Determination of chloride by stripping chronopotentiometry with silver-film electrode

The experimental parameters of cathodic stripping chronopotentiometry of chloride at a silver-film electrode are investigated and optimized. The chloride preconcentration is achieved in the form of silver chloride by a controlled potential oxidation of the working electrode under vigorous stirring. Cathodic stripping of the deposit is obtained by the constant current, under the condition of diffusive mass transfer. Deaeration of the solution is not necessary. A detection limit of 35 μ dm⁻³ (10⁻⁶ mol dm⁻³) is obtained at a deposition time of 180 s, with a reproducibility of 6.7 % (expressed as relative standard deviation, RSD).     

Protein secondary structure conformations and associated hydrophobicity scales

We have developed conformational preference functions and a hierarchy of algorithms that can evaluate the success of each hydrophobicity scale in predicting protein secondary conformation. The results of such evaluation are shown for fiftyfive different scales with respect to their ability to predict -helix, -sheet and coil structure in three testing sets of proteins: five integral membrane proteins, twelve -class and sixteen -class soluble proteins. Our scale of conformational parameters is the best predictor of secondary structure segments in membrane proteins and -class proteins. The success rate and correlation coefficient for -helix conformation in membrane proteins are 76% and 0.46 respectively, which is superior to the performance measures attained with other prediction schemes. Evaluation of solution hydrophobicity scales, often used to predict transmembrane segments in membrane proteins, indicated absence of correlation in prediction of helix segments and experimental results for the conformation of membrane proteins. Such scales have better performance (correlation coefficient around 0.30) in predicting sheet conformation in the -class proteins.     

On the z-counting polynomial for edge-weighted graphs

The construction of the Z-counting polynomial for edge-weighted graphs is discussed.Dedicated to Professor Haruo Hosoya (Tokyo) on the occasion of his 55th birthday for enriching chemical graph theory with the elegant concept of the Z-counting polynomial.     

Use of x-ray fluorescence spectrometry in ion-exchange studies of leach liquors

Equilibrium and kinetic studies of ion exchange of a complex leach liquor from coal ash, with strongly basic resins and a liquid ion exchanger are reported. The multi-component system was studied by determining the composition of the effluents and by non-destructive analysis of the loaded ion exchangers by XRF spectrometry. In the case of column operation, the elution processes were investigated by the same method, which was very appropriate for non-destructive monitoring of resin regeneration steps done repeatedly with the same resin sample.     

On lower bounds of number of perfect matchings in fullerene graphs

Couting perfect matchings in graphs is a very difficult problem. Some recently developed decomposition techniques allowed us to estimate the lower bound of the number of perfect matchings in certain classes of graphs. By applying these techniques, it will be shown that every fullerene graph with p vertices contains at least p/2+1 perfect matchings. It is a significant improvement over a previously published estimate, which claimed at least three perfect matchings in every fullerene graph. As an interesting chemical consequence, it is noted that every bisubstituted derivative of a fullerene still permits a Kekulé structure.     

Can colloidal particles flocculate at two energy minima simultaneously in a quiescent medium?

The effect of electrostatic repulsion forces on the gravity-induced flocculation of non-Brownian particles in a quiescent medium is analyzed, and the phenomenon that colloidal particles can simultaneously flocculate at both the primary minimum and the secondary minimum of the total interaction curve of DLVO theory is proved. A narrow strip region describing this "combined" flocculation behavior exists on the stability diagram, and the concept of a limiting trajectory can be applied to explain this interesting result.     

Delphinidin–Aluminum(III) Complexes in Aqueous and Non-Aqueous Media: Spectroscopic Characterization and Theoretical Study

Summary. The study of delphinidin complexation with trivalent aluminum in acidic aqueous buffered (pH 3.0 and 3.8) and methanolic solutions was performed utilizing electronic absorption spectroscopy and quantum chemical calculations. In its structure delphinidin possesses several chelating sites in competition towards aluminum(III). Molar ratio plots denoted the formation of only one aluminum(III):delphinidin complex of stoichiometry of 1:1 in both investigated media. Semiempirical calculations, performed at the restricted HF AM1 level, enabled the determination of the structural features of free delphinidin and structural modifications caused by chelation of aluminum(III). Considering the pigment molecular structure and the results of the theoretical calculations it is possible to equally implicate C3′–C4′ and C4′–C5′ hydroxyl groups as those with the predominant chelating power.     

Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH A 2Sigma+ (v = 4) radicals

Fluorescence-dip infrared spectroscopy, an UV-IR double-resonance technique, is employed to characterize the line positions, linewidths, and corresponding lifetimes of highly predissociative rovibrational levels of the excited A (2)Sigma(+) electronic state of the OH radical. Various lines of the 4 <--2 overtone transition in the excited A (2)Sigma(+) state are observed, from which the rotational, centrifugal distortion, and spin-rotation constants for the A (2)Sigma(+) (v = 4) state are determined, along with the vibrational frequency for the overtone transition. Homogeneous linewidths of 0.23-0.31 cm(-1) full width at half maximum are extracted from the line profiles, demonstrating that the N = 0 to 7 rotational levels of the OH A (2)Sigma(+) (v = 4) state undergo rapid predissociation with lifetimes of < or =23 ps. The experimental linewidths are in near quantitative agreement with first-principles theoretical predictions.     

Vanadium determination in rat tissues and biological reference materials by neutron activation analysis

Vanadium was determined in adrenal gland, brain, ileum, kidney, liver, lung, muscle, myocard, skin, spleen, gonads, thyroid, and tibia of rats fed with normal diet and exposed to high vanadium doses in drinking water. Both radiochemical neutron activation analysis (RNAA) and instrumental neutron activation analysis (INAA) were employed. The RNAA procedure consisted in dry ashing samples prior to irradiation and vanadium separation from the irradiated samples by extraction with N-benzoyl-N-phenylhydroxylamine (BPHA) in toluene from 5 mol·l^–1 HCl. Vanadium accumulation as a function of a type of the tissue, exposure time, sex of rats, and administration of V(IV) or V(V) was studied. For quality assurance purposes, the biological (standard) reference materials NBS SRM 1571 Orchard Leaves, NBS SRM 1577a Bovine Liver, IAEA H-4 Animal muscle, and Bowen's Kale were analyzed.Presented at the 2nd Balkan Conference on Activation Analysis and Nuclear-Related Analytical Techniques, Bled, 4–6 October, 1989.     

Concentrations of some elements in the coastal sea sediments: Bays with marinas

Summary Surface sediments and sediment cores from two bays in the Adriatic sea (Punat Bay and Soline Bay, Croatia) have been analyzed for a number of elements, in particular: Ti, V, Mn, Fe, Cu, Zn, As and Pb, by using XRF. Maps of elemental distribution in surface sediments show increased concentrations for some elements present in antifouling paints (Cu, Zn, Pb) near the service areas in the villages or marinas. Core profiles for these elements were used to evaluate the environmental impact of newly constructed marinas. Source partition indicates the influence of other sources located in near by villages. The critical factor in these considerations was shown to be water exchange with the open sea.