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81.
Reza Dabestani Robert D. Hall Robert H. Sik Colin F. Chignell 《Photochemistry and photobiology》1990,52(5):961-971
The photochemistry (Type I and II) of anthralin and its photo-oxidation product 1,8-dihydroxyanthraquinone (1,8-DHAQ) has been studied in ethanol, acetonitrile and dimethylsulfoxide using spin-trapping and direct detection of singlet oxygen (1O2) luminescence techniques. In ethanol, where it exists in its neutral form (AN), anthralin does not undergo either Type I or II reactions upon UV-irradiation. In contrast, irradiation of anthralin in acetonitrile, a solvent in which anthralin is partially converted to its corresponding mono-anion (AN-), generates both superoxide and singlet oxygen. Irradiation of anthralin in dimethylsulfoxide, where the AN- form is present in substantial quantity, generates superoxide and solvent derived radicals but no detectable singlet oxygen. UV-irradiation of 1,8-DHAQ in ethanol and acetonitrile produces both superoxide and singlet oxygen in significant yields. In dimethylsulfoxide, on the other hand, only superoxide and solvent derived radicals are observed. The 1O2 quantum yield for AN- and 1,8-DHAQ in acetonitrile were determined to be 0.14 and 0.88 relative to rose bengal in the same solvent. These findings suggest that the AN photosensitization occurs via Type I and II pathways, is solvent dependent and involves AN- as well as its oxidation product 1,8-DHAQ, which is a more potent generator of both singlet oxygen and superoxide. 相似文献
82.
JPC – Journal of Planar Chromatography – Modern TLC - 相似文献
83.
Shamsipur M Chaichi MJ Karami AR 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2003,59(3):511-517
The chemiluminescence arising from the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of acriflavine has been studied. The relationship between the chemiluminescence intensity and concentrations of TCPO, H2O2, acriflavine and the base sodium salicylate are reported. The kinetic parameters for the peroxyoxalate-chemiluminescence (PO-CL) of acriflavine were evaluated from the computer fitting of the corresponding chemiluminescence intensity-time plots. 相似文献
84.
Ahmad Shaabani Abbas Rahmati Soheila Naderi 《Monatshefte für Chemie / Chemical Monthly》2007,138(6):553-557
Summary. Reaction of triphenylphosphine and an electron deficient acetylenic ester in the presence of strong N–H acid such as alkyl
and aryl sulfamides or acetamide produces phosphorous ylides at room temperature in CH2Cl2. The aryl sulfamide phosphoranes undergo a smooth transformation reaction in boiling toluene and produce iminophosphoranes. 相似文献
85.
F. Ahmad 《Journal of the Iranian Chemical Society》2004,1(2):99-105
A method is suggested to determine valid and authentic values of thermodynamic stability parameters of proteins from their heat-induced conformational transition curves. We show (a) that the estimate of ΔHm van, the enthalpy change on denaturation at Tm, the midpoint of denaturation, is significantly less than ΔHm cal, the value obtained by the calorimetric measurements, if the analysis of the conformational transition curve uses the conventional method which assumes a linear temperature-dependence of the pre- and post-transition baselines; and (b) that there exists an excellent agreement between ΔHm van and ΔHm cal values of proteins, if the analysis of thermal denaturation curves assumes that the temperature-dependence of pre- and post-transition baselines is described by a parabolic function. The latter analysis is supported by our observations that the temperaturedependencies of the absorption and circular dichroism properties of protein groups are indeed nonlinear. It is observed that the estimate of ΔCp, the constant-pressure heat capacity change is independent of the model used to describe the temperaturedependence of the pre- and post-transition baselines. An important conclusion is that for proteins which exhibit a two-state character, all stability parameters are measured with the same error as that observed with a calorimeter. 相似文献
86.
Araúzo-Bravo MJ Fujii S Kono H Ahmad S Sarai A 《Journal of the American Chemical Society》2005,127(46):16074-16089
Sequence dependence of DNA conformation plays a crucial role in its recognition by proteins and ligands. To clarify the relationship between sequence and conformation, it is necessary to quantify the conformational energy and specificity of DNA. Here, we make a systematic analysis of dodecamer DNA structures including all the 136 unique tetranucleotide sequences at the center by molecular dynamics simulations. Using a simplified conformational model with six parameters to describe the geometry of adjacent base pairs and harmonic potentials along these coordinates, we estimated the equilibrium conformational parameters and the harmonic potentials of mean force for the central base-pair steps from many trajectories of the simulations. This enabled us to estimate the conformational energy and the specificity for any given DNA sequence and structure. We tested our method by using sequence-structure threading to estimate the conformational energy and the Z-score as a measure of specificity for many B-DNA and A-DNA crystal structures. The average Z-scores were negative for both kinds of structures, indicating that the potential of mean force from the simulation is capable of predicting sequence specificity for the crystal structures and that it may be used to study the sequence specificity of both types of DNA. We also estimated the positional distribution of conformational energy and Z-score within DNA and showed that they are strongly position dependent. This analysis enabled us to identify particular conformations responsible for the specificity. The presented results will provide an insight into the mechanisms of DNA sequence recognition by proteins and ligands. 相似文献
87.
A heterocyclic peptide nanotube 总被引:3,自引:0,他引:3
An open-ended hollow tubular structure is designed based on hydrogen-bond-directed self-assembly of a chimeric cyclic peptide subunit comprised of alternating alpha- and epsilon-amino acids. The design features a novel 1,4-disubstituted-1,2,3-triazole epsilon-amino acid and its utility as a peptide backbone substitute. The N-Fmoc-protected epsilon-amino acid was synthesized in high yield and optical purity in three steps from readily available starting materials and was employed in solid-phase peptide synthesis to afford the desired cyclic peptide structure. The cyclic peptide self-assembly has been studied in solution by (1)H NMR and mass spectrometry and the resulting tubular ensemble characterized in the solid state by X-ray crystallography. 相似文献
88.
S. Ahmad M. S. Chaudhary A. Mannan I. H. Qureshi 《Journal of Radioanalytical and Nuclear Chemistry》1983,78(2):375-383
The concentration of certain toxic and essential elements in various brands of tea consumed locally were determined using instrumental neutron activation analysis. Transference of these elements into the drinkable portion was studied by brewing and boiling tea leaves for two minutes and the approximate intake was calculated from this data. The toxic effects of Hg, Se, As, Br and Sb are briefly discussed. 相似文献
89.
Summary A method for semiquantitative determination of thiourea, thiocyanate, and sulfide is described. It involves making circular spots or rings on chromatoplates using the circular thin layer Chromatographic apparatus. The method is based on sensitivity and specificity of spot reactions and comparison of the color intensity of a circular spot with that of sensitivity standards. The accuracy of the method is ± 5% and the procedure is rapid, simple and economical.
Zusammenfassung Ein Verfahren zur halbquantitativen Bestimmung von Thioharnstoff, Thiocyanat und Sulfid wurde beschrieben. Es beruht auf der chromatographischen Herstellung kreisförmiger Flecken oder Ringe auf Dünnschichtplatten. Mit empfindlichen und spezifischen Tüpfelreaktionen wurden die Flecken gefärbt und die Farbintensität mit Standardproben verglichen. Die Genauigkeit beträgt ± 5%.相似文献
90.
Reza Dabesrani Robert H. Sik Donald G. Davis George Dubay Colin F. Chignell 《Photochemistry and photobiology》1993,58(3):367-373
Abstract— The photochemistry, photophysics, and photosensitization (Type I and II) of indomethacin (IN) (N-[p-chlorobenzoyl]-5-methoxy-2-methylindole-3-acetic acid) has been studied in a variety of solvents using NMR, high performance liquid chromatography-mass spectroscopy, transient spectroscopy, electron paramagnetic resonance in conjunction with the spin trapping technique, and the direct detection of singlet molecular oxygen (l O2) luminescence. Photodecomposition of IN (λex > 330 nm) in degassed or air-saturated benzene proceeds rapidly to yield a major (2; N-[p-chlorobenzoyl]-5-methoxy-2-methyl-3-methylene-indoline) and a minor (3; N-[p-chlorobenzoyl]-5-methoxy-2, 3-dimethyl-indole) decarboxylated product and a minor indoline (5; 1-en-5-methoxy-2-methyl-3-methylene-in-doline), which is formed by loss of the p-chlorobenzoyl moiety. In air-saturated solvents two minor oxidized products 4 (N-[p-chlorobenzoyl]-5-methoxy-2-methylindol-3-aldehyde) and 6 (5-methoxy-2-methyl-indole-3-aldehyde) are also formed. When photolysis was carried out in 18O2-saturated benzene, the oxidized products 4 and 6 contained 18O, indicating that oxidation was mediated by dissolved oxygen in the solvent. In more polar solvents such as acetonitrile or ethanol, photodecomposition is extremely slow and inefficient. Phosphorescence of IN at 77 K shows strong solvent dependence and its emission is greatly reduced as polarity of solvent is increased. Flash excitation of IN in degassed ethanol or acetonitrile produces no transients. A weak transient is observed at 375 nm in degassed benzene, which is not quenched by oxygen. Irradiation of IN (λex > 325 nm) in N2-gassed C6H6 in the presence of 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) results in the trapping of two carbon-centered radicals by DMPO. One adduct was identified as DMPO/.COC6H4-p-CI, while the other was probably derived from a radical formed during IN decarboxylation. In air-saturated benzene, (hydro) peroxyl and alkoxyl radical adducts of DMPO are observed. A very weak luminescence signal from 1O2 at 1268 nm is observed initially upon irradiation (λex= 325 nm) of IN in air-saturated benzene or chloroform. The intensity of this 1O2 signal increases as irradiation is continued suggesting that the enhancement in 1O2 yield is due to photoproduct(s). Accordingly, when 2 and 3 were tested directly, 2 was found to be a much better sensitizer of 1O2 than IN. In air-saturated ethanol or acetonitrile no IN 1O2 luminescence is detected even on continuous irradiation. The inability of IN to cause phototoxicity may be related to its photo stability in polar solvents, coupled with the low yield of active oxygen species (1O2, O2?-) upon UV irradiation. 相似文献