Kinetic measurements as well as B3LYP/ and MP2/6-31G(d,p)
calculations provide evidence that carbonyl
oxides formed in the gas-phase ozonolysis of alkylated alkenes are an
important source of OH radicals. In the
gas-phase ozonolysis of propene, cis-2-butene,
trans-2-butene, tetramethylethene, and isoprene, 18, 17, 24,
19% OH radicals (relative to reacted ozone, error margin ≤4%) are
measured using CO as a scavenger for OH. The
quantum chemical calculations show that OH radical production depends
on syn positioned methyl (alkyl) groups
and their interaction with the terminal O atom of a carbonyl oxide.
For example, in the gas-phase ozonolysis of
ethene only 5% OH radicals are measured while for a carbonyl oxide
with syn-positioned methyl (alkyl) group, a
much larger amount of OH radicals is formed. This is due to the
fact that 1,4 H migration and the formation of an
intermediate hydroperoxy alkene, that is prone to undergo OO bond
cleavage, is energetically more favorable than
isomerization to dioxirane. In the case of syn-methyl,
dimethyl, and isopropenyl carbonyl oxide calculated
enthalpies at 298 K are 14.8, 14.4, and 15.5 kcal/mol compared to the
corresponding dioxirane isomerization barriers
of 23.8, 21.4, and 23.0 kcal/mol, respectively. The OO cleavage
reactions of the hydroperoxy alkenes formed in
these cases are just 11, 12.8, and 10.3 kcal/mol.
Neoangiogenesis is required for tumor development and progression. Many solid tumors induce vascular proliferation by production of angiogenic factors, prominently vascular endothelial growth factor (VEGF). Because nutrition is a causative factor for tumor prevention and promotion, we determined whether secondary plant constituents, i.e., flavonoids, tocopherols, curcumin, and other substances regulate VEGF in human tumor cells in vitro. VEGF release (concurrent with synthesis) from MDA human breast cancer cells and, for comparison, U-343 and U-118 glioma cells was measured by ELISA. Of 21 compounds tested, 9 showed significant inhibitory activity at 0.1 micromol/L in MDA human breast cancer cells. The rank order of inhibitory potency was naringin > rutin > alpha-tocopheryl succinate (alpha-TOS) > lovastatin > apigenin > genistein > alpha-tocopherol >or= kaempferol > gamma-tocopherol; chrysin and curcumin were inactive except at a concentration of 100 micromol/L. Glioma cells were similarly sensitive, with U343 more than U118, especially for alpha-TOS and tocopherols. Among the tocopherol derivatives, alpha-TOS (0.1 micromol/L) was the most effective in reducing VEGF release. Overall, the glycosylated flavonoids (i.e., naringin, a constituent of citrus fruits, and rutin, a constituent of cranberries) induced the greatest response to treatment at the lowest concentration in MDA human breast cancer cells. Inhibition of VEGF release by flavonoids, tocopherols, and lovastatin in these models of neoplastic cells suggests a novel mechanism for mammary cancer prevention.
Significant gas phase bromine enrichment is experimentally verified on flowing gaseous HOBr over ice surfaces doped with sodium halides of sea salt composition. It is argued that formation of Br,Cl-in the condensed phase followed by its dissociation with release of Br, into the gas phase accounts for this phenomenon.
The development of a computer-controlled electrochemical impedance spectrometer, based on a Fourier transform algorithm is described. Together with a fast potentiostat the system can be used in the frequency range 1 mHz to 100 kHz. The perturbation signal is a superposition of sine waves with properly chosen frequencies. The overall measurement time is limited only by the lowest frequency in the spectrum and by the data transfer to the computer, thus, time-resolved impedance spectra measurements can be performed. The principle of operation and technical details are presented and discussed.
Adsorption of electron acceptors on partially reduced TiO2 leads to the localization of mobile conduction band electrons which is indicated by the appearance of an ESR signal with g = 2.003. The localization process is accompanied by an electron transfer from donor centers in reduced TiO2 (Ti3+ ions) to adsorbed molecules. The ESR spectra show a decrease of the Ti3+ signal intensity at gr -1.96. Additionally, in the case of O2, SO2 and C6H5NO2 adsorption, the ESR signals of the respective anion radicals are observed. Illumination of samples with an electron acceptor adsorbed enhances the g - 2.003 signal. The activation energy corresponding to the temperature dependence of the localization process has been determined to be ~ 1 kcal/mole in the temperature range - 140 to - 40 °C. In the case of O2 and N2O the signal at g = 2.003 could be annihilated chemically by using propylene which reacted with the adsorbed electron acceptor and thus removed the species which reduced the electron mobility
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