A novel spectrophotometric method for batch and flow injection determination of sulfite in beverages

A novel spectrophotometric assay method for batch and flow injection determination of sulfite in beverages is described. The method involves a reaction with diaquacobyrinic acid heptamethyl ester (diaquacobester, DACbs) in acetate buffer of pH 3 to form a highly stable sulfite cobester complex (SO₃Cbs). In the absence of sulfite, the reagent displays three absorption maxima at 349, 409 and 525 nm. Addition of sulfite is associated with the development of a new absorption band at 313 nm, an increase in the intensity of the band at 425 nm and a decrease in the absorbance of the bands at 349 and 525 nm. Variations of the absorptions at 313, 349, 425 and 525 nm are linearly proportional to sulfite concentrations over the range of 0.05–25 μg ml⁻¹with a detection limit of 0.01–0.2 μg ml⁻¹. Negligible interferences are caused by most common ions. Validation of the method according to the quality assurance standards shows suitability for quality control assessment of sulfite in complex matrices without prior treatment. The method has the advantages of high selectivity, good sensitivity, fast reaction, high stability of the reagent and reaction product and absorbance measurements at four different wavelengths in the same run. The method is successfully applied to determine the sulfite contents of some beverages. The results compare fairly well with data obtained using the standard method.     

Electronic properties of austenite and martensite Fe-9%Mn alloys

We calculate the electronic properties of austenite and martensite Fe-9%Mn alloys using the self consistent full-potential linearized-plane-wave method under the generalized gradient approximation full lattice relaxation. By minimizing total-energy, the lattice constants in their ground states were determined. We discuss the total energy dependence of the volume, and density of states (DOS).      

Hydrogenated carbon clusters produced by highly charged ion impact on solid

The emission of small (hydrogenated) carbon cluster ions C_nHm ⁺ (n =2-22) upon highly charged Xe^q+ (q =20-44) impact on C₈₄ surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation of a certain carbon cluster ion Cn ⁺ is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted as an entity in agreement with “equilibrium” and “shock wave” models. Received 4 February 2000     

Modern pathology: protein mis-folding and mis-processing in complex disease

Electrostatic and electrochemical properties of bio-molecules, such as proteins, are governed by energy parameters that are, in part dependent on its folding. Disruption of this process can lead to the development of complex, multisystem diseases whose presentation may be organ-dependent. Examples include cystic fibrosis, alpha-1 antitrypsin deficiency, and Alzheimer disease. In addition to explaining exotic pathologic syndromes, an understanding of protein folding mechanisms may facilitate the understanding of less complex diseases and allow the development of novel therapeutic approaches.     

On the Cauchy problem for some abstract nonlinear differential equations

In the present paper, we study the Cauchy problem in a Banach spaceE for an abstract nonlinear differential equation of form $$\frac{{d^2 u}}{{dt^2 }} = - A\frac{{du}}{{dt}} + B(t)u + f(t,W)$$ whereW = (A ₁(t)u,A ₂(t)u,?,A _?(t)u), (A _i (t),i = 1, 2, ?,?), (B(t),t ∈I = [0,b]) are families of closed operators defined on dense sets inE intoE, f is a given abstract nonlinear function onI ×E ^? intoE and ?A is a closed linear operator defined on dense set inE intoE, which generates a semi-group. Further, the existence and uniqueness of the solution of the considered Cauchy problem is studied for a wide class of the families (A _i(t),i = 1, 2, ?,?), (B(t),t ∈I). An application and some properties are also given for the theory of partial diferential equations.     

Ligand substitution behavior of a simple model for coenzyme B12

The ligand substitution reactions of trans-[CoIII(en)2(Me)H2O]2+, a simple model for coenzyme B12, were studied for cyanide and imidazole as entering nucleophiles. It was found that these nucleophiles displace the coordinated water molecule trans to the methyl group and form the six-coordinate complex trans-[Co(en)2(Me)L]. The complex-formation constants for cyanide and imidazole were found to be (8.3 +/- 0.7) x 10(4) and 24.5 +/- 2.2 M-1 at 10 and 12 degrees C, respectively. The second-order rate constants for the substitution of water were found to be (3.3 +/- 0.1) x 10(3) and 198 +/- 13 M-1 s-1 at 25 degrees C for cyanide and imidazole, respectively. From temperature and pressure dependence studies, the activation parameters delta H++, delta S++, and delta V++ for the reaction of trans-[CoIII(en)2(Me)H2O]2+ with cyanide were found to be 50 +/- 4 kJ mol-1, 0 +/- 16 J K-1 mol-1, and +7.0 +/- 0.6 cm3 mol-1, respectively, compared to 53 +/- 2 kJ mol-1, -22 +/- 7 J K-1 mol-1, and +4.7 +/- 0.1 cm3 mol-1 for the reaction with imidazole. On the basis of reported activation volumes, these reactions follow a dissociative mechanism in which the entering nucleophile could be weakly bound in the transition state.     

Effect of cold work on the dissolution kinetics in the precipitating Al-2 wt% Mg alloy

The fracture stress _f was taken to assess the mechanism of dissolution of the -phase in the Al-2 wt% Mg alloy. The role of pre-cold working the polycrystalline samples was investigated. The dissolution of the -phase was carried out at different temperatures. The relation between the fracture stress and the time of dissolution showed peaky curves. The rise of _f towards a maximum was attributed to the progressive dissolution of the precipitates. The position of the peak was found to shift towards lower times of dissolution with the increase of the dissolution temperature or the increase of the pre-cold work. Considering the peak position indicative of a certain degree of dissolution, equivalent times and temperatures of the dissolution mechanism were thus found and the activation energy was calculated. Pre-cold working the samples before the dissolution act decreased the activation energy responsible for the process from 1·95×10^–19 J to about 0·48× 10^–19 J. The initial drop in _f observed for heavily cold worked samples pointed towards the possible effect of excess vacancies created by cold work.The authors are really grateful for the enlightening discussions with Prof. R. Kamel.     

Kinetics and mechanism of the reactions of Pd(II) complexes with azoles and diazines. Crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O

Substitution reactions of the complexes [Pd(bpma)(H2O)]2+, [Pd(bpma)Cl]+, [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+, where bpma = bis(2-pyridylmethyl)amine and dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as triazole, pyrazole, pyrimidine, pyrazine and pyridazine, were studied in an aqueous 0.10 M NaClO4 at pH 2.8 using variable-temperature and -pressure stopped-flow spectrophotometry. The second-order rate constants indicate that the Pd(II) complexes of bpma, viz. [Pd(bpma)(H2O)]2+ and [Pd(bpma)Cl]+, are more reactive than the complexes of dien, viz. [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+. Also, the aqua complexes, [Pd(bpma)(H2O)]2+ and [Pd(dien)(H2O)]2+, are much more reactive than the corresponding chloro complexes. The most reactive nucleophile of the five-membered rings is triazole and for the six-membered rings the most reactive one is pyridazine. Activation parameters were determined for all reactions and the negative entropies and volumes of activation (Delta S++, Delta V++) support an associative ligand substitution mechanism. The crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O was determined by X-ray diffraction. Crystals are monoclinic with the space group P2(1)/c. The coordination geometry of [Pd(bpma)(H2O)]2+ is distorted square-planar. The Pd-N (central) bond distance, 1.958(5) A, is shorter than the other two Pd-N distances, 2.007(5) and 2.009(5) A. The Pd-O distance is 2.043(5) A.     

Mechanisms of atomic layer deposition on substrates with ultrahigh aspect ratios

Atomic layer deposition (ALD) appears to be uniquely suited for coating substrates with ultrahigh aspect ratios (> or similar 10(3)), including nanoporous solids. Here, we study the ALD of Cu and Cu3N on the inner surfaces of low-density nanoporous silica aerogel monoliths. Results show that Cu depth profiles in nanoporous monoliths are limited not only by Knudsen diffusion of heavier precursor molecules into the pores, as currently believed, but also by other processes such as the interaction of precursor and reaction product molecules with pore walls. Similar behavior has also been observed for Fe, Ru, and Pt ALD on aerogels. On the basis of these results, we discuss design rules for ALD precursors specifically geared for coating nanoporous solids.