Determination of pKa values of some hydroxylated benzoic acids in methanol-water binary mixtures by LC methodology and potentiometry

An accurate estimation of pK_a values in methanol-water binary mixtures is very important for several separation techniques such as liquid chromatography and capillary electrophoresis that use these solvent mixtures. In this study, the pK_a values of 11 polyphenolic acids have been determined in methanol-water binary mixtures (10%, 20% and 30% (v/v)) by potentiometry, liquid chromatography (LC) and LC-DAD methodology.The results show a similar trend for the pK_a values of all the studied compounds, as they increase with increasing concentration of organic modifier, which allows a linear relationship between pK_a values and mole fraction of methanol to be obtained. The pK_a values obtained in aqueous medium have been compared with those given in the literature, and also with the values predicted by the SPARC on-line pK_a calculator. The data obtained have been used to test the feasibility of an estimation of dissociation constants in a methanol-water medium from the relationship between pK_a values and the organic cosolvent fraction in the mixtures.     

RNA integrity as a quality indicator during the first steps of RNP purifications : A comparison of yeast lysis methods

Background  

The completion of several genome-sequencing projects has increased our need to assign functions to newly identified genes. The presence of a specific protein domain has been used as the determinant for suggesting a function for these new genes. In the case of proteins that are predicted to interact with mRNA, most RNAs bound by these proteins are still unknown. In yeast, several protocols for the identification of protein-protein interactions in high-throughput analyses have been developed during the last years leading to an increased understanding of cellular proteomics. If any of these protocols or similar approaches shall be used for the identification of mRNA-protein complexes, the integrity of mRNA is a critical factor.     

THE CHLOROPHYLL-SENSITIZED REACTION BETWEEN BENZOQUINONE AND ETHANOL*

The electron-transfer reaction between triplet excited chlorophyll and quinones has been extensively studied as a model of the primary reaction in photosystem II. There has also been reported a minor reaction in which the chlorophyll cation radical ostensibly oxidizes the alcohol solvent or even water, leading to a gradual net reduction of quinone, but the exact mechanism and even the existence of this reaction has been uncertain. We have examined the consequences of prolonged irradation of ethyl chlorophyllide and benzoquinone in acidulated ethanol, and find a chlorophyllide-sensitized reaction which is not analogous to the better-known autosensitized reduction of quinones in blue or UV light. In the chlorophyllide-sensitized reaction, benzoquinone is apparently converted to ethoxy-substituted quinones and quinols, and polymeric material. Ethyl chlorophyllide (or chlorophyll) is simultaneously oxidized to more polar products which themselves continue to photosensitize the reaction of quinones. The production of acetaldehyde could not be demonstrated in the sensitized reaction. Chlorophyllide-sensitized reaction of (l-hydroxyethyl)benzoquinone, ethoxybenzoquinone and 2.5-diethoxybenzo-quinone were examined for additional information. A reaction sequence, tentatively proposed to accommodate the known facts, starts with oxidative attack by quinone on an oxidized chlorophyllide radical formed by loss of a hydroxyl proton from alcohol bound as a ligand to Mg²⁺. It is not likely that this reaction is closely related to events at the oxidizing side of photosystem II.     

Two-dimensional polyacrylamide gel electrophoresis, with immobilized pH gradients in the first dimension, of barley seed proteins: discrimination of cultivars with different malting grades.

The suitability of high-resolution two-dimensional gel electrophoresis for barley cultivar discrimination and for classification with respect to their malting properties was studied. Seed proteins of 14 barley cultivars with different malting qualities were extracted with urea/dithiothreitol/Nonidet P-40 buffer and subjected to two-dimensional gel electrophoresis with immobilized pH gradients in the first dimension (IPG-DALT). The results of IPG-DALT were compared to the protein patterns obtained by a standard technique, sodium dodecyl sulfate polyacrylamide gel electrophoresis of hordeins. Sodium dodecyl sulfate-gel electrophoresis yielded seven different "B" and four different "C" hordein patterns; "A" and "D" hordein patterns were uniform in all cultivars tested. Four cultivars could be distinguished unequivocally, the others were classified into three groups containing between two and five cultivars. In contrast to these findings. IPG-DALT yielded three different "A", eight different "B", four different "C" and two different "D" hordein patterns. When the "A", "B", "C" and "D" hordein patterns were combined, ten cultivars exhibited unique hordein patterns whereas the remaining ones were classified into two groups containing two cultivars each. Moreover, when albumin and globulin proteins were used for evaluation in addition to the hordeins, all cultivars could be discriminated by IPG-DALT. IPG-DALT, performed on small-scale and/or ready-made gels, proved to be an ideal complementary system to one-dimensional electrophoretic methods for routine seed testing purposes because of its speed, reliability, and simplicity. IPG-DALT was also applied to study the relationship between the different polypeptide patterns and the malting quality. Although cultivars with identical one-dimensional protein patterns but different malting quality could be successfully differentiated by IPG-DALT, a direct correlation between specific protein spots or protein patterns to the malting quality was not found within the cultivars tested.     

Study on the interaction of platinum(IV), gold(III) and silver(I) ions with DNA

Fluorescence, absorption spectra have been produced by the interaction of platinum(IV), silver(I) and gold(III) ions with the berberine–DNA system (berberine, Scheme 1). Platinum(IV) and gold(III) ions show different effects from that of silver(I) ion on the spectral characteristics of the berberine–DNA system. Quenching fluorescence is seen with platinum(IV) and gold(III) ions addition, whereas increasing fluorescence is observed for silver(I) ions. The addition of gold(III) and silver(I) ions cause an increase in absorption of the berberine–DNA system. The above results suggest that different metal ions exhibit different affinities when binding to DNA correlates well with the ions’ charge, structure and the coordination ability.     

In-capillary micro solid-phase extraction and capillary electrophoresis separation of heterocyclic aromatic amines with nanospray mass spectrometric detection

A miniaturised technique to analyse and detect heterocyclic aromatic amines (HAs) using micro solid-phase extraction (SPE) coupled on-line (in-capillary) to capillary electrophoresis (CE) separation with nanospray (nESI) mass spectrometry (MS) detection has been developed. HAs are mutagenic and carcinogenic compounds formed at low levels in protein-rich food during cooking. Due to the low concentrations of HAs and the high complexity of the matrix in which they exist, sensitive and selective analytical methods are required for quantification. SPE was performed on a packed bed of C₁₈ particles inside the CE capillary, which minimised the dead volume. The on-line coupling of SPE, CE and nESI-MS reduced the time for extraction and identification to less than half an hour, which will allow for screening of several samples per day. The new technique provides short analysis time, low sample and solvent consumption, and HAs in standard solutions were easily detected at 12–17 fmol injections, and in spiked urine samples at 750–810 fmol injections.     

High-performance liquid chromatographic determination of cocaine and benzoylecgonine by direct injection of human blood plasma sample into an alkyl-diol-silica (ADS) precolumn

A column-switching high-performance liquid chromatographic method with UV detection for the determination of cocaine (COC) and benzoylecgonine (BZE) in human blood plasma samples is described. The method uses an alkyl-diol-silica ADS-C18 extraction precolumn. A 50- micro L plasma sample was introduced to the ADS precolumn in order to separate the analytes from proteins and endogenous compounds. The fraction containing COC and BZE was back-flushed and transferred to an Alltech mixed-mode C(18)/cation-exchange analytical column for final separation. The validation of the method revealed quantitative recoveries from 95.0 to 99.0% for COC at three different concentrations (0.5, 1.0 and 2.0 micro g mL(-1)), and from 96.0 to 99.0% for BZE at the same concentration levels with coefficients of variation <4.00% (n=5). The detection limit (signal to noise ratio (S/N)>3) was 0.03 micro g mL(-1) for all the compounds with an injection volume of 50 micro L. However, it was possible to enhance the sensitivity further by injecting larger plasma volumes, up to 200 micro L, at the same optimal conditions. The overlap of sample preparation, analysis and reconditioning of the extraction column, increase the overall sample throughput to 5 samples h(-1). The developed method has been applied to human blood plasma samples from subjects suspected of cocaine abuse.     

Determination of S-carboxymethyl-L-cysteine and some of its metabolites in urine and serum by high-performance liquid chromatography using fluorescent pre-column labelling.

Pre-column labelling techniques are described for the determination of S-carboxymethyl-L-cysteine (CMC) and its metabolites in urine and plasma samples by high-performance liquid chromatography (HPLC) without prior extraction. All substances containing an amino group were converted into fluorescent fluorenylmethyl derivatives with 9-fluorenylmethyloxycarbonyl chloride (FMOC). Deaminated or N-acetylated carbocysteine metabolites were coupled with 1-pyrenyldiazomethane (PDAM) to give fluorescent PDAM esters. Similar results were obtained with the two commercially available and stable diazomethane derivatives PDAM and 9-anthryldiazomethane (ADAM). Following double derivatization with PDAM and FMOC, in a single chromatographic run with two fluorescence detectors connected in series, amines and amino(carboxylic) acids could be detected by their FMOC residues and, simultaneously, carboxylic acids were detected as fluorescent PDAM esters. The (R) and (S) enantiomers of the sulphoxides of CMC, of methylcysteine and of N-acetyl CMC were separated, although the reversed-phase HPLC system did not contain a chiral additive or stationary phase designed for the separation of enantiomers. The methods do not include liquid extraction steps and can therefore be performed either manually or automatically using an HPLC autosampler. These methods were used for the investigation of a disputed pharmacogenetic polymorphism of S-oxidation of CMC in humans, which until now has most often been studied using paper chromatography. The described techniques were applied to the determination of CMC and its metabolites in human urine and plasma samples.     

Substituent control in the diastereoselectivity of dipolar cycloadditions of nitrones and their Zn(II) complexes with N-arylmaleimides

The π-π stacking interactions between maleimide's and nitrone's aromatic rings during the 1,3-dipolar cycloaddition were assumed to control the exo-endo selectivity of the reaction. The exo-endo ratios change during the reactions until they reach a constant value, which depends on the substituent. Electron-withdrawing groups favour the exo adduct while electron-donating groups favour the endo adduct. The nitrone ZnBr₂ complexes react much more slowly than the free nitrone and the cycloaddition is exo selective in all cases independent of the substituents on the maleimide's aromatic ring. Thermal retrocycloaddition of the cycloadducts produce the corresponding nitrones. The ring opening in the presence of secondary amines did not induce imine formation. endo Adducts were shown for the first time to be the stable paramagnetic compounds.