyl)-2-lactams. 2-pyrrolidone (lactam) and glycolic acid are produced from direct hydrolysis of the open form by means of the amide exocyclic cleavage. The [NBA]/[lactam] ratio increases at higher pH since the NBA production is second order with respect to [OH,] while the corresponding lactam formation is only first order. The obtained kobs is hence the sum of the rate constants that yield lactam and NBA, respectively. This kobs is uncatalyzed and specific base catalyzed with unusually high rate constants of 2.1,×,10,6,s,1 and 0.025,M,1,s,1, respectively. The stability of the corresponding tetrahedral intermediate formed and the intramolecular alkoxy nucleophilic attack on the lactam carbonyl group combined with an effective protonation of the lactam nitrogen that promotes the CN cleavage, contribute to increase the reaction rates and lactam opening. Rate constants for the two parallel reactions are obtained from kobs and [NBA]/[lactam] versus pH plots. Copyright © 2006 John Wiley & Sons, Ltd. [source] Photoinduced Excited State Intramolecular Proton Transfer of New Schiff Base Derivatives with Extended Conjugated Chromophores: A Comprehensive Theoretical SurveyCHINESE JOURNAL OF CHEMISTRY, Issue 6 2010Qi Wang Abstract This paper presented comprehensive theoretical investigation of excited state intramolecular proton transfer (ESIPT) of four new large Schiff base derivatives with extended conjugated chromophores. The properties of the ground state and the excited state of phototautomers of C1 to C4 [C1: 2-(4,-nitro-stilbene-4-ylimino)methylphenol; C2: 2-(4,-cyano-stilbene-4-ylimino)methylphenol; C3: 2-(4,-methoxyl-stilbene-4-ylimino)methylphenol; C4: 2-(4,- N,N -diethylamino-stilbene-4-ylimino)methylphenol], which included geometrical parameter, energy, rate constant, frontier orbit, Mulliken charge, dipole moment change, were studied by DFT (density functional theory), CIS (configuration interaction singles-excitation), TDDFT (time-dependent DFT) methods to analyze the effects of chromophore part on the occurrence of ESIPT and the role of substituent groups. The structural parameter calculation showed that the shorter RHN and larger ROH from enol to enol* form, and less twisted configuration in the excited state implied that these molecules could undergo ESIPT as excitation. Stable transition states and a low energy barrier were observed for C1 to C4. This suggested that chromophore part increased some difficulty to undergo ESIPT for these molecules, while the possibility of occurrence of ESIPT was quite high. The negative ,E* (,9.808 and ,9.163 kJ/mol) of C1 and C2 and positive ,E* (0.599 and 1.029 kJ/mol) of C3 and C4 indicated that withdrawing substituent groups were favorable for the occurrence of ESIPT. The reaction rate constants of proton transfer of these compounds were calculated in the S0 and S1 states respectively, and the high rate constants of these compounds were observed at S1 state. C1 even reached at 1.45×1015 s,1 in the excited state, which is much closed to 2.05×1015 s,1 of the parent moiety (salicylidene methylamine). Electron-donating and electron-withdrawing substituent groups had different effects on the electron density distribution of frontier orbits and Mulliken charges of the atoms, resulting in different dipole moment changes in enol*,keto* process. These differences in turn suggested that C1 and C2 had more ability to undergo ESIPT than C3 and C4. The ultraviolet/visible absorption spectra, normal fluorescence emission spectra and ESIPT fluorescence emission spectra of these compounds were predicted in theory. [source] | ||||||||||