Effect of visible light on selected enzymes, vitamins and amino acids.

In most cases, the presence of an endogenous photosensitizer is a requirement for visible light modification of biomolecules in animal tissues. Riboflavin (RF) is present in all aerobic cells and is a very efficient photosensitizer, presenting a complex photochemistry with a mixed type I-type II mechanisms. Visible light irradiation in the presence of RF diminished the enzymatic activity of horse radish peroxidase (HRP) only when this glyco-enzyme was deglycosilated. In contrast, the activity of catalase is inactivated via singlet oxygen, and that of lysozyme was efficiently inactivated by a mixed type I-type II mechanisms. The reactive species involved in the RF sensitized photoconversion of lysozyme and the aromatic amino acids tryptophan and tyrosine (both free in solution) is discussed. The role of ascorbate and the effect of RF photosensitized processes in biological complex systems, such as the ocular lens and tumoral cell in culture, is analyzed.     

Elliptic operators with unbounded coefficients: construction of a maximal dissipative extension

We prove maximal dissipativity of some dissipative systems in where is an invariant measure. Received May 23, 2000; accepted June 10, 2000.     

Gas chromatography-optical fiber detector for the speciation of aromatic hydrocarbons in confined areas

An analytical method, based on separation with gas chromatography (GC) and detection with optical fiber (OF), was used for the separation, detection and quantification of benzene, toluene, ethylbenzene, p-xylene, m-xylene and o-xylene. The use of OF as a detector is based on the variations of the reflected optical power detected when the aromatic compounds eluted from the GC column are sorbed in a thin polymeric film on a single-mode OF. General figures of merit, such as the analytical time, analytical error and analytical performance of GC-OF were similar to those of the classical analytical methods, such as a gas chromatography-flame ionization detector (GC-FID). However, the developed GC-OF method constitutes a much less expensive alternative for the speciation of aromatic hydrocarbons compounds, with high accuracy, and being most suitable for actual monitoring work on confined environments.     

Intermediate coupling model description of55Fe and57Fe

The properties of the negative parity states of⁵⁵Fe and⁵⁷Fe are investigated in the framework of the intermediate coupling model. In the model, a neutron or a quasineutron is coupled to anharmonic vibrations of the core. Anharmonicities of the vibrations are estimated through the observed properties of the core. Energy levels, spectroscopic factors and electromagnetic properties have been calculated. The results of the present calculations are also compared with available experimental results and other theoretical results. The model reasonably accounts for many of the properties of the low-lying states.     

Automatic stopped-flow determination of l-cysteine

An automatic stopped-flow method is proposed for the routine determination of microgram amounts of l-cysteine, based on its oxidation by 2,6-dichlorophenolindophenol (DPIP) in a weakly basic medium. The oxidation reaction is monitored by measuring the rate of the absorbance decrease at 615 nm, the wavelength of the maximum absorption for DPIP. Under optimal conditions, the calibration graph is linear over the range 2-32 mug/ml and the detection limit is 300 ng/ml. Most of the amino-acids tested have no effect on the rate of the oxidation reaction and are tolerated at high concentration levels. The sample throughput achieved, 80 samples per hr, allows the proposed method to be used for the routine determination of l-cysteine.     

A study of amino-protecting groups using the polarizable continuum model (PCM)

In order to better understand the performance of 1,2-dimethyl-5-acetyl barbituric acid (DMB) as an amino protecting group relative to 5,5-dimethylcyclohexane-1,3-dione (DMD), ab initio calculations were performed. pK_a calculations using the PCM model indicated that both molecules are more acidic in the enol form. Therefore, the protecting reaction of these molecules should involve the anions formed from the loss of a proton from the enol compounds. Contrary to what would be expected, the larger efficiency exhibited by the DMB molecule cannot be attributed to an extension of the electronic conjugation effect. In the absence of any other noticeable effect that could be responsible for the greater efficiency of the DMB molecule, we are inclined to believe that the difference could be accounted for by the presence of two independent centers of conjugation.This paper is dedicated to Jacopo Tomasi in recognition of his outstanding contribution to the field of computational chemistry in solution. The authors are honored to contribute to this volume; especially so for two of them (COS and MACN) who have the privilege of his friendship.Acknowledgements The authors would like to thank the Brazilian research agencies CNPq, CAPES and FAPERJ for the financial support. C. O. da Silva thanks the Dipartimento di Chimica e Chimica Industriale, University of Pisa, where the MCSCF calculations were performed.     

Determination of copper in water samples by atomic absorption spectrometry after cloud point extraction

Cloud point extraction employing the new reagent 6-[2′-(6′-methyl-benzothiazolylazo)]-1,2-dihydroxy-3,5-benzenedisulfonic acid as complexing agent and Triton X-114 as the surfactant is proposed for copper determination. A sample volume of 10 mL was used. Dilution of the surfactant-rich phase with acidified methanol was performed after phase separation, and the copper contents were measured by flame atomic absorption spectrometry. Variables affecting the system were optimized using factorial design and Doehlert matrix. Signals were measured as peak height using an instrument software. Using the experimental conditions defined in the optimization, the method allowed copper determination with a detection limit of 1.5 μg L⁻¹. The calculated enrichment factor is 14. The effects of foreign ions are reported. The accuracy of the procedure was tested by analyzing certified reference material. The method was successfully applied to copper determination in natural and drinking water samples.