Uniform a priori estimates for positive solutions of the Lane–Emden equation in the plane

We prove that positive solutions of the Lane–Emden equation in a two-dimensional smooth bounded domain are uniformly bounded for all large exponents.     

Adsorption of some mixtures of surface active substances at the mercury electrode: Kalousek commutator measurements

Adsorption behaviour at the mercury electrode of some mixtures of surface-active substances, relevant and important for natural and polluted seawater, has been studied on the basis of capacity current measurements using the polarographic method of discontinuously changing potential known as the Kalousek commutator technique.The total adsorption effects of different mixtures of selected surface-active substances (albumin, lecithin, oleic acid, Triton-X-100, sodium dodecyl sulphate) were determined in the seawater as a supporting electrolyte and compared with the values calculated from the individual isotherms.     

The effect of electrolytes on the surface potential at the rutile/aqueous interface

The method for evaluation of surface potential from the electrode potential of single crystal electrodes is refined. The effect of NaCl and LiCl concentrations on the dependency of the surface potential of TiO₂ on pH was examined. The point of zero potential was shifted to lower pH values in the presence of NaCl and to higher pH values in the presence of LiCl. The interpretation of the data based on the Surface Complexation Model suggested that at 0 0 1 crystal plane of rutile, sodium ions as counterions have more pronounced affinity for association with negatively charged surface groups with respect to the lithium ions.     

Entwicklung und Überprüfung von Photoelektroden zur Wasserzersetzung

Polycrystallinen-TiO₂ semiconductor photoanodes were produced by controlled thermal oxidation, anodic oxidation of Ti-foils and vapour deposition of TiO₂. Samples of both former series were subsequently reduced in H₂-atmosphere (700°C, 30 min) in order to enhance the donor density in the semiconductor layer. All TiO₂-samples were investigated with respect to their crystal structure, the relationship between thickness and photoefficiency, frequency dependence ofMott-Schottky plots and corrosion resistance at long-time operation. Using 10 ns LASER-pulses (=353 nm; 0.6 mJ) some preliminary kinetic studies of the photocurrent were also reported.

Herrn Prof. Dr.K. Schlögl mit herzlichen Glückwünschen zum 60. Geburtstag gewidmet.     

Experimental design for the estimation of photophysical parameters of the two-state excited-state proton-exchange reaction in the presence of pH buffer

The excited-state proton-exchange reaction of commonly used fluorescent pH probes at physiological pH becomes reversible upon addition of pH buffer. Using computer-generated fluorescence decay surfaces, we investigated under which experimental conditions (pH, buffer concentration, and excitation and emission wavelengths) the rate constants describing the excited-state processes (k(ij)) and the spectral parameters related to excitation ((~)b(1)) and emission ((~)c(1)) can be accurately and precisely estimated by global compartmental curve fitting. It was found that a minimum of three fluorescence decay traces should be collected for the pH probe in the presence of buffer. These three decays should be characterized by at least two different pH values and at least two different buffer concentrations. In addition to these three traces, a minimum of one trace corresponding to the pH probe without buffer has to be recorded. Furthermore, for the accurate estimation of k(ij), (~)b(1), and (~)c(1), at least two of these traces should be collected at the same pH and excitation and emission wavelengths. The experimental conditions should be chosen in such a way that decays with unambiguous biexponential character are obtained. For fluorescent pH probes with pK(a) approximately equal to 7 that are responsive in the near-neutral pH range, it is advisable to use buffers with pK(B)(a) values comparable to or higher than the pK(a) of the probe. Because the changes in the decay times are already apparent with a small quantity of buffer, there is no need to use excessively high buffer concentrations. From a practical point of view, the best experimental design is attained when one combines in a single fluorescence decay surface traces originating from samples characterized by different pH values at the same buffer concentration with traces characterized by different buffer concentrations at the same pH and decays of samples without buffer measured at several pH values.     

Reaction of Hydroxyurea with Iron(III): Products and the Stoichiometry of the Redox Reaction

Products and reaction stoichiometry of the interaction between iron(III) and hydroxyurea (HU) in aqueous solutions were studied. Under condition of an excess of iron(III) over hydroxyurea, initially formed mono(hydroxyureato)iron(III) complex decomposed through redox processes, forming two moles of Fe^II, one mole of NH₄⁺, one mole of CO₂, and 0.5 mole of N₂O per 1 mole of initially present HU. Hydroxyurea radical, H₂N‐CO‐NHO^?, was detected in the course of the reaction. The possible mechanism of the formation of the products is discussed and compared with that obtained for horseradish peroxidase mediated oxidation of hydroxyurea with hydrogen peroxide. The possibility of direct reaction of hydroxyurea with the iron center of protein R2 of the ribonucleotide reductase is proposed. The reaction examined may serve as a simple model for testing the suggested hydroxyurea like activity for chemical substances.     

One-electron oxidation of mitomycin C and its corresponding peroxyl radicals. A steady-state and pulse radiolysis study

The one-electron oxidation of Mitomycin C (MMC) as well as the formation of the corresponding peroxyl radicals were investigated by both steady-state and pulse radiolysis. The steady-state MMC-radiolysis by OH-attack followed at both absorption bands showed different yields: at 218 nm G_i (-MMC) = 3.0 and at 364 nm G_i (-MMC) = 3.9, indicating the formation of various not yet identified products, among which ammonia was determined, G(NH₃) = 0.81. By means of pulse radiolysis it was established a total κ (OH + MMC) = (5.8 ± 0.2) × 10⁹ dm³ mol⁻¹ s⁻¹. The transient absorption spectrum from the one-electron oxidized MMC showed absorption maxima at 295 nm (ε = 9950 dm³ mol⁻¹ cm^t-1), 410 nm (ε = 1450 dm³ mol⁻¹ cm⁻¹) and 505 nm ( ε = 5420 dm³ mol⁻¹ cm⁻¹). At 280–320 and 505 nm and above they exhibit in the first 150 μs a first order decay, κ₁ = (0.85 ± 0.1) × 10³ s⁻¹, and followed upto ms time range, by a second order decay, 2κ = (1.3 ± 0.3) × 10⁸ dm³ mol^-1 s⁻¹. Around 410 nm the kinetics are rather mixed and could not be resolved.

The steady-state MMC-radiolysis in the presence of oxygen featured a proportionality towards the absorbed dose for both MMC-absorption bands, resulting in a G_i (-MMC) = 1.5. Among several products ammonia-yield was determined G(NH₃) = 0.52. The formation of MMC-peroxyl radicals was studied by pulse radiolysis, likewise in neutral aqueous solution, but saturated with a gas mixture of 80% N₂O and 20% O₂. The maxima of the observed transient spectrum are slightly shifted compared to that of the one-electron oxidized MMC-species, namely: 290 nm (ε = 10100 dm³ mol⁻¹ cm⁻¹), 410 nm (ε = 2900 dm³ mol⁻¹ cm⁻¹) and 520 nm (ε = 5500 dm³ mol⁻¹ cm⁻¹). The O₂-addition to the MMC-one-electron oxidized transients was found to be at 290 to 410 nm gk(MMC·OH + O₂) = 5 × 10⁷ dm³ mol⁻¹ s⁻¹, around 480 nm κ = 1.6 × 10⁸ dm³ mol⁻¹ s⁻¹ and at 510 nm and above, κ = 3 × 10⁸ dm³ mol⁻¹ s⁻¹. The decay kinetics of the MMC-peroxyl radicals were also found to be different at the various absorption bands, but predominantly of first order; at 290–420 nm κ₁ = 1.5 × 10³ s⁻¹ and at 500 nm and above, κ = 7.0 × 10³ s⁻¹.

The presented results are of interest for the radiation behaviour of MMC as well as for its application as an antitumor drug in the combined radiation-chemotherapy of patients.     

Stereoselective in-vitro aromatic-ring oxygenations of chiral 1,4-benzodiazepin-2-ones

Biological N(1)-demethylation and C(3)-hydroxylation of two enantiomeric 1,4-benzodiazepin-2-ones 1 and 2 (cf. scheme 2) were found to be nonstereoselective. Aromatic-ring hydroxylation, however, took place in the (S)-series only, leading to 3′- and 4′-hydroxylated, N(1)-demethylated, metabolites ( 54 and 56 , cf. scheme 5: these structures were unambiguously confirmed by comparing their UV., CD., and mass spectra with those of authentic specimens). Several compounds, theoretically conceivable as products of hydroxylation in the aromatic A-ring of 1 and 2 by mechanisms including the NIH-shift (cf. scheme 3), were synthesized but none of these compounds was yet isolated from in vitro incubation mixtures.     

Determination of lead in a minute blood sample by the spectrophotometric method

Summary The mono-colour dithizone method for the determination of small amounts of lead in blood (5 ml samples) has been improved to a certain extent and adapted for smaller blood samples (2 ml). Blood is mineralized by wet oxidation with nitric acid and hydrogen peroxide. The lead content is determined spectrophotometrically as Pb dithizonate after extraction with dithizone from the mineralized solution. The modified method has several advantages over the original one: (a) the sensitivity of the method is increased; (b) the amount of the blood necessary for the analysis is smaller; (c) the time needed for the lead determination is shorter. The results show that the modified method is accurate and reliable enough to meet the requirements of practical toxicology.

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Zusammenfassung Die für die Bestimmung kleiner Bleimengen in Blut mit Dithizon entwickelte Methode wurde verbessert und für sehr kleine Blutmengen modifiziert. Das organische Material wird mit Salpetersäure und Wasserstoffperoxid zerstört. Der Bleigehalt wird nach Extraktion aus der mineralisierten Lösung als Bleidithizonat spektrophotometrisch bestimmt.Gegenüber der ursprünglichen Methode ergaben sich einige Vorteile: a) die Empfindlichkeit der Methode wurde gesteigert; b) die für eine Analyse erforderliche Blutmenge ist geringer; c) die Dauer einer Bleibestimmung ist kürzer. Die Resultate zeigen, daß die modifizierte Methode für die Ansprüche der praktischen Toxikologie genügend genau und verläßlich ist.