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 369,292 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The equilibrium and relative rate of rotamer interconversion around the bond joining the 2,2'-bipyrrolyl and pyrromethene moieties in a synthetic analogue of immunosuppressant prodigiosin are investigated as a function of pHapp in a water/acetonitrile mixture (1/1 by volume). Two chromatographically separable isomeric forms are obtained in acid solutions (pHapp < 4), whereas rapid interconversion occurs above neutrality. Furthermore, pH modulates the conformational preference of the molecule according to nitrogen protonation on the three pyrrole rings system (pKa = 7.2). At high pHapp (neutral form), the same conformer that is observed in pure acetonitrile prevails, whereas the other one is preferred by the protonated form. The nuclear magnetic resonance data indicate that the structures of the two conformers mainly differ in the value of the torsion angle around the aforementioned C-C bond. Kinetic and equilibrium data are quantitatively interpreted with a cyclic mechanism including two protonation (pKa1 = 8.23 +/- 0.03, pKa2 = 5. 4 +/- 0.2) and two conformational rearrangement steps. A molecular interpretation of the observed behavior includes, for the preferred conformer at low pH, formation of a new hydrogen bond between the exocyclic oxygen and the neighboring pyrrole NH upon protonation of the three pyrrole rings system.
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PMID:Equilibrium and kinetics of rotamer interconversion in immunosuppressant prodigiosin derivatives in solution. 987 5

The singlet oxygen quenching rate constants (kq) for a range of hydroxycinnamic acids in acetonitrile and D2O solutions were measured using time resolved near infrared phosphorescence in order to establish their antioxidant activity. The magnitude of kq observed depends on both the nature of the substituent groups and solvent polarity. The variations in kq depend on the energy of the hydroxycinnamic acid/molecular oxygen charge transfer states, (O2delta- ...HCAdelta+). In D2O the values of kq range from 4x10(7) M(-1) s(-1) to 4x10(6) M(-1) s(-1) for caffeic acid and o-coumaric acid respectively. In acetonitrile, the charge transfer energy levels are raised and this is reflected in lower singlet oxygen quenching rate constants with a kq value of 5x10(6) M(-1) s(-1) for caffeic acid. The phenoxyl radical spectra derived from the hydroxycinnamic acids were determined using pulse radiolysis of aqueous solutions and the reduction potentials were found to range from 534 to 596 mV. A linear correlation is observed between reduction potential, and hence free energy for electron transfer, and log kq. These correlations suggest a charge transfer mechanism for the quenching of singlet oxygen by the hydroxycinnamic acids.
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PMID:Singlet oxygen quenching and the redox properties of hydroxycinnamic acids. 1038 Nov 91

The formation of complexes with different ligands in the interlayer space of montmorillonite saturated in Na(+), Mg(2+), Ca(2+), Co(2+), Cu(2+), Ni(2+), Fe(3+), and Cr(3+) was studied. Acetone, acetonitrile, dimethyl sulfoxide, and trimethylphosphate were used as ligands. The nature of the complexes was studied by means of X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, microcalorimetry, and ab initio quantum mechanical methods. In all cases, the organic ligands penetrate into the interlayer space at room temperature, forming complexes stable in vacuum with the interlayer cations. The ligand-cation ratio depends on the valence of the saturating cation. The cation-ligand interaction in these complexes has an ion-dipole electrostatic nature. The complexes are formed by the direct interaction of the oxygen or nitrogen atom of ligand and the interlayer cation. Using the quantum mechanical approach, allow us to determine the disposition of the ligand in these complexes. In all cases, only one layer of ligands is present in the stable complexes. Copyright 2000 Academic Press.
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PMID:Adsorption Mechanism and Structure of the Montmorillonite Complexes with (CH(3))(2)XO (X=C, and S), (CH(3)O)(3)PO, and CH(3)-CN Molecules. 1065 34

We have studied the catalytic two-electron reduction of 3-chloro-2,4-pentanedione by cobalt(I) salen electrogenerated at a glassy carbon cathode in acetonitrile containing tetramethylammonium tetrafluoroborate. When cobalt(I) salen is electrogenerated at -0.65 V (a potential that is 30 mV more negative than the peak potential for the reversible one-electron reduction of cobalt(II) salen), the carbon-chlorine bond of 3-chloro-2,4-pentanedione is catalytically cleaved to form 2,4-pentanedion-3-ate; this anion can be protonated either by adventitious water or by a deliberately added proton donor to produce 2,4-pentanedione, or the anion can be trapped with iodoethane to give 3-ethyl-2,4-pentanedione. However, when cobalt(I) salen is electrogenerated at -0.40 V (a potential at which the rate of generation of cobalt(I) salen is relatively small), the 2,4-pentanedion-3-ate, resulting from the catalytic two-electron cleavage described above, can deprotonate unreduced starting material to form 3-chloro-2,4-pentanedion-3-ate and 2,4-pentanedione. In further work, we have found that 2,4-pentanedion-3-ate can be oxidized directly to form the corresponding radical which couples to yield 3,4-diacetyl-2,5-hexanedione. Chemically produced 2,4-pentanedion-3-ate reacts with electrogenerated cobalt(III) salen to give a dionylcobalt(III) salen species which undergoes a one-electron reduction to liberate cobalt(II) salen and the dionate. In addition, cobalt(II) salen reacts with molecular oxygen to give cobalt(III) salen and superoxide, and the latter reduces 3-chloro-2,4-pentanedione to form chloride ion, the 2,4-pentanedion-3-yl radical, and molecular oxygen.
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PMID:Electrochemical behavior of 3-chloro-2,4-pentanedione in the presence of cobalt salen. 1069 81

Catalytic activities of catalase (CAT) immobilized on graphite--GMZ, soot--"NORIT" and "PM-100" to mediate decomposition of 3-Cl-C6H4COOOH (3-CPBA) in acetonitrile have been investigated. Under these conditions, the kinetic parameters Km, k, Ea, Vmax, and Z0 were calculated. Conclusions on a probable mechanism of the catalytic process observed were drawn from the calculated values of deltaG*, deltaH*, and deltaS*. A quantitative UV-spectrophotometrical approach was used as the basic analytical tool. The electrochemical reduction of oxygen generated in enzyme catalysed 3-CPBA decomposition was examined with polarization curves method.
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PMID:Catalytic decomposition of 3-chloroperoxybenzoic acid by immobilized catalase in a non-aqueous medium. 1073

Photochemical reactions (lambda irr = 254 nm) of substituted benzyldiphenylphosphine oxides 1a-e have been investigated in benzene and acetonitrile. alpha-Cleavage from the singlet excited state is proposed as the primary process, and products formed both before and after escape of the primary intermediates from the solvent cage result. Radicals 2 and 3 are observed by nanosecond flash photolysis following excitation of 1a at 266 nm in acetonitrile. Acetone sensitization of 1a,b fails to improve the efficiency of product formation. The quantum yields of disappearance of 1a,b are unaffected by O2, and mechanisms for product formation from 1a in the presence of oxygen are proposed.
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PMID:Photochemistry of substituted benzyldiphenylphosphine oxides 1077 39

A high-performance liquid chromatography (HPLC) analysis system based on a water-acetonitrile gradient program was established for simultaneous quantification of shikimate-derived secondary metabolites in cultured cells of Lithospermum erythrorhizon. The cells cultured in pigment production medium (M-9) are capable of producing five highly hydrophilic compounds such as p-hydroxybenzoic acid-O-glucoside and lithospermic acid B, as well as eleven lipophilic compounds including echinofuran B and acetylshikonin. In addition to the wide polarities of those compounds, many of them are unstable under light, dryness, oxygen and heating. Thus, a new extraction procedure for all these compounds was also established by use of ultrasonication under ice-water chilling with MeOH as the solvent. This procedure was applied to the quantitative analyses of these compounds in cell cultures and hairy root cultures of Lithospermum, and in the intact plants as well.
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PMID:Simultaneous analysis of shikimate-derived secondary metabolites in Lithospermum erythrorhizon cell suspension cultures by high-performance liquid chromatography. 1077 21

Organic disulfides with both alkyl and aryl substituents are oxidized by hydrogen peroxide when CH(3)ReO(3) (MTO) is used as a catalyst. The first step of the reaction is complete usually in about an hour, at which point the thiosulfinate, RS(O)SR, can be detected in nearly quantitative yield. The thiosulfinate is then converted, also by MTO-catalyzed oxidation under these conditions, to the thiosulfonate and, over long periods, to sulfonic acids, RSO(3)H. In the absence of excess peroxide, RS(O)SR (R = p-tolyl), underwent disproportionation to RS(O)(2)SR and RSSR. Kinetics studies of the first oxidation reaction established that two peroxorhenium compounds are the active forms of the catalyst, CH(3)ReO(2)(eta(2)-O(2)) (A) and CH(3)ReO(eta(2)-O(2))(2).(OH(2)) (B). Their reactivities are similar; typical rate constants (L mol(-)(1) s(-)(1), 25 degrees C, aqueous acetonitrile) are k(A) = 22, k(B) = 150 (Bu(2)S(2)) and k(A) = 1.4, k(B) = 11 (Tol(2)S(2)). An analysis of the data for (p-XC(6)H(4))(2)S(2) by a plot of log k(B) against the Hammett sigma constant gave rho = -1.89, supporting a mechanism in which the electron-rich sulfur attacks a peroxo oxygen of intermediates A and B.
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PMID:Oxidation of symmetric disulfides with hydrogen peroxide catalyzed by Methyltrioxorhenium(VII) 1081 2

Singlet-oxygen quenching constants were measured for 19 cyanine dyes in acetonitrile. The most efficient quenchers were 1-butyl-2-[2-[3-[(1-butyl-6-chlorobenz-[cd]indol-2(1H)- ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd] indolium and 6-chloro-2-[2-[3-(6-chloro-1-ethylbenz[cd]indol-2(1H)-ylidene) ethylidene]-2-phenyl-1-cyclopenten-1-yl]ethenyl]-1-ethyl-benz[cd]indolium, having quenching constants with diffusion-controlled values of 2.0 +/- 0.1 x 10(10) and 1.5 +/- 0.1 x 10(10) M-1 s-1, respectively. There was a trend toward increased quenching constants for cyanine dyes with the absorption band maxima at longer wavelengths. However, the quenching constants correlated better with the oxidation potentials of the cyanine dyes, suggesting that quenching proceeds by charge transfer rather than energy transfer. The quenching constants for 1,1',3,3,3',3'-hexamethylindotricarbocyanine perchlorate and 1,1'-diethyl-4,4'-carbocyanine iodide were measured in several solvents as well as in aqueous solutions of detergent micelles. In different solvents, the quenching constants varied by as much as a factor of 50. The quenching constants were largest in solvents with the highest values on the pi* scale of Kamlet, Abboud, Abraham and Taft. This was consistent with quenching occurring by charge transfer. Within cells, cyanine dyes concentrate in membrane-bound organelles. The quenching constants were substantial within detergent micelles. To the extent that micelles are models for biological membranes, cyanine dyes may be effective biological singlet-oxygen quenchers.
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PMID:Structural and environmental requirements for quenching of singlet oxygen by cyanine dyes 1082 85

The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate constant, k(2), values of several para-substituted phenyl methyl sulfides correlate linearly with Hammett sigma constants and the rho values are in the range of -1.3 to -2.7 with different substituted oxo(salen)chromium(V) complexes. The reactivity of different alkyl sulfides is in accordance with Taft's steric substituent constant, E(S). A mechanism involving direct oxygen atom transfer from the oxidant to the substrate rather than electron transfer is envisaged. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reaction of various sulfides with a given oxo(salen)chromium(V) complex and vice versa. Mathematical treatment of the results shows that this redox system falls under strong reactivity-selectivity principle (RSP).
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PMID:Mechanism of selective oxidation of organic sulfides with Oxo(salen)chromium(V) complexes 1084 14


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