CONTENTS & ABSTRACTS
In English. Summaries in Estonian
Proceedings of the Estonian Academy of Sciences.
Volume 51 No. 1 March 2002
Solvatochromism of fullerene C60 in solvent mixtures: application of the preferential solvation model; 3–18
Urmas Pille, Koit Herodes, Ivo Leito, Peeter Burk, Viljar Pihl, and Ilmar Koppel
Abstract. Absorption spectra of fullerene C60 were recorded between 300 and 700 nm in seven different binary solvent mixtures (toluene–acetic acid, toluene–methanol, toluene–acetonitrile, toluene–dimethyl sulphoxide, toluene–dimethylformamide, pyridine–methanol, and pyridine–water). The appearance of the UV-vis spectra of fullerene C60 was found to be influenced by the composition of the mixture. The influence of the composition of the mixture on the position of four absorption bands and was studied. The solvent-induced shifts of the absorption maxima were generally in the range of several nanometers, the longest being 10.3 nm in the mixture of toluene and dimethylformamide. The data were analysed in terms of the preferential solvation model. Fullerene was found to be strongly preferentially solvated by toluene and pyridine compared to the more polar components of the mixtures. The selective solvation model was found to be applicable to the description of solvatochromism of C60 in binary solvent mixtures, at least formally. Particularly interesting solvent effects – marked changes in the shape of the spectrum – were found in the mixtures toluene–dimethylformamide and pyridine–water.
Key words: binary mixtures, UV-vis spectroscopy, solvent-induced shifts, absorption maxima.
Isobaric vapour–liquid equilibria of the ternary system hexan-2-one + o-xylene + nonane; 19–28
Enn Siimer, Helle Kirss, Mati Kuus, and Ludmilla Kudryavtseva
Abstract. Boiling temperatures (T) vs liquid-phase mole fractions (x) were measured for the system hexan-2-one + o-xylene + nonane and for two constituent binaries containing hexan-2-one at pressures 26.66, 53.33, 79.99, and 101.32 kPa. Hexan-2-one formed minimum boiling azeotrope with nonane while the binary system hexan-2-one + o-xylene was nonazeotropic. The modified Wilson model was used to correlate the binary T–x data and to predict vapour–liquid equilibria in the ternary system. The excess Gibbs energy calculated by the modified Wilson model and experimental HE data at 318.15 K of the ternary system are treated simultaneously.
Key words: phase equilibria modified Wilson equation.
A computerized flow injection analysis manifold for interference determination in flame atomic absorption spectrometry; 29–37
Riina Lahne, Anu Viitak, Helvi Hödrejärv, Maili Treumann, Margus Friedenthal, and Mihkel Kaljurand
Abstract. A computerized flow injection analysis (FIA) manifold was set up and its performance is described for automatic determination of the influence of interferences in flame atomic absorption (FAA) spectrometry. The manifold operates on the principle of multiple injection of the sample solution of the metal to be determined with the interference to the FAA-spectrometer. The sample is simultaneously diluted with a pure solution of the analyte, which allows covering a wide range of interference concentrations. As an example, calcium determination with phosphate as the interference and lanthanum as the interference suppressor is demonstrated. The method was applied to study calcium concentration in milk.
Key words: atomic absorption spectrometer, flow injection, interference phosphate, calcium, milk.
Non-steady-state processes in amperometric biosensors: modelling studies; 38–48
Siiri Velling, Kaido Tammeveski, Alexey Mashirin, and Toomas Tenno
Abstract. The non-steady-state processes of a biosensor based on a diffusion-limited oxygen sensor were investigated. The effect of various processes occurring in the biosensor on its overall response was studied. A mathematical model of transient processes of the biosensor was elaborated. The maximum rate of current change was taken as a measure of sensor response to substrate concentration. The validity of the model was tested by using a biosensor with immobilized microorganisms.
Key words: microbial biosensor, agarose-gel film mathematical modelling, non-steady-state process, dynamic method.
Kinetic studies on the mechanism of haematoporphyrin derivative photobleaching; 49–70
Lyudmila Chekulayeva, Igor Shevchuk, Vladimir Chekulayev, and Raissa Jäälaid
Abstract. Haematoporphyrin derivative (HPD), a sensitizer used in photodynamic therapy (PDT) of tumours, is progressively destroyed (photobleached) during illumination. However, the mechanism of the sensitizer photobleaching remains unclear, although its degradation presents both potential problems and potential advantages. This paper surveys the effects of reaction conditions, photooxidizable biomolecules, electron acceptors, and other agents on the quantum yield (QY) and kinetics of the photodestruction of HPD in solution. The initial QY of HPD photobleaching in pH 7.4 phosphate buffer in air was measured as 3.6 ´ 10–5. The yield decreased significantly in organic solvents with a low dielectric constant and in the presence of various surfactants. Ionic strength, pH, and temperature had relatively slight effects on the photobleaching yield of HPD. For example, raising the temperature from 10 to 43 °C caused only a moderate (about 2-fold) increase in the QY of HPD photobleaching. It was found that oxygen is needed for the photobleaching of HPD. However, the QY of HPD photodestruction increased only slightly (by 55%) in D2O. Sodium azide, an efficient physical quencher of singlet oxygen (1O2), had only a slight effect on the photobleaching yield, even at 50 mM. Our data suggest that besides 1O2, free radical reactions are involved in the photodegradation of HPD in aqueous solution. In fact, the QY of HPD photobleaching decreased in the presence of hydroxyl radical scavengers, such as mannitol, sodium benzoate, ethanol, and deferoxamine. In addition, the photodestruction of HPD could be associated with the formation of very reactive cation radicals of the sensitizer. Some photooxidizable substrates and model electron acceptors increased markedly the photobleaching efficiency of HPD. At certain concentrations, the QY of HPD photobleaching was enhanced by the presence of histidine, reduced glutathione, dithiothreitol, and lecithin. Electron acceptors, such as metronidazole and flavin mononucleotide, also increased the photobleaching yield. In contrast, NADH and cysteine, which are electron donors, inhibited the rate of HPD photodestruction. High concentrations (> 1 mM) of biological antioxidants, such as ascorbic acid and α-tocopherol, also increased the photostability of HPD in solution. These results suggest that the mechanism(s) of the photobleaching of HPD in cells and tissues during PDT may be complex.
Key words: haematoporphyrin derivative, photobleaching, photodynamic therapy, tumour.
Instructions to authors; 71–73
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