Bis(amino acid) derivatives of 1,4-diamino-2-butyne that adopt a C2-symmetric turn conformation

1,4-Diamino-2-butyne was prepared from 1,4-dichloro-2-butyne via 1,4-diazido-2-butyne. Bis(amino acid) derivatives of 1,4-diamino-2-butyne having the general structure (Boc-Xxx-NHCH2C[triple bond])2 (Xxx = Ala, Phe and Met) were prepared and examined by 1H NMR spectroscopy. Using chemical shift, coupling constant and DMSO titration data it is found that these compounds adopt a C2-symmetric turn conformation featuring two intramolecular hydrogen bonds.     

A theoretical investigation of the preferred conformations of glutathione and its constituent amino acid residues

The preferred conformations of the tripeptide glutathione have been investigated by performing quantum mechanical calculations using the PCILO method. A series of model compounds representing fragments of the tripeptide has been studied as well as the complete molecule. The results are compared with the available experimental data.     

AN INEXPENSIVE STIRRING DEVICE FOR THE 'MERRY-GO-ROUND' PHOTOREACTOR FOR THE DETERMINATION OF REACTION QUANTUM YIELDS

A simple bar magnet is employed to effect stirring of the contents of reaction cells in a'merry-go-round'photoreactor.     

Collaborative monitoring study of airborne particulate matters among seven Asian countries

Summary Seven Asian countries have been collaborating in collecting airborne particulate matter (APM) in their individual countries and analyzing them by neutron activation analysis as a common analytical tool. APM samples were collected into two fractions of fine and coarse grains (PM2 and PM2-10, respectively). Analytical data were compared from several viewpoints such as particulate sizes, locality of sampling sites (either urban or rural) and geographical location of participating countries. Chemical composition and their monthly variations as well as mass concentrations appear to be highly characteristic for individual sampling sites, suggesting that NAA data are suitable for evaluating the air quality in each site.     

Feasibility of using solid sampling graphite furnace atomic absorption spectrometry for speciation analysis of volatile and non-volatile compounds of nickel and vanadium in crude oil

A method for the direct determination of volatile and non-volatile nickel and vanadium compounds in crude oil without previous treatment using direct solid sampling graphite furnace atomic absorption spectrometry is proposed. The crude oil samples were weighed directly onto solid sampling platforms using a microbalance and introduced into a transversely heated solid sampling graphite tube. In previous work of our group losses of volatile nickel and vanadium compounds have been detected, whereas other nickel and vanadium compounds were thermally stable up to 1300 and 1600 °C, respectively. In order to avoid this problem different chemical modifiers (conventional and permanent) have been investigated. With 400 μg of iridium as permanent modifier, the signal started to drop already after two atomization cycles, possibly because of an interaction of nickel (which is a catalyst poison) with iridium. Twenty micrograms of palladium applied in each determination was found to be optimum for both elements. The palladium was deposited on the platform and submitted to a drying step at 150 °C for 75 s. After that the sample was added onto the platform and submitted to the furnace program. The influence of sample mass on the linearity of the response and on potential measurement errors was also investigated using four samples with different nickel content. For the sample with the lowest nickel concentration the relationship between mass and integrated absorbance was found to be non-linear when a high sample mass was introduced. It was suspected that the modifier had not covered the entire platform surface, which resulted in analyte losses. This problem could be avoided by using 40 μL of 0.5 g L⁻¹ Pd with 0.05% Triton X-100. Calibration curves were established with and without modifier, with aqueous standards, oil-in-water emulsions and the certified reference material NIST SRM 1634c (trace metals in residual fuel oil). The sensitivity for aqueous standards and emulsions was close to that for SRM 1634c, making possible the use of aqueous standards for calibration. The limits of detection and quantification obtained for nickel and vanadium under this condition were found to be 0.02 and 0.06 μg g⁻¹, respectively, for both elements, based on 10 mg of sample. Nickel and vanadium were determined in the samples with (total Ni and V) and without the use of Pd (thermally stable compounds), and the concentration of volatile compounds was calculated by difference. The results were compared with those obtained by high-resolution continuum source graphite furnace atomic absorption spectrometry by emulsion technique; no significant differences were found for total Ni and V at the 95% confidence level according to a Student's t-test.     

Hydride generation atomic fluorescence spectrometric determination of ultratraces of selenium and tellurium in cow milk

A sensitive procedure has been developed for selenium and tellurium determination in milk by hydride generation atomic fluorescence spectrometry (HG-AFS) after microwave-assisted sample digestion. The method provides sensitivity values of 1591 and 997 fluorescence units ng⁻¹ ml⁻¹ with detection limits of 0.005 and 0.015 ng ml⁻¹ for Se and Te, respectively. The application of the developed methodology to the analysis of cow milk samples of the Spanish market evidenced the presence of concentration ranges from 11.1 to 26.0 ng ml⁻¹ for Se, and from 1.04 to 9.7 ng ml⁻¹ for Te having found a good comparability with data obtained after dry-ashing of samples.     

Application of preparative high-speed counter-current chromatography for the separation of flavonoids from the leaves of Byrsonima crassa Niedenzu (IK)

The methanolic extract of the leaves of the medicinal plant Byrsonima crassa (Malpighiaceae) contain flavonoids with antioxidant activity. They were separated in a preparative scale using high-speed counter-current chromatography. The optimum solvent system used was composed of a mixture of ethyl acetate-n-propanol-water (140:8:80 (v/v/v)) and led to a successful separation between monoglucosilated flavonoids (quercetin-3-O-alpha-L-arabinoside, quercetin-3-O-beta-D-galactoside) and the biflavonoid amentoflavone in only 3.5 h. The purities of quercetin-3-O-alpha-L-arabinoside (95 mg), quercetin-3-O-beta-D-galactoside (16 mg) and the biflavonoid amentoflavone (114 mg) were all isolated at purity over 95%. Identification was performed by 1H NMR, 13C NMR and UV analyses.     

Thermal study of CoxZn7-xSb2O12 spinel obtained by pechini method using different alcohols

With the aim of obtaining materials with applications in pigments, Co_xZn_7-xSb₂O₁₂ spinels were synthesized using the Pechini method. This method consists in the formation of a polymeric net, where the metallic cations are homogeneously distributed. In this work, two types of alcohol (ethyl glycol and ethylene glycol) were used for the synthesis of a zinc antimoniate spinel, Co_xZn_7-xSb₂O₁₂ (x=0-7). The materials were characterized by termogravimetry (TG) and differential thermal analysis (DTA). TG results indicated a decrease in total mass loss when cobalt was added to the solution substituting zinc, for samples prepared using the two different alcohols. Decomposition temperatures, obtained by TG and DTA, presented a decreasing behavior as cobalt was added to the material. In relation to the alcohols, all results indicated a better polymerization of the resin when ethylene glycol was used, being the most indicated one for cation immobilization. X-ray diffraction did not show differences between the two alcohols - both presented the spinel phase (Co, Zn)_2.33Sb₀.₆₇O₄. Samples with higher quantity of cobalt also presented ilmenite phase (Co, Zn)Sb₂O₆. This revised version was published online in August 2006 with corrections to the Cover Date.