Studies on pre-hump and main fractions of cellulose-2,5-acetate in acetone

Technical cellulose-2.5-acetates (CA 2.5) were characterized regarding their carbohydrate composition in comparison to the raw material. The association of the CA 2.5 samples in acetone was studied by size exclusion chromatography (SEC) using various acetone grades and styrene divinylbenzene copolymer columns. In HPLC grade acetone with and without addition of 1% water up to three different pre-humps eluted in front of the main fraction of the polymer. The evaluation of the main peak by light scattering measurements resulted in high molar masses indicating that for these technical CA 2.5 samples even the main fraction is not dissolved without association. No pre-humps or association phenomena were observed after addition of 1 ppm LiBr to HPLC grade acetone or with p.a. grade acetone. In addition pre-hump enriched and pre-hump free fractions were isolated by fractionated precipitation. The carbohydrate composition of these fractions was determined and correlated with their association pattern in SEC investigations.     

Zur Berechnung Stekloffscher Eigenwerte

Zusammenfassung Es wird ein neues Verfahren zur numerischen Berechnung des ersten nichttrivialen Stekloffschen Eigenwertes beschrieben. Dieses Verfahren arbeitet nur mit oberen Schranken.

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We describe a new procedure for the numerical evaluation of the first nontrivial Stekloff eigenvalue. This procedure works only with upper bounds.

     

The vitamin D3 pathway in human skin and its role for regulation of biological processes

The skin is the only tissue yet known in which the complete ultraviolet-B (UV-B)-induced pathway from 7-dehydrocholesterol to hormonally active calcitriol (1alpha,25-dihydroxyvitamin D(3)) occurs under physiological conditions. Epidermal synthesis of calcitriol could be of fundamental relevance because calcitriol regulates important cellular functions in keratinocytes and immunocompetent cells. Because of their antiproliferative and prodifferentiating effects, calcitriol and other vitamin D analogs are highly efficient in the treatment of psoriasis vulgaris. The known antipsoriatic effect of UV-B light could, at least in part, be mediated via UV-B-induced synthesis of calcitriol. In addition, mounting evidence indicates that cutaneous vitamin D(3) synthesis is of high importance for the prevention of a broad variety of diseases, including various malignancies. New but controversially discussed sun-protection guidelines were established for the prevention of internal cancers. A better understanding of the metabolism of vitamin D in the skin opens new perspectives for therapeutic applications of vitamin D analogs.     

Two new 5-deoxyflavones from Albizia odoratissima

Two new 5-deoxyflavones, 7,8-dimethoxy-3',4'-methylenedioxyflavone (1) and 7,2',4'-trimethoxyflavone (2) together with a known flavone, 7,4'-dimethoxy-3'-hydroxyflavone (3) were isolated from the rootbark of Albizia odoratissima. The structures of these new compounds were elucidated by electrospray ionization mass spectrometry (ESI-MS) and 1D and 2D-NMR spectral studies including (1)H-(1)H correlation spectroscopy (COSY), heteronuclear single quantum coherence (HSQC), heteronuclear multiple bond connectivity (HMBC) and nuclear Overhauser enhancement spectroscopy (NOESY).     

Total synthesis of dictyodendrin B

A concise total synthesis of dictyodendrin B (1) is reported, a scarce marine alkaloid endowed with promising telomerase inhibitory activity. Key steps of the chosen route are a reductive cyclization of ketoamide 11 to indole 12 mediated by low-valent titanium (from TiCl3 and KC8) followed by a photochemical 6pi-electrocyclization, which was performed in the presence of Pd/C and nitrobenzene to effect concomitant dehydrogenation/aromatization of the product initially formed. Regioselective bromination of the resulting pyrrolocarbazole 13 followed by lithium/bromine exchange and quenching of the resulting organolithium species with p-methoxybenzaldehyde installed the side chain at C2. Oxidation of the benzylic alcohol 15 thus obtained to ketone 17 was best achieved with catalytic amounts of tetra-n-propylammonium perruthenate (TPAP) and N-methylmorpholine-N-oxide (NMO) in dilute CH2Cl2 solution to avoid the formation of undue amounts of the unsymmetrical dimer 16. Ketone 17 was elaborated into the natural product by selective cleavage of the isopropyl ether with BCl3, introduction of the sulfate moiety with the aid of trichloroethyl chlorosulfuric acid ester, deprotection of all lateral methyl ether groups, and final reductive cleavage of the trichloroethyl ester moiety. The spectroscopic data of synthetic dictyodendrin B thus formed matched those of an authentic sample in all regards. Moreover, it was shown that global deprotection of the peripheral -OH groups in pyrrolo[2,3-c]carbazole 13 is accompanied by spontaneous air-oxidation to form the quinone core of dictyodendrin C.     

Temperature-dependent behavior of a symmetric long-chain bolaamphiphile with phosphocholine headgroups in water: from hydrogel to nanoparticles

The temperature-dependent self-assembly of the single-chain bolaamphiphile dotriacontan-1,1'-diyl-bis[2-(trimethylammonio)ethyl phosphate] (PC-C32-PC) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray scattering, rheological measurements, and dynamic light scattering (DLS). At room temperature this compound, in which two phosphocholine headgroups are connected by a C(32) alkyl chain, proved to be capable of gelling water very efficiently by forming a dense network of nanofibers (Kohler et al. Angew. Chem., Int. Ed. 2004, 43, 245). A specific feature of this self-assembly process is that it is not driven by hydrogen bonds but solely by hydrophobic interactions of the long alkyl chains. The nanofibers have a thickness of roughly the molecular length and show a helical superstructure. A model for the molecular structure of the fibrils which considers the extreme constitution of the bolaamphiphile is proposed. Upon heating the suspensions three different phase transitions can be detected. Above 49 degrees C, the temperature of the main transition where the alkyl chains become "fluid", a clear low-viscosity solution is obtained due to a breakdown of the fibrils into smaller aggregates. Through mechanical stress the gel structure can be destroyed as well, indicating a low stability of these fibers. The gel formation is reversible, but as a drastic rearrangement of the molecules takes place, metastable states occur.     

Atomic Layer Deposition of Cobalt Phosphide for Efficient Water Splitting

Transition‐metal phosphides (TMP) prepared by atomic layer deposition (ALD) are reported for the first time. Ultrathin Co‐P films were deposited by using PH₃ plasma as the phosphorus source and an extra H₂ plasma step to remove excess P in the growing films. The optimized ALD process proceeded by self‐limited layer‐by‐layer growth, and the deposited Co‐P films were highly pure and smooth. The Co‐P films deposited via ALD exhibited better electrochemical and photoelectrochemical hydrogen evolution reaction (HER) activities than similar Co‐P films prepared by the traditional post‐phosphorization method. Moreover, the deposition of ultrathin Co‐P films on periodic trenches was demonstrated, which highlights the broad and promising potential application of this ALD process for a conformal coating of TMP films on complex three‐dimensional (3D) architectures.