New cytotoxic salinosporamides from the marine Actinomycete Salinispora tropica

An extensive study of the secondary metabolites produced by the obligate marine actinomycete Salinispora tropica (strain CNB-392), the producing microbe of the potent proteasome inhibitor salinosporamide A (1), has led to the isolation of seven related gamma-lactams. The most important of these compounds were salinosporamide B (3), which is the deschloro-analogue of 1, and salinosporamide C (4), which is a decarboxylated pyrrole analogue. New SAR data for all eight compounds, derived from extensive testing against the human colon carcinoma HCT-116 and the 60-cell-line panel at the NCI, indicate that the chloroethyl moiety plays a major role in the enhanced activity of 1.     

Structure and dynamics of solvated Sn(II) in aqueous solution: an ab initio QM/MM MD approach

Structural and dynamical properties of the hydrated Sn(II) ion have been investigated by ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations at double-zeta HF quantum mechanical level. The results indicate Sn(II)aq to be a rather peculiar, if not unique, case of a hydrated ion: four of its eight first-shell ligands do not take place in the otherwise frequent ligand-exchange processes, forming an approximately tetrahedral cage around the ion. The remaining ligands, however, exchange at a rate that is rather comparable to monovalent than divalent ions. This very surprising behavior of ligand exchange not yet observed in any previous simulation of over 30 hydrated metal ions is consistently confirmed by vibrational spectra, bond lengths, and a detailed analysis of the trajectories of the simulation.     

Correlations between the viscoelastic behavior of pyrene-labeled associative polymers and the associations of their fluorescent hydrophobes

The viscoelastic behavior of a series of three pyrene-labeled hydrophobically modified alkali swellable emulsion copolymers (Py-HASEs) was investigated. All Py-HASEs thickened the aqueous solutions with viscosities orders of magnitude larger than that of a HASE control which displayed no pyrene hydrophobe. This fact demonstrated that the pyrene molecule is a good hydrophobe for associative thickeners such as HASEs. The Py-HASE solutions exhibited shear thinning, whose magnitude was found to increase with increasing pyrene content. A large shear-thinning effect indicates that a large fraction of the elastically active cross-links has been severed. Fluorescence measurements on the Py-HASEs confirmed that the smaller the pyrene content of the Py-HASE, the more intermolecular associations it formed, in agreement with the results obtained by rheology. Above the overlap concentration of the polymers, the zero-shear viscosity of the Py-HASE solutions increased steeply with increasing polymer concentration. The onset concentration where viscosity increases matches the onset concentration where intermolecular associations are being formed, as probed by fluorescence. Oscillatory rheological measurements were carried out to determine the terminal relaxation time, Td, and the storage modulus at the infinite time limit, G0, of the Py-HASE network. G0 was found to increase with decreasing pyrene contents, indicating that Py-HASEs with lower pyrene contents exhibited a higher density of elastically active chains. This result is in agreement with the trends obtained by the fluorescence and steady-state rheology measurements. A model is suggested that accounts for the fluorescence and rheology results.     

A simple route to Δ2-butenolides from conjugated aldehydes

A variety of Δ²-butenolides may be synthesized by oxidation of the O-trimethylsilylcyanohydrins of α,β-unsaturated aldehydes using pyridinium dichromate in dimethylformamide.     

Über das bang-bang-Prinzip bei Kontrollproblemen aus der Wärmeleitung

Ohne ZusammenfassungDiese Note wurde verfaßt mit der Unterstützung des S.F.B. 72 der DFG.     

Multicomponent redox gradients on photoactive electrode surfaces

Redox gradients have been used to tailor the arrangement of photoactive ITO-electrodes at the molecular level.     

Clustered bottlenecks in mRNA translation and protein synthesis

Using a model based on the totally asymmetric exclusion process, we investigate the effects of slow codons along messenger RNA. Ribosome density profiles near neighboring clusters of slow codons interact, enhancing suppression of ribosome throughput when such bottlenecks are closely spaced. Increasing the slow codon cluster size beyond approximately 3-4 codons does not significantly reduce the ribosome current. Our results are verified by both extensive Monte Carlo simulations and numerical calculation, and provide a biologically motivated explanation for the experimentally observed clustering of low-usage codons.     

Ordered clusters and dynamical states of particles in a vibrated fluid

Fluid-mediated interactions between particles in a vibrating fluid lead to both long range attraction and short range repulsion. The resulting patterns include hexagonally ordered microcrystallites, time-periodic structures, and chaotic fluctuating patterns with complex dynamics. A model based on streaming flow gives a good quantitative account of the attractive part of the interaction.     

Orbit cardinals: On the effective cardinalities arising as quotient spaces of the formX/G whereG acts on a Polish spaceX

We prove an Ulm-type classification theorem for actions inL(ℝ), thereby answering a question of Becker and Kechris, and investigate the effective cardinalities which can be induced by various classes of Polish groups. Research partially supported by NSF grant DMS 96-22977.     

Algorithms for Measuring Perturbability in Matroid Optimization

perturbability function of a matroid measures the maximum increase in the weight of its minimum weight bases that can be produced by increases of a given total cost on the weights of its elements. We present an algorithm for computing this function that runs in strongly polynomial time for matroids in which independence can be tested in strongly polynomial time. Furthermore, for the case of transversal matroids we are able to take advantage of their special structure to design a faster algorithm for computing the perturbability function. Received: June 13, 1997