Vanadium(V)-doped MCM-41 synthesised by a novel one-step procedure

 A novel procedure to synthesise V-doped surfactant–templated mesoporous materials in one step using a V(V) complex is described. The resulting V(V)–MCM-41 material had a bimodal pore size distribution. Received: 25 April 2001 Accepted: 20 June 2001     

On-line electrothermal atomic absorption spectrometry configurations: recent developments and trends

In the present mini-review, an account of the actual state-of-the-art and future possibilities offered by on-line ET-AAS is presented. Topics such as: (1) on-line analyte preconcentration (by means of precipitation, sorption, solvent extraction, and solid phase extraction); (2) analyte separation by means of chromatography, and electrochemical, microdialysis and chemical vapor generation processes; and (3) sample treatment (by microwave sample digestion, sample emulsification and dilution processes) are used to illustrate the versatility of flow injection, sequential injection analysis, stop flow and continuous flow, when coupled to a graphite furnace. The use of some of the on-line systems for speciation and the simultaneous determination of different analytes is underlined.     

CHEMIENERGIZED SPECIES IN PEROXIDASE SYSTEMS

Abstract— Several hemeprotein-catalyzed reactions generate products of the type expected from the cleavage of a high energy intermediate. For some systems, the formation, in high yield, of a carbonyl compound in its excited triplet state has been firmly established on the basis of (i) equivalence of the chemiluminescence and phosphorescence spectra of the expected products; (ii) energy transfer to sensitizers containing heavy atoms and (iii) occurrence of photoproducts. The excited species appears to be generated within the enzyme and shielded from quenching by oxygen. It may be quenched, however, via long-range triplet-singlet energy transfer.
This work strongly supports our hypothesis that excited electronic states are also formed in biological systems which are not necessarily bioluminescent. One of the functions which peroxidases may thus fulfill might be the utilization of the potential of photochemistry in the absence of light.     

Monosaccharides and the VO(IV) metal ion: Equilibrium,thermal studies and hypoglycemic effect

Protonation and complexation equilibriums of monosaccharides and the VO(IV) metal ion in aqueous solution were studied as well as their effect on the hyperglycemia of diabetic rats. The complexes formed were characterized by potentiometric titrations, paramagnetic resonance spectroscopy (EPR) and thermogravimetric–differential scanning calorimetry (TGA–DSC). The system involving d-gluconic acid (HGlu) and oxovanadium(IV) (VO²⁺) was chosen to study the serum glucose levels in alloxan-induced diabetic rats. A binuclear species was detected in small quantities, which was formed by coordination of two HGlu molecules and two VO²⁺ ions through a hydroxide bridge. The mononuclear species formed by HGlu and VO²⁺ were confirmed by EPR. The anisotropic spectra obtained from aqueous frozen solutions (77 K) are characteristic of mononuclear VO-hexoses. The cyclic sugars d-ribone-1,4-lactone (Riblac), d-galactone-1,4-lactone (Galac) and 2-deoxy-d-glucopyranose (dGlu) showed weak interactions with the metal ion and they are not able to hold the metal in solution above pH 4.6 resulting in hydrolysis of the metal ion. Also, the acute treatment with sugar complexes of HGlu–VO led to a significant hypoglycemic effect (23% and 18% by intraperitoneal or oral gavage treatment, respectively) in diabetic rats. These results show the potential effectiveness of VO–HGlu complexes as anti-hyperglycemic agents through intraperitoneal injection in alloxan-induced diabetic rats.     

On-line generation and hydrolysis of methyl borate for the spectrophotometric determination of boron in soil and plants with azomethine-H

A continuous-flow system for boron determination in soils and plants with spectrophotometric detection using the azometihine-H-boron complex method was developed. In order to avoid the interferences of concomitants present in samples and to increase the sensitivity, the element was separated on-line from the matrix by methyl borate generation. For this purpose, a concentrated sulfuric acid sample solution was combined with methanol in 1:3 ratio which produce enough heating for the esterification reaction without external source. Subsequently, the methyl borate produced was stripped by the addition of a nitrogen flow and separated from the bulk solution in a gas-liquid separator to be then hydrolyzed in an ammonium-phosphate buffer solution (pH 6.8). Finally, the new bulk of phases were separated in a second gas-liquid separator and the liquid phase was combined with azomethine-H to form a boron complex for its detection at 420 nm. The effects of a number of possible interferents, both anionic and cationic were evaluated. The most severe depressions were caused by fluoride and potassium for which a concentration of 100 μg ml⁻¹ caused a 5% depression on the signal. A linear response was obtained between the detection limit of 0.05 μg ml⁻¹ (3σ of the blank) and 50 μg ml⁻¹ of boron. The precision (R.S.D.%) for 10 consecutive readings of the same solution (5.0 μg ml⁻¹ of boron) was 2.6%. Recoveries of boron added to the samples before the extraction process were 94, 97, and 101% for soil, fruit tissue, and leaf tissue, respectively. The developed system was applied to the determination of boron in soil, fruits tissue, and leaves tissue of coffee plantations from different towns of Mérida State, Venezuela.     

Aqueous sodium oleate–sodium dehydrocholate mixtures at low concentration

The aqueous mixed system sodium dehydrocholate (NaDHC)–sodium oleate (NaOL) was studied by several methods to determine the influence of the hydrophobic structure of both surfactants in the mixed micellization and the formation of the mixed monolayer adsorbed at the air–water interface. The molecular area at the critical micelle concentration in pure surfactant solutions suggests that the adsorbed oleate chain was folded to allow the double bond in the middle of the molecule to remain in contact with water, and that the NaDHC molecule was situated with its plane laying parallel to the water surface, allowing the three carbonyl groups in the hydrocarbon backbone to form hydrogen bonds with water. The interaction was repulsive at the surface, and in the mixed monolayer some molecules must move away the less hydrophilic groups from water (double bond of NaOL, carbonyl groups of NaDHC). The interaction in mixed micelles was strongly attractive, showing a preferential composition roughly equimolar. The hydrolysis in mixed micelles was augmented in comparison with pure surfactants systems, which could be explained by assuming the existence of a more hydrophobic mixed micelle core. The mixed micelle degree of ionization was below that of the pure micelles, thus indicating a high surface charge density.     

Multi-syringe flow injection system with in-line microwave digestion for the determination of phosphorus

A multi-syringe system for spectrophotometric determination of total phosphorus involving in-line digestion is proposed. Sample and digestion solution were dispensed and directed towards a digestion vessel located inside a domestic microwave oven (MWO) where sample digestion took place. Afterwards, the digested sample was merged with the necessary reagents for the colorimetric determination based on the molybdenum blue method. Several digestion conditions were studied regarding composition of digestion solution, digestion time and power set on the MWO. The system was applied to waste water samples and results shown a good agreement with the reference method. Repeatable results (R.S.D.<2.41%) and determination frequency of 12 h⁻¹ were obtained.     

Spectroscopic binding studies of novel fluorescent distamycin derivatives

Novel distamycin-porphyrin conjugates were synthesized and their interaction with calf thymus DNA was studied. Minor groove binding of the distamycin part of the molecule was confirmed. The porphyrin part of the conjugates exhibited intercalation and the non-specific electrostatic interaction with the phosphate groups of DNA.     

Numerical stability in the calculation of eigenfrequencies using integral equations

We comment on a phenomenon of instability that appears while computing eigenfrequencies using the integral equation framework. More precisely, it is currently known that the real symmetric matrices are well, and sometimes the best, adapted to numerical treatment. However, we show that this is not the case, if we wish to determine with high accuracy the spectrum of elliptic, and other related operators, using integral representations.