Preparation and Structure of β-Peptides Consisting of Geminally Disubstituted β2,2- and β3,3-Amino Acids: A Turn Motif for β-Peptides

We report on the synthesis of new and previously described β-peptides ( 1 – 6 ), consisting of up to twelve β^2,2- or β^3,3-geminally disubstituted β-amino acids which do not fit into any of the secondary structural patterns of β-peptides, hitherto disclosed. The required 2,2- and 3,3-dimethyl derivatives of 3-aminopropanoic acid are readily obtained from 3-methylbut-2-enoic acid and ammonia (Scheme 1) and from Boc-protected methyl 3-aminopropanoate by enolate methylation (Scheme 2). Protected (Boc for solution-, Fmoc for solid-phase syntheses) 1-(aminomethyl)cycloalkanecarboxylic-acid derivatives (with cyclopropane, cyclobutane, cyclopentane, and cyclohexane rings) are obtained from 1-cyanocycloalkanecarboxylates and the corresponding dihaloalkanes (Scheme 3). Fully ¹³C- and ¹⁵N-labeled 3-amino-2,2-dimethylpropanoic-acid derivatives were prepared from the corresponding labeled precursors (see asterixed formula numbers and Scheme 4). Coupling of these amino acids was achieved by methods which we had previously employed for other β-peptide syntheses (intermediates 18 – 23 ). Crystal structures of Boc-protected geminally disubstituted amino acids ( 16a – d ) and of the corresponding tripeptide ( 23a ), as well as NMR and IR spectra of an isotopically labeled β-hexapeptide ( 2a* ) are presented (Figs. 1 – 4) and discussed. The tripeptide structure contains a ten-membered H-bonded ring which is proposed to be a turn-forming motif for β-peptides (Fig. 2).     

Coupling of SPME and GC-AED for the Determination of Organometallic Compounds

 The solid-phase microextraction (SPME) technique coupled with gas chromatography and atomic emission detection was successfully applied for the determination of selected organometallic species of Pb, As and Hg in aqueous samples. To obtain a high extraction yield, the SPME conditions were optimised for each element by fibre selection and varying the exposure time, stirring rate, pH range and desorption time. All the organometallic compounds tested were extracted from the aqueous phase using SPME. The preconcentration factors attained ranged between 40 and 150, depending on the compound. Detection limits in the pg/L and ng/L ranges were achieved. Received January 18, 2000. Revision April 11, 2000.     

Density-functional-based molecular-dynamics simulations of molten salts

The physicochemical properties of two molten salts, namely, KCl and NaCl, have been studied with a molecular-dynamics approach using a density-functional-based tight-binding (DFTB) model. The obtained results have been compared with a number of previously reported simulations, carried out on smaller systems and using classical force-field techniques. A good agreement has been found for both structural parameters and macroscopic properties, such as self-diffusion coefficients. Furthermore, our DFTB results are very close to the available experimental data. From a more general point of view, our results demonstrate the applicability of DFTB as an efficient tool in the modeling of melts. At the same time, the quality of the obtained results supports the use of this as a reliable alternative to the more expensive ab initio dynamics approaches, if accurate parameters are provided.     

Downstream Fabry-Perot interferometer for acoustic wave monitoring in photoacoustic tomography

An optical detection setup consisting of a focused laser beam fed into a downstream Fabry-Perot interferometer (FPI) for demodulation of acoustically generated optical phase variations is investigated for its applicability in photoacoustic tomography. The device measures the time derivative of acoustic signals integrated along the beam. Compared to a setup where the detection beam is part of a Mach-Zehnder interferometer, the signal-to-noise ratio of the FPI is lower, but the image quality of the two devices is similar. Using the FPI in a photoacoustic tomograph allows scanning the probe beam around the imaging object without moving the latter.     

On the Hochschild cohomology of tame Hecke algebras

We explicitly calculate a projective bimodule resolution for a special biserial algebra giving rise to the Hecke algebra H_q(S₄){{\mathcal H}_q(S_4)} when q = −1. We then determine the dimensions of the Hochschild cohomology groups.     

C28 fullerites-structure, electronic properties and intercalates

Mechanical and electronic properties of hypothetical carbon nanostructures, on the basis of C28 building blocks, hyperdiamond and hyperlonsdaleite, have been investigated with DFT based methods. The low mass density and large internal surface suggest applications as catalyst, nanosieve and gas storage material. We estimate the active volume accessible by H2. Special emphasis is given to the possibility to tune their properties by endo- and exohedral intercalation with Zn, Ti and K. While endohedral intercalation with Zn does not affect the overall structure, endohedral Ti intercalation has different consequences on the structural stability of the two allotropes. Exohedral intercalation with K leads to an ionic fullerite phase with metallic conductivity.     

Quantitative Bestimmung primärer aliphatischer Alkohole (C6-C10) mit Hilfe einer pyridinnukleotidunabhängigen Alkoholdehydrogenase ausPseudomonas putida

A new enzymatic method for microdetermination of longer chain alcohols has been established withNAD(P)-independent alcohol dehydrogenase from an alkane-oxidizing strain ofPseudomonas putida. The alcohols were estimated spectrophotometrically by measuring the formazan formation from the corresponding tetrazolium salt. Amounts of longer chain alcohols up to 3 nmol could be assayed by reading the endpoint of the reaction, even in presence of a 1000-fold exess of ethanol.