WKB-type approximations for second-order differential equations in -algebras

Rigorous asymptotic approximations of the WKB (or Liouville-Green) type are obtained for a basis of solutions to in the framework of -algebras. Both cases and are included, thus generalizing the classical theory for scalar equations developed by F.W.J. Olver to matrix as well as to infinite-dimensional equations.

     

Combined cysteine and homocysteine quantitation in plasma by trap and release membrane introduction mass spectrometry

Recently, a new and efficient method for total homocysteine (tHcy) quantitation in plasma using trap and release membrane introduction mass spectrometry (T&R-MIMS) with a versatile removable direct introduction membrane probe (DIMP) was described [R. Haddad, M. A. Mendes, N. F. Hoehr and M. N. Eberlin, Analyst, 2001, 126, 1212]. Herein we report on the use of the DIMP-T&R-MIMS technique for total cysteine (tCys) quantitation; hence combined tCys and tHcy quantitation in plasma or serum can be achieved. The method employs Cys and Hcy derivatization with ethyl chloroformate (after disulfide bond reduction with dithiothreitol and protein precipitation with trichloroacetic acid), preconcentration in a capillary silicone membrane, and their thermal desorption to the gas phase inside the ion source region of a mass spectrometer, at a point exactly between the two ionization filaments. Thermal desorption uses the uniform heat radiation provided by the two ionization filaments. The analytes are then ionized by electron ionization and both Cys and Hcy are quantitated by mass spectrometry using selected ion monitoring. For tCys quantitation, good linearity and reproducibility was observed for concentrations ranging from 5 to 350 microM, recovery was near 95%, and the limit of detection (LOD) was of 2 microM. This LOD is well below the mean Cys concentration in plasma, and serum samples from a large group of healthy people showed a mean tCys concentration of 132 +/- 45 microM.     

Diastereoselective synthesis of 2-phenylselenenyl-1,3-anti-diols and 2-phenylselenenyl-1,3-anti-azido-alcohols via hydroxy and azido-selenenylation reactions

A method to synthesize 2-phenylselenenyl-1,3-anti-diols and 2-phenyl- selenenyl-1,3-anti-azidoalcohols via hydroxy- or azido-selenenylation of trans-allylic alcohols is reported. Moreover, the first example of hydroxyl-selenenylation of an allylic azide is presented. Yields ranging from moderate to good and diastereomeric ratios up to 95:5 are achieved.     

Synthesis of Aristotelia-Type Alkaloids. Part XIII. Total syntheses of (+)-makonine, (+)-aristotelinone,and (+)-11,12-didehydroaristoteline

The oxidative transformation of synthetic (+)-aristoteline ((+)- 6 ) into other metabolites which had been isolated from Aristotelia species was investigated. Thus, treatment of (+)- 6 with I₂ as the single oxidant furnished the naturally occurring indole alkaloids (+)-makonine ((+)- 9 ),(+)-aristotelinone ((+)- 11 ), or (+)-11, 12-didehydroaristoteline ((+)- 7 ) in good yields, the selectivity of the oxidation process depending on the chosen reaction conditions.     

Studies on decarboxylation reactions. Part 6 . Kinetic study of decarboxylation of 5-amino-1,3,4-oxadiazole-2-carboxylic acid and its N-phenyl derivatives at high hydrochloric acid concentrations

The dissociation constants (K₁) of both acids 4a-c and esters 5a-c and the rate constants of the decarboxylation reaction of acids 4a-c have been measured at various high concentrations of hydrochloric acid (0.5-8.0 M range). The results obtained have enabled us to suggest the probable structure of the zwitterion which undergoes decarboxylation.     

Diffusion layer titration of dipyrone in pharmaceuticals at a dual-band electrochemical cell

This paper describes the use of a thin-layered dual-band electrochemical cell operating at flow conditions to determine dipyrone (sodium salt of 1-phenil-2,3-dimethyl-4-methylaminomethanesulfonate-5-pyrazolone) by reaction with electrogenerated iodine. The electrolytic cell consisted of two closely spaced gold electrodes, the upper stream electrode serving as the generator electrode and the downstream electrode working as the collector electrode. A linear dynamic range from 2 to 15 μmol l⁻¹ dipyrone was obtained by using a sample volume of 100 μl, with a detection limit of 1.1 μmol l⁻¹. Standard deviation (S.D.) of 3.4% for 20 repetitive injections of a 40 μmol l⁻¹ dipyrone solution and sampling frequency of 90 h⁻¹ were achieved. The results obtained with the thin-layered dual-band electrochemical cell for dipyrone determination in three different pharmaceutical samples compared well with those found by iodimetry with coulometrically generated iodine.     

Kinetic study of the reduction of hydroxylamine by Cu(I)

We have studied the reduction of Se^IV in acidic medium (1 M HCl and 1 M HClO₄) by classical and alternating current polarography and single sweep linear voltammetry with dropping mercury electrode and hanging mercury drop (HMDE). Two steps are observed distinctly: (1) The reduction of Se^IV→Se^o leads to a deposit of adsorbed elementary selenium. A mathematical expression is shown for the variation of current which is related to the surface covered at the HMDE. (2) The second step is the reduction of Se^o→Se^2? which takes place at more negative potentials. The accumulation of Se^o may be used for the determination of traces in trace analysis by cathodic stripping. It appears that the behavior of selenium in these two mineral acids is similar. But sometimes it behaves in a different manner, especially in a.c. polarography (in this connection the influence of frequency and demodulation angle are important).     

Heterogenized rhodium complexes as hydrogen transfer catalysts

Cationic rhodium(I) norbornadiene complexes with polystyrene-immobilized imidazole, tetramethylbiimidazole or phosphine ligands in the presence of potassium hydroxide catalyze hydrogen transfer from isopropanol to acetophenone and 1-hexene. [Rh(NBD) (poly-PPh₂)₂] ClO₄ complexes are particularly efficient for the reduction of acetophenone, but slow decomposition to rhodium metal is observed.

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(I) , , , 1-. [Rh(NBD) (-PPh₂)₂]ClO₄ , .

     

Simple and efficient chemoselective mild deprotection of acetals and ketals using cerium(III) triflate

A new and chemoselective method for the cleavage of alkyl and cyclic acetals and ketals at room temperature in wet nitromethane by using catalytic cerium(III) trifluoromethane sulfonate is presented. The high yields, the observed selectivity, the very gentle reaction conditions, and the almost neutral pH make this procedure particularly attractive for multistep synthesis.     

Development of a laccase-based flow injection electrochemical biosensor for the determination of phenolic compounds and its application for monitoring remediation of Kraft E1 paper mill effluent

This paper describes the development of a new system for amperometric determination of phenolic compounds, and its application for monitoring these compounds in paper mill effluent. The method was based on a flow system, a dialysis sampler, and a laccase-based biosensor. The performance of this system was investigated with respect to pH, ionic strength, working potential, and flow-rate dependence. The biosensor showed an excellent long-term stability allowing measurements for over than 3 months. The sensitivity of laccase-based biosensor was tested for phenol, p-chlorophenol, guaiacol and chloroguaiacol; the detector presented selective measurements of micromolar concentration of these compounds. The integration of a dialysis membrane sampling in the system protected the biosensor surface from fouling and gave independence of sample conditions that commonly influence the biosensor performance. These favorable characteristics allowed its application for direct measurements in complex media with no sample pretreatment. This ability was confirmed employing this system in a continuous analysis of phenolic compounds during the remediation of paper mill effluent by ozonization process.