Assessment of Four Inter-Comparison Exercises in Organic Elemental Analysis

 Four inter-comparison exercises on organic elemental analysis were carried out between 1997 and 2001 by the Department of Analytical Chemistry of the University of Barcelona, together with the Microanalysis Service and the Institute of the Marine Sciences, which both belong to the CSIC in Barcelona, and the University of A Coru?a. More than sixty laboratories participated in these exercises. Here we describe the design and characteristics of the trials, the samples and the homogeneity tests applied. We report the results obtained for the analysis of carbon, hydrogen, nitrogen, sulphur and oxygen, their statistical analysis, and the most relevant aspects of the technical discussion meetings. Received December 20, 2001; accepted March 18, 2002; published online July 22, 2002     

Use of a continuous flow solid-phase spectroscopic sensor using two sensing zones: determination of thiamine and ascorbic acid

A simple, rapid, inexpensive, and automated flow-through solid-phase spectroscopic sensing device is proposed for the sequential determination of 2 vitamins: thiamine and ascorbic acid. The vitamins are concentrated on ion-exchange gels, thiamine on Sephadex SP C-25, and ascorbic acid on Sephadex QAE A-25; both solid supports are packed in 2 different flow cells. The absorbance is monitored directly on the solid phase with a double-beam spectrophotometer at 250 nm, without derivatization or additional elution. With the use of 2 carrier/self-eluting solutions (0.1 5M sodium acetate/acetic acid and 0.18M citric acid/K2HPO4) and a sample volume of 1000 microL, the sensor responds linearly in the range of 0.5-15 and 3-50 microg/mL with detection limits of 0.14 and 0.36 microg/mL for thiamine and ascorbic acid, respectively. When the method was applied to synthetic samples and pharmaceutical preparations, precise and accurate values were obtained.     

Sequence-dependent conformational energy of DNA derived from molecular dynamics simulations: toward understanding the indirect readout mechanism in protein-DNA recognition

Sequence dependence of DNA conformation plays a crucial role in its recognition by proteins and ligands. To clarify the relationship between sequence and conformation, it is necessary to quantify the conformational energy and specificity of DNA. Here, we make a systematic analysis of dodecamer DNA structures including all the 136 unique tetranucleotide sequences at the center by molecular dynamics simulations. Using a simplified conformational model with six parameters to describe the geometry of adjacent base pairs and harmonic potentials along these coordinates, we estimated the equilibrium conformational parameters and the harmonic potentials of mean force for the central base-pair steps from many trajectories of the simulations. This enabled us to estimate the conformational energy and the specificity for any given DNA sequence and structure. We tested our method by using sequence-structure threading to estimate the conformational energy and the Z-score as a measure of specificity for many B-DNA and A-DNA crystal structures. The average Z-scores were negative for both kinds of structures, indicating that the potential of mean force from the simulation is capable of predicting sequence specificity for the crystal structures and that it may be used to study the sequence specificity of both types of DNA. We also estimated the positional distribution of conformational energy and Z-score within DNA and showed that they are strongly position dependent. This analysis enabled us to identify particular conformations responsible for the specificity. The presented results will provide an insight into the mechanisms of DNA sequence recognition by proteins and ligands.     

Pyrazino[2,3-c][1,2,6]thiadiazine 2,2-dioxides. Sulfur dioxide analogs of pteridines

Several pyrazino[2,3-c][1,2,6]thiadiazine 2,2-dioxide derivatives have been synthesized for the first time by condensation of suitable 4,5-diamino-1,2,6-thiadiazine 1,1-dioxides and symmetrical 1,2-dicarbonyl compounds. Structures of these compounds have been characterized by their elementary analyses, ¹H-nmr and uv spectra as well as their pK_a values. The most striking differences between this series and the corresponding pteridines are discussed.     

Thermal and FTIR spectroscopic analysis of the interactions of aniline adsorbed on to MCM-41 mesoporous material

The adsorption of aniline on Na-AlMCM-41 synthesized by us has been characterized by infrared spectroscopy, temperature programmed desorption (TPD), and differential thermal analysis methods. Aniline would be mostly bound to the mesostructure through weak pi interactions. On the mesostructure containing adsorbed water, the co-adsorption of aniline could occur by weak hydrogen bonding through surface water molecules. For water, two possible modes of adsorption have been identified. Different associations between aniline and hydrated and nonhydrated mesostructures have been evaluated in order to favor the posterior in situ polymerization of adsorbed aniline.     

An NMR study of merocyanine-type dyes derived from barbituric acid

The 13C NMR of two solvatochromic dyes derived from a barbituric acid acceptor and dimethylaminophenyl donor fragments, compound 1 and the related merocyanine 2, were recorded in various solvents. The observed chemical-shift variations were used to interpret their structural differences and solvatochromic behavior in solution.     

Monitoring of the metal displacement from the recombinant mouse liver metallothionein Zn(7)-complex by capillary zone electrophoresis with electrospray MS detection

A solution of a Zn-complex of recombinant mouse MT-1 isoform (Zn(7)-MT-1) was prepared and titrated with Cd(2+) ions. A method based on the coupling of capillary zone electrophoresis (CZE) with electrospray MS detection was developed for the analysis of the stoichiometry of the species formed during the titration. The method offered the possibility of the on-line removal of up to 100 mM Tris or phosphate buffer solutions that would otherwise suppress the electrospray signal. By allowing the determination of the metal stoichiometry of the complex species present in solution the method was shown to be complementary to circular dichroism and UV-VIS spectrophotometry conventionally used for similar studies. The titration of the Zn(7)-MT complex with Cd(II) showed the sequential displacement of the Zn by Cd. The unusually high stability of the Cd(6)Zn(1)-MT species was observed which suggests a structural role of the remaining Zn(II) ion.     

Synthesis of tetrahydroimidazo[1,5-b]-β-carboline derivatives with complex basic substituents

Novel tetrahydroimidazo[1,5-b]-β-carbolines derivatives 6 bearing complex basic chains as substituents on the imidic nitrogen have been prepared in a one-pot reaction. This simple experimental procedure overcomes the direct handle of isocyanates which can be favorably generated “in situ” from carbonyldiimidazole (CDI) and different amines. The stereochemistry of the novel compounds was determined by nmr experiments.     

Investigating the possibility of simultaneously finding an electron-hole and an electron-pair in a molecule: Delocalization,competition of ionic vs. covalent character,and related effects in push–pull ethylenes

This work examines the possibility of finding an electron-hole and an electron-pair simultaneously in a π-system substituted by an electron-donating (NH₂) and/or electron-withdrawing (NO₂) group. The contributions of various ionic [(+), ( ↑ ↓ )] structures are calculated from ab initio SCF –CI wave functions, using a recently developed general multielectron population analysis. The molecules studied are two monosubstituted ethylenes, aminoethylene and nitroethylene, and a disubstituted ethylene, the 2-nitroethenamine (push–pull ethylene) in its two configurational forms. The influence of the NH₂ and/or NO₂ group in delocalization and ionic (vs. covalent) character of the C?C double bond are investigated, along with examining the experimental chemist formalism of electron-pair “displacements” in several resonance structures. Analysis of mutual dependence of an electron-hole and an electron-pair, at short and long distances, leads to the conclusion that a push–pull π-system can stimulate the simultaneous existence of an electronhole and an electron-pair even for nonvicinal positions. The relationship between the electronpair distributions and contributions of the corresponding structures are also examined; the effects of electronic correlation are analyzed as well. © 1992 John Wiley & Sons, Inc.