Riemannsche Arealstrukturen und multiquadratische Formen

It is well known (and rather trivial to prove) that the square F² of a Finsler norm F:TM on a differentiable manifold M is differentiable at the zero section if and only if F is the norm function of a Riemannian metric. However, the corresponding question for a general p-areal on M is far less trivial to settle and leads to interesting algebraic and combinatorial problems concerning multiquadratic forms. For p=2, the results are closely related to known properties of the curvature tensor of a Riemannian metric.     

Isolation,Structure Determination and Synthesis of New Acetylenic Steroids from the Sponge Calyx nicaaensis

Two acetylenic steroids, cholest-5-en-23-yn-3β-ol ( 5 ) and 26,27-dinorcholest-5-en-23-yn-3β-ol ( 3 ), and another unsaturated steroidalcohol, stigmasta-5,23-dien-3β-ol ( 7 ), were isolated from the sponge Calyx nicaaensis. The structures of these two acetylenic steroids were established by synthesis. Several attempts to synthesize the marine steroids alcohol calysterol ( 1 ), with a cyclopropene-containing side chain, starting from cholest-5-en-23-yn-3β-ol are also recorded. Addition of ethyl-diazo-acetate to the triple bond was performed, but the reduction to the methyl derivative yielded decomposition products.     

Synthesis of C-3' methyl taxotere (docetaxel)

Protected (3R,4S)-N-Boc-3-hydroxy-4-methyl-4-phenylazetidin-2-one has been synthesized stereoselectively and used to esterify protected 10-desacetylbaccatin III to give, following removal of the protecting groups, novel C-3' methyl taxotere (docetaxel).     

A spirocyclic system comprising both phosphazane and phosphazene rings

The reaction of hexakis(cyclohexylamino) cyclotriphosphazene [NP(CyNH)(2)](3), 1, with phosphorus trichloride yields [NP(CyN)(2)PCl](3), 2, which contains three four-membered phosphazane rings fused in spirocyclic fashion to a central six-membered phosphazene ring and constitutes the first structurally characterized compound that comprises both phosphazene and phosphazane rings. The peripheral P atoms feature stereoactive lone pairs, and, thus, 2 exists in isomeric C(3h) and C(s) forms. The spirocyclic phosphazene-phosphazane derivative 2 carries three reactive PCl functions in peripheral positions, promising an interesting precursor molecule for the synthesis of extended phosphorus nitrogen structures of high rigidity. Extension of the PN moiety can be achieved by reaction of 2 with a primary amine yielding [NP(CyN)(2)PN(H)(t)Bu](3), 3, which features a central scaffold of 6 phosphorus and 12 nitrogen centers and aggregates via N-H...P hydrogen bonds in the solid state. On the contrary, the reaction of 1 with SbCl(3) undergoes incomplete proton abstraction, resulting in the formation of the tricyclic compound N(3)P(3)(CyNH)(4)(CyNSbCl(2))(2), 4, which contains two four-coordinate Sb centers chelated by N(exo)-N(ring) sites of the phosphazene.     

Charge neutralization of ions from benzene

Cross-sections have been measured for the charge neutrilization if ions from benzene in kiloelectron-volt collisions with benezene target molecules. Measured values range from 65 Ų for the symmetric [C₆H₆]⁺? C₆H₆ resonant reactions to 8 Ų for [C₃H₃]⁺? C₆H₆ reactions. Cross-sections computed using a simple resonance charge transfer model compare favourably with experimental data for the symmetric reactions. The cross-sections for asymmetric reactions are smaller that those for they symmetric system and magnitudes of the asymmetric cross-sections are correlated with recombination energies of the respective ions.     

Position dependent heavy atom effect in physical triplet quenching by electron donors

The radical yields and rate constants in the quenching reaction of thionine triplet with the complete series of monohalogen substituted anilines as electron donors were determined by flash spectroscopy. Whereas the quenching rate constants show little and unsystematic variation, the radical yields decrease with increasing spin—orbit coupling constant of the halogen substituent. This effect is very sensitive to the position of the halogen in the donor. The results are explained in terms of a heavy atom effect on the intersystem crossing rate constant in a triplet exciplex.     

EXCITATION ENERGY TRANSFER IN THE CRYPTOPHYTES. FLUORESCENCE EXCITATION SPECTRA AND PICOSECOND TIME-RESOLVED EMISSION SPECTRA OF INTACT ALGAE AT 77 K

Excitation spectra of chlorophyll- a (Chl- a ) fluorescence in intact cells of Cryptomonas ovata, Chroomonas pauciplastida and Chroomonas salina were determined at 77 K. For all species the excitation spectra for emission from Chl- a associated with photosystem II (PSII) showed increased contributions by a carotenoid (493 nm) and phycobiliproteins, and decreased contributions by carotenoid (417 nm, 505 nm) and Chl- a (445 nm) as compared to excitation spectra for emission from Chl- a associated with photosystem I (PSI). Excitation spectra of C. salina and C. ovata showed an increased contribution by Chl- c ² to PSII Chl- a fluorescence emission. In all three species the absorbance band positions of Chl- a , as determined from the excitation spectra, were similar to those previously described in green plants. green algae and phycobilisome-containing organisms. Time-resolved 77 K fluorescence emission spectra of C. ovata and C. salina showed successive emission from both phycoerythrin and Chl- c ², PSII Chl- a , and PSI Chl- a. C. pauciplastida showed successive emission from phycocyanin, PSII Chl- a , and PSI Chl- a. Spectral red-shifts with time were observed for the phycobiliprotein peaks in all three species. The fluorescence decay of phycoerythrin in C. ovata and C. salina was faster than that of phycocyanin in C. pauciplastida. The results are discussed in relation to the organization of the antenna pigments of PSII and PSI in the cryptophyte algae.     

INCREASED QUANTUM YIELD OF PHOTOREDUCTION OF DYES (A CATALYTIC EFFECT)

Abstract— While studying the photoreduction of some dyes (D) by reducing agents (R), it was observed that the quantum yield of the photoreduction increases considerably upon addition of a third substance (C), whereas it is very small when the dye is photoreduced by C alone (catalytic effect), (see Table 1).
The system thionine (D), allylthiourea (R), and azulene (C) was investigated in detail using both flash photolysis and continuous illumination. On photolysis, thionine is converted into its photo-reduced form, leucothionine. Azulene reacts with the basic form of the thionine triplet ³ TH ⁺ to produce the semithionine radical. In the system thionine and azulene, most of these radicals revert back to thionine. When ATU (˜ 10^2- M ) is added to thionine and azulene (3 × 10^-4 M ), the semithionine radicals are reduced to leucothionine; the quantum yield of this reduction is considerably higher than in the system thionine and allylthiourea. Flash experiments demonstrate that allylthiourea does not react with the semithionine radicals.
At very high ATU concentrations (≥ 10^-1 M ), however, the primary reaction is between thionine triplet and allylthiourea; under these conditions the quantum yield is not influenced by azulene (3 × 10^-4 M ).     

On-line determination of mercury in river water at the German monitoring station Schnackenburg/Elbe

A monitor is described which provides the on-line determination of mercury in river water at concentrations from 20 to 1000 ng/L. The measurement includes an on-line digestion with Br-/BrO3- and UV-radiation. Each determination is controlled by an on-line addition of 50 and 100 ng/L mercury carried out by pre-dilution of a 500 and 1000 ng/L stock solution using sequential injection analysis (SIA). One cycle of analysis takes 20 min and results in nine signals. A five days stand-alone operation has been performed successfully. Details are also published at web page: "http/www.rzbd.fh-hamburg.de/-prmercol".