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Peter Schwarz

Publications and source records attributed to Peter Schwarz.

23 records · Page 2Linked to original sources
PubMed2003

Ruthenium(II)-tris-bipyridine/titanium dioxide codoped zeolite Y photocatalysts: II. Photocatalyzed degradation of the model pollutant 2,4-xylidine, evidence for percolation behavior.

A considerably arduous test of a novel class of composite materials consisting of [Ru(bpy)3]2+ and TiO2 codoped zeolites Y is presented here. The [Ru(bpy)3]2+ and TiO2 codoped zeolites Y served as photocatalysts in the oxidation of the model compounds 2,4-dimethylaniline (2,4-xylidine) by H2O2 in an acidic aqueous medium. Zeolite-embedded TiO2 (nano)particles play an important role in the degradation mechanism. The first step in this complex mechanism is the photoelectron transfer from photoexcited [Ru(bpy)3]2+*, located inside the supercage of zeolite Y, to a neighboring TiO2 nanoparticle. During this electron transfer process, electron injection into the conduction band of TiO2 is achieved. The second decisive step is the reaction of this electron with H2O2, which was previously chemisorbed at the surface-region of the TiO2 nanoparticles. In this reaction, a TiO2 bound hydroxyl radical (TiO2-HO.) is created. This highly reactive intermediate initiates then the oxidation of 2,4-xylidine, which enters the zeolites framework in its protonated form (Hxyl+). The formation of 2,4-dimethylphenol as first detectable reaction product indicated that this oxidation proceeds via an electron transfer mechanism. Furthermore, [Ru(bpy)3]3+, which was created in the initiating photoelectron transfer reaction between [Ru(bpy)3]2+* and TiO2, also takes place in the oxidation of Hxyl+. [Ru(bpy)3]2+ is recycled in that reaction, which also belongs to the group of electron transfer reactions. In addition to the primary steps of this particular Advanced Oxidation Process (AOP), the dependence of the efficiency of the 2,4-xylidine degradation as a function of the [Ru(bpy)3]2+ and TiO2 loadings of the zeolite Y framework is also reported here. The quenching of [Ru(bpy)3]2+* by H2O2 as well as the photocatalytic activity of the [Ru(bpy)3]2+ and TiO2 codoped zeolite Y catalysts both follow a distinct percolation behavior in dependence of their TiO2 content.

Journal Article↗
PubMed2003

Manipulation of calciumhomeostatic mechanisms in man: what are the possibilities?

The aim of our studies on parathyroid hormone dynamics were to establish standardized methods for induction of hypocalcaemia, sequential hypercalcaemia and normocalcaemia and sequential hypocalcaemia and hypercalcaemia suitable for careful and detailed evaluation of the PTH(1-84) secretion in vivo. We found that at least two distinctly different mechanisms of PTH(1-84) secretion serve to protect normal humans against hypo- and hypercalcaemia. First, an initial decrement of B-Ca2+ leads to a large transient release of preformed PTH(1-84) from the cellular depots, whereas, an initial increment of B-Ca2+ leads to almost immediate suppression of PTH(1-84) release. The change in PTH(1-84) release is rate dependent in either direction and demonstrable even at small decrements or increments of B-Ca2+. This mechanism of delta regulation provide a strong homeostatic mechanism for maintaining a stable extracellular calcium level during slow as well as rapid changes in B-Ca2+. Second, a mechanism of steady state regulation for continued secretion takes over, being dependent on the absolute B-Ca2+ concentration, which probably controls the synthesis of PTH(1-84) molecules. Selective investigation of the steady state response to hypocalcaemia demands elimination of preformed PTH(1-84). With this precaution, we described the inverse sigmoidal relationship in vivo between the steady state pairs of B-Ca2+ and S-PTH(1-84) in normal humans. The calcium set-points of Brown measured by this computer method were significantly lower than Parfitt's calcium set-points in normal humans, but strikingly well correlated. This observation supporting the view that Brown and Parfitt describe two different points on the same sigmoidal curve, corresponding to 50% and about 85% inhibition of PTH(1-84) in normal humans.

Calcium Chloride↗
PubMed2002

Ultrastructural alterations in acquired aortic and mitral valve disease as revealed by scanning and transmission electron microscopical investigations.

Scanning and transmission electron microscopical investigations were carried out on explanted human aortic and mitral valves to study the prevalence of hyperplastic and degenerative lesions in acquired valvular dysfunction. Biopsies were taken from 67 aortic and 23 mitral valves. All of the valves examined showed degenerative lesions including a loose binding of the endothelial cells, a partial denudation of the endothelial cover and areas of fibrous hyperplasia surrounded by calcium deposits. Additionally, the formation of various excrescences was detected by means of scanning electron microscopy. Of all excrescences identified, 90% were localized at the free margin of the leaflet, 3% in the subnodular region and 7% in the nodule of Arantius. The ratio of filiform to lamellar forms of hyperplastic lesions was approximately 80% in most of the samples examined. The results presented demonstrate the complex ultrastructural features of surgically explanted human valves showing both degenerative and hyperplastic lesions in the same valve.

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