Characterization of humic substances: Implications for trihalomethane formation

Humic substances are precursors of carcinogenic trihalomethanes (THMs) formed during disinfection by chlorination in water treatment processes. In an effort to understand the relationship between trihalomethane formation potential (THMFP) and physicochemical properties of humic substances, UV-visible absorbance, fluorescence in emission and synchronous scan modes, and NMR spectra were measured for several aquatic fulvic and humic acids. For comparison, a soil fulvic acid was also examined using these methods. The feasibility of the gradient modified spin-echo (GOSE) NMR experiment to selectively measure singlet resonances arising from isolated protons was examined. In addition, diffusion coefficients were measured for DMSO solutions of the fulvic acids using BPPLED and GOSE-edited pulse sequences. Although none of the methods tested produced results that correlated with THMFP, the GOSE intensities determined for different regions of the NMR spectra did reflect the relative abundance of different types of functional groups produced by lignin oxidation. In addition, the GOSE-edited diffusion results suggest that the isolated protons, those most reactive to chlorination, are more likely contained in the larger molecular weight fractions of fulvic acids.     

Kinetic spectrophotometric methods for the determination of dothiepin hydrochloride in bulk and in drug formulation

Two simple and sensitive kinetic methods for the determination of dothiepin hydrochloride are described. The first method is based on kinetic investigation of the oxidation reaction of the drug with alkaline potassium permanganate at room temperature for a fixed time of 25 min. The absorbance of the colored manganate ions is measured at 610 nm. The second method is based on the reaction of dothiepin hydrochloride with 4-chloro-7-nitrobenzofurazan (NBD-Cl) in the presence of 0.1 mol L^–1 sodium bicarbonate. Spectrophotometric measurement was achieved by recording the absorbance at 470 nm for a fixed time of 60 min. All variables affecting the development of the color were investigated and the conditions were optimized. Plots of absorbance against concentration in both procedures were rectilinear over the ranges 4–24 and 50–250 g mL^–1, with mean recoveries 99.33±0.42 and 99.88±0.53, respectively. The proposed methods were successfully applied for the determination of dothiepin hydrochloride in bulk powder and in capsule dosage form. The results obtained were found to agree statistically with those given by the non-aqueous B.P. method. Furthermore the methods were validated according to USP guidelines and also assessed by applying the standard addition technique. The determination of dothiepin hydrochloride by the fixed concentration method is feasible with the calibration equations obtained, but the fixed time method proves to be more applicable.     

Identification of selenium species in urine by ion-pairing HPLC–ICP–MS using laboratory-synthesized standards

This study focused on the detection/identification of possible selenium metabolites in human urine. Organoselenium compounds not commercially unavailable were synthesized and characterized by electrospray mass spectrometry. Separation of selenomethionine, methylselenomethionine, trimethylselonium, selenoethionine, and selenoadenosylmethionine was achieved by ion-pairing HPLC with a mobile phase of 2 mmol L^–1 hexanesulfonic acid, 0.4% acetic acid, 0.2% triethanolamine (pH 2.5), and 5% methanol. The column effluent was introduced on-line to inductively coupled plasma–mass spectrometry for selenium-specific detection (⁷⁷Se and ⁷⁸Se). For selenium speciation in urine, solid-phase extraction was carried out using C₁₈ cartridges modified with hexanesulfonic acid. Selective retention of cationic species was observed from acidified urine (perchloric acid, pH 2.0). After elution with methanol, evaporation, and dissolution in the mobile phase, the sample was introduced to the HPLC–ICP–MS system and the chromatographic peaks were assigned by adding standards. The species identified in urine were selenomethionine, trimethylselonium ion, and selenoadenosylmethionine. The last species was detected for the first time and our results suggest that selenomethionine might enter the metabolic pathway of its sulfur analog in the activated methylation cycle.Kazimierz Wrobel and Katarzyna Wrobel are on the leave from the Institute of Scientific Research, University of Guanajuato, L. de Retana No. 5, 36000 Guanajuato, Gto., Mexico     

The study of terbium generated bacterirhodopsin

The localization of Terbium (Tb³⁺) cations binding to deionized bacteriorhodopsin (bR) has been studied by using spectroscopic methods. It was found that adding Tb³⁺ cations to deionized bR affects the fluorescence lifetimes of tryptophan (Trp) in bR, the wavelength of fluorescence peak shifts “blue” and the peak value of fluorescence decreases. It was also found that adding one Tb³⁺ cation to deionized bR can restore the purple state from its blue state obviously. The measurements of absorbance, fluorescence and lifetime of fluorescence also show that when more than three Tb³⁺ cations are added, no further changes can be found. It is suggested that one Tb³⁺ specific binding site for the color-controlling is located on the exterior of the bR trimer structure to negatively charged lipids near Trp-10 and Trp-12. Three Tb³⁺ cations binding per bR is needed for the regenerated bR.     

Efficient desulfurization of 2-thiopyrimidine nucleosides to the corresponding 4-pyrimidinone analogues using trans-2-(phenylsulfonyl)-3-phenyloxaziridine

A brief treatment of 2-thiopyrimidine nucleosides (s²U^∗) with trans-2-phenylsulfonyl-3-phenyloxaziridine (PSO) results in efficient substrate desulfurization leading to the corresponding 4-pyrimidinone analogues (H²U^∗). The key transformation proceeds through oxidation of the 2-thiocarbonyl group to a sulfur oxyacid derivative and subsequent elimination of sulfur dioxide. 4-Pyrimidinone 1-β-d-riboside (H²U) has been transformed into the respective phosphoramidite, a ready-to-use monomer for the introduction of a modified nucleoside into an oligonucleotide chain. Moreover, the effective desulfurization of the 2-thiouridine nucleotide could be achieved directly at the oligonucleotide level, by treatment of the TdA(s²U)dGdC oligonucleotide with PSO, as verified by MALDI-TOF mass spectrometry.     

Modifying the thermosensitivity of copolymers of sodium styrene sulfonate and<Emphasis Type="Italic"> N</Emphasis>-isopropylacrylamide with dodecyltrimethylammonium chloride

The interactions between copolymers of sodium styrene sulfonate (SSS) and N-isopropylacrylamide (NIPAM), anionic polyelectrolytes, and dodecyltrimethylammonium chloride (DTAC), a cationic surfactant, were studied in aqueous solutions of various ionic strengths. The copolymers were found to be thermoresponsive, showing a lower critical solution temperature (LCST). The influence of the polymer composition, the surfactant concentration, and the ionic strength on the LCST was studied. The surfactant was found to interact strongly with the polymer, forming mixed polymer-surfactant micelles. The critical aggregation concentration (cac) of the polymer-surfactant system was found from fluorescence spectroscopy using pyrene as a fluorescent probe. A strong dependence of the anionic polyelectrolyte-cationic surfactant interactions on the structure of the ionic comonomer was observed.     

Synthesis of highly substituted meso-tetraarylporphyrins

meso-Tetraphenylporphyrin (and its derivatives), in the reaction with fuming yellow nitric acid (d=1.53), form either 5-(4-nitroaryl)-10,15,20-triarylporphyrin, 5,10-bis(4-nitroaryl)-15,20-diarylporphyrin, or 5,10,15-tris(4-nitroaryl)-20-arylporphyrin, depending on the reaction temperature (0-20 °C), amounts of the acid used, and reaction time. The above nitroporphyrins react, in the presence of a base (t-BuOK) at 0 °C, with carbanions (which bear nucleophugal groups at the carbanionic center: ⁻CH(Cl)SO₂Tol, ⁻CH(Br)SO₂Tol, and ⁻CH(Cl)SO₂NMe₂), leading to the nucleophilic substitution of hydrogen in one or more of the meso-nitroaryl rings. By this route, the preparation of the highly substituted ‘synthetic’ porphyrins (bearing up to ten O-, N-, Cl-, or C-substituents) was demonstrated.     

Some applications of silica aerogels

Silica aerogels are very highly divided materials which are synthesised through the association of a chemical step, the so-called sol–gel chemistry, with a physical step which is a particular way of drying the wet gel, namely under supercritical conditions with respect to the liquid phase filling its porosity. This drying process preserves the texture of the dry material: in practice it strongly reduces the pore collapse. The resulting hyperporous solids that have bulk densities of the same magnitude as air develop new and very interesting physical and even chemical properties. Owing to their poor chemical reactivity, very large surface areas (of the order of 1,000 m ²/g), unusual porous volumes (greater than 95%), morphologies (monoliths or powders), optical properties (transparent, opaque or translucent), and very low thermal conductivity, they find high added-value applications in the physics of high-energy particles (Cherenkov emitters), transparent and superinsulating double windows, life and space science as well.