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 15,165 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The effects of six organic modifiers (urea, methanol, dioxane, tetrahydrofuran, acetonitrile and 2-propanol) on the retention mechanism and separation selectivity of the bulk buffer in micellar electrokinetic capillary chromatography (MECC) with sodium dodecyl sulfate (SDS) micelles as pseudo-stationary phase have been investigated through linear solvation energy relationships (LSERs). It is found that the retention value in MECC systems with or without organic modifier is primarily dependent on the solvophobic interaction and the hydrogen bonding interaction with the solute as proton acceptor, while the dipolar interaction and the hydrogen bonding interaction with the solute as proton donor play minor roles. The effects of the organic modifiers on the solvophobic, dipolar and hydrogen bonding interactions are evaluated in terms of the relationship between regression coefficient of the LSER equations and the modifier concentration. The variations of the solvophobic interaction and the dipolar interaction with change of the modifier concentration can be approximately explained using the solubility parameter and the dipolarity/polarizability parameter of the organic modifier, respectively. However, the relationships between the hydrogen bond acidity and basicity of the bulk buffer and the organic modifiers are rather complicated. Those results may be caused from the displacement of organic modifiers to the water adsorbed on the micellar surface as well as changes in the acidity and basicity of the bulk buffer with the addition of organic modifiers. In addition, it is found that the phase ratio is influenced significantly by the use of organic modifier.
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PMID:Effects of organic modifiers on retention mechanism and selectivity in micellar electrokinetic capillary chromatography studied by linear solvation energy relationships. 1059 65

A variety of empirical and calculated data from the largest tank at the New Jersey State Aquarium were used to quantify the fluxes of carbon and nitrogen before and after the installation of denitrification in this facility. Before denitrification, the stock of dissolved inorganic carbon (DIC) in Ocean Tank exhibited a decrease of 6.9 kg C/month and sodium bicarbonate had to be added to maintain DIC in steady state. We were able to explain the DIC decrease by two non-conservative processes: the formation of carbonaceous precipitates (removes 4.2 kg C/month) and outgassing of carbon dioxide due to acidity from nitrification (independently determined to remove 3.2 kg C/month). Nitrogen budget in Ocean Tank before denitrification is in contrast to that of carbon, and it shows an increase of 4.8 kg N/month in the form of nitrate. Denitrification is currently removing 53.3 kg N/month (in the form of nitrogen gas), so this element should eventually reach steady state. The use of methanol for denitrification has resulted in a flux of 26.3 kg C/month into the aquarium and, as predicted, an increase in Ocean Tank DIC stock has been observed without any additions of sodium bicarbonate. Our approach can be used to model carbon and nitrogen balances in closed seawater facilities that host heterotrophic organisms and operate either with or without a biological denitrification system.
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PMID:Carbon and nitrogen fluxes in a closed seawater facility. 1072 Nov 43

The solvatochromic comparison method has been used to probe the interactions of solutes with binary solvent mixtures of methanol-water and acetonitrile-water. The solute spectra recorded in these mixtures are composed of the additive spectral contributions of the different solvated species of the solute, i.e., the water-solvated species, the cosolvent-solvated species, and the species solvated by water-solvent complexes. Multivariate curve resolution-alternating least squares has been used to model the solvation of the solutes as a function of the composition of the binary solvent mixture. Spectra and concentration profiles of the dye surrounded by the different solvation environments have been isolated. For the first time, solute spectra solvated exclusively by methanol-water and acetonitrile-water complexes have been obtained, and the solvatochromic parameters of dipolarity/polarizability and hydrogen-bonding acidity have been estimated for these complex species.
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PMID:Characterization of methanol-water and acetonitrile-water association using multivariate curve resolution methods 1081 51

The decarboxylation rate of the tetramethylguanidinium salt of 3-carboxy-6-nitrobenzisoxazole in 24 pure solvents and 36 dimethyl sulfoxide binary mixtures with diglyme, acetonitrile, benzene, dichloromethane, chloroform, and methanol was analyzed in the light of the SPP, SA, and SB pure solvent scales. The results allow one to rationalize the high sensitivity of this kinetics to the reaction medium and to assess the potential use of this compound as a probe in biochemical environments. The natural environment for comparison of this kinetics was found to be the gas phase rather than the aqueous medium. In the latter, the process is much faster owing to such high polarity, which, however, is strongly diminished by the high acidity of the medium. Based on our calculations, the rate constant for the decarboxylation kinetics in the gas phase must be in the region of 2 x 10(-10) s(-1) (i.e., 3 orders of magnitude smaller than in water).
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PMID:Effects of medium on decarboxylation kinetics: 3-carboxybenzisoxazoles and their potential use as environmental probes in biochemistry. 1084 24

Two remarkable crystal structures are reported of a cyclic receptor 1, containing two metalloporphyrin units. The overall crystal structure of 1 provides the first direct evidence that pi-stacking between two metalloporphyrins reduces the Lewis acidity of the metal ion and thereby dramatically reduces the affinity of zinc for external ligands; this effect was previously suggested indirectly by solution state binding studies. In addition, crystallising 1 from a different combination of solvents and the ability of 1 to distort its structure leads to the remarkable observation of a supramolecular dimer of inter-penetrating macrocycles, 4, held together by clusters of hydrogen-bonded methanol molecules.
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PMID:Hydrogen-bonding clusters leading to formation of supramolecular dimers of metalloporphyrin receptors: modulation of Lewis acidity by pi-pi interactions 1092 15

Retention factors determined for 31 solutes of widely different types on five columns of different chromatographic characteristics have been used to calculate the regression coefficients of the linear free energy relationship (LFER) equations. The mobile phases investigated consisted of acetonitrile-water and methanol-water, respectively, in a composition range of 20-70% (v/v) of organic modifiers. The regression coefficients of the LFER equations are characteristic of the given phase system (stationary phase, organic modifier and mobile phase composition) and represent the extent of the various molecular interactions contributing to the retention process. The effect of the characteristic of the stationary phase, the type of the organic modifier and the mobile phase composition is demonstrated and discussed. Alpha selectivity factors have been determined for various pairs of compounds. Hydrophobic or methylene selectivity can be described by the variation of the upsilon coefficient in Eq. (3) representing the difference in hydrophobicity between the stationary phase and the mobile phase. The polar or chemical selectivity of a phase system varies with the b coefficient in Eq. (3) representing the difference in acidity between the stationary phase and the mobile phase. Polar selectivity, i.e. the relative retention of polar solutes to that of a non-polar solute, e.g. toluene decreases with increasing polarity of the mobile phase. It depends also significantly on the polar characteristics of the columns. Specific selectivity, i.e. the relative retention of various polar solutes depends on the acidic or basic properties of the solutes to be separated and the chemical properties of the columns. The b regression coefficients can be used to describe the effect of mobile phase composition on the variation of specific selectivities. We have demonstrated that the LFER method provides a useful estimate of selectivity under different operating conditions by using the solvation parameters describing the different molecular interactions and the regression coefficients of the LFER equation characterizing the phase system.
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PMID:Characterization of reversed-phase columns using the linear free energy relationship. III. Effect of the organic modifier and the mobile phase composition. 1107 93

Ethnobotanical studies have revealed that Bidens pilosa is used in the traditional management of wounds and chronic gastro-duodenal ulcers. This led us to screen the methanol, cyclohexane and methylene chloride extracts of the plant for anti-ulcerogenic activity using the HCl/ethanol gastric necrotizing solution. The methylene chloride extract, which showed the highest activity (100% inhibition) at a dose of 750 mg/kg compared with the methanol and cyclohexane extracts (41 and 46% inhibition, respectively), was further tested using the indomethacin-HCl/ethanol-, absolute ethanol- and pylorus ligation-induced ulcer methods. Pre-treatment with indomethacin significantly reduced the protective effect of the extract against HCl/ethanol solution to 31%. The extract had very little gastric mucosal protection against absolute ethanol (9.8% inhibition at 750 mg/kg) compared with the controls and neither reduced gastric acid secretion in vivo nor the acidity of gastric juice following in vitro incubation.
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PMID:Effects of methanol, cyclohexane and methylene chloride extracts of Bidens pilosa on various gastric ulcer models in rats. 1109 Sep 94

The processing technology and characteristics of orubisi/amarwa, an opaque beer commonly consumed in Kagera region in the north-western part of Tanzania is described in detail. The protein content of orubisi increased from 2.0 to 2.7% after 120 hours of fermentation. The maximum alcohol content of orubisi as determined by specific gravity method was 2.5%. The alcohol profile of orubisi analysed by gas liquid chromatography (GLC) was found to contain ethanol and iso-butanol. The test for methanol was negative. Orubisi was characterised as product with relatively high acidity ranging from 0.35-0.89 g/100 ml and a final pH of 3.7. The levels of fermentable sugars--sucrose, maltose, glucose and fructose--were 0.5, 0.7, 1.8 and 0.6 g/100 ml after 120 hours of fermentation, respectively. High microbial counts were encountered in orubisi. The viable counts included yeasts: 2.0 x 10(7) cfu/ml, moulds 7.4 x 10(6) cfu/ml, coliforms 1.18 x 10(2) cfu/ml, lactic acid bacteria 6.5 x 10(7) cfu/ml and total aerobic count 2.95 x 10(7) cfu/ml. Based on these results, orubisi poses a serious danger to public health due to the presence of high numbers of total count and coliforms. In order to improve the safety of orubisi the pasteurisation step is recommended in the process of preparing orubisi. Hygienic handling of the final product is necessary in order to avoid contamination before consumption. The presence of trace amount of iso-butanol can lead to irritation of mucous membranes and depression of central nervous system.
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PMID:Aspects of manufacture, composition and safety of orubisi: a traditional alcoholic beverage in the north-western region of Tanzania. 1110 5

Retention data for a set of 69 compounds using rapid gradient elution are obtained on a wide range of reversed-phase stationary phases and organic modifiers. The chromatographic stationary phases studied are Inertsil (IN)-ODS, pentafluorophenyl, fluoro-octyl, n-propylcyano, Polymer (PLRP-S 100), and hexylphenyl. The organic solvent modifiers are 2,2,2-trifluoroethanol (TFE); 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP); isopropanol; methanol (MeOH); acetonitrile (AcN); tetrahydrofuran; 1,4-dioxane; N,N-dimethylformamide; and mixed solvents of dimethylsulfoxide (DMSO) with AcN and DMSO with MeOH (1:1). A total of 25 chromatographic systems are analyzed using a solvation equation. In general, most of the systems give reasonable statistics. The selectivity of the reversed phase-high-performance liquid chromatographic (HPLC) systems with respect to the solute's dipolarity-polarity, hydrogen-bond acidity, and basicity are reflected in correspondingly large coefficients in the solvation equation. We wanted to find the most orthogonal HPLC systems, showing the highest possible selectivity difference in order to derive molecular descriptors using the gradient retention times of a compound. We selected eight chromatographic systems that have a large range of coefficients of interest (s, a, and b) similar to those found in water-solvent partitions used previously to derive molecular descriptors. The systems selected are IN-ODS phases with AcN, MeOH, TFE, and HFIP as mobile phase, PLRP-S 100 phase with AcN, propylcyano phase with AcN and MeOH, and fluorooctyl phase with TFE. Using the retention data obtained for a compound in the selected chromatographic systems, we can estimate the molecular descriptors with the faster and simpler gradient elution method.
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PMID:Characterizing the selectivity of stationary phases and organic modifiers in reversed-phase high-performance liquid chromatographic systems by a general solvation equation using gradient elution. 1110 74

This paper describes the results of the evaluation of retention dependence on the physicochemical properties of solutes in linear gradient elution by reversed-phase liquid chromatography (RPLC) based on linear solvation energy relationships (LSERs). Retention time data on Inertsil ODS(3) column by linear gradient elution were collected for both acetonitrile-water and methanol-water binary mobile phases under various gradient steepness. Based on the LSERs, the retention times were linearly correlated with the physicochemical properties (size, dipolarity, and hydrogen bond donor-acceptor acidity and basicity) of solutes. As predicted by LSERs, very acceptable linear relationships are observed for both mobile phases. While the magnitudes of the coefficients are modified by the gradient steepness, their signs are consistent with those obtained by isocratic elution. As obtained for isocratic elution, the dominant factors to retention in linear gradient elution of RPLC are the solutes' size and hydrogen bond acceptor basicity. The conclusions of the study allow us to predict retention in chromatographic method development by gradient elution.
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PMID:Evaluation of the retention dependence on the physicochemical properties of solutes in reversed-phase liquid chromatographic linear gradient elution based on linear solvation energy relationships. 1120 4


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