Synthesis, structure and conformation of partially-modified retro- and retro-inverso psi[NHCH(CF3)]Gly peptides

Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction. Remarkable stability of turn-like conformations, comparable to that of parent malonyl-based retropeptides, was evidenced, as a likely consequence of two main factors: 1) severe torsional restrictions about sp(3) bonds in the [CO-CH(2)-CH(CF(3))-NH-CH(R)-CO] module, which is biased by the stereoelectronically demanding CF(3) group and the R side chain; 2) formation of nine-membered intramolecularly hydrogen-bonded rings, which have been clearly detected both in CHCl(3) solution and in some crystal structures. The former factor seems to be more important, as turn-like conformations were found in the solid-state even in the absence of intramolecular hydrogen bonding. The relative configuration of the -C*H(CF(3))NHC*H(R)- stereogenic centers has a major effect on the stability of the turn-like conformation, which seems to require a syn stereochemistry. X-ray diffraction and ab initio computational studies showed that the [-CH(CF(3))NH-] group can be seen as a sort of hybrid between a peptide bond mimic and a proteolytic transition state analogue, as it combines some of the properties of a peptidyl -CONH- group (low NH basicity, CH(CF(3))-NH-CH backbone angle close to 120 degrees, C-CF(3) bond substantially isopolar with the C=O) with some others of the tetrahedral intermediate [-C(OX)(O(-))NH-] involved in the protease-mediated hydrolysis reaction of a peptide bond (high electron density on the CF(3) group, tetrahedral backbone carbon).     

Zur Bestimmung und Trennung des Titans

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Untersuchung von Mehl- und Backwaren

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Schnelle Bestimmung des Cyans in den komplexen Eisencyaniden

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Summary of the CMS potential for the Higgs boson discovery

The European Physical Journal C - This work summarizes the studies for the Higgs boson searches in CMS at the LHC collider. The main discovery channels are presented and the potential is given for...     

First results of investigation of radiation from positrons in a crystalline undulator

Radiation emitted by positrons moving in a periodically deformed crystal has been experimentally observed for the first time. Radiation spectra have been measured in a wide energy range. Experimental evidence has been obtained for an undulator peak in a radiation spectrum, which is qualitatively consistent with calculations. Crystalline undulators ensure an equivalent magnetic field of 1000 T and a period in the submillimeter range and can therefore be used to generate x-ray and gamma radiation that is a hundred times harder than radiation in usual undulators.     

The Renazzo meteorite

Summary An imaging spectrometer developed for a space mission (Mars 94) is being used to study meteorites with solar illumination. It is shown that for a carbonaceous chondrite like the Renazzo meteorite it is possible to study also the internal composition of large chondrules, by making use of all the information of the image cube obtained. Inside chondrules, 3 typical materials are identified. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

Leading logarithmic corrections to the weak leptonic and semi-leptonic low-energy hamiltonian

If one defines the parameters of the Weinberg-Salam theory at a momentum scale M = O(M_W, M_Z), the weak effective hamiltonian at a momentum scale μ ? M has logarithmically enhanced corrections, of order αln(M²/μ²). We present a computation of these corrections, for that part of the hamiltonian which leads to detectable weak-electromagnetic interference effects. The largest correction can be absorbed into a running sin²θ(μ). Other, smaller, corrections are estimated, taking into account the effect of strong interactions.An estimate of the non-logarithmically enhanced corrections is also given, by evaluating them in the limit sin²θ → 0. From the SLAC e - d asymmetry it was found sin²θ = 0.224 ± 0.020 at μ² ? 2 GeV². In correspondence, we find sin²θ(M) = 0.217 ± 0.020. This value, however, is subject to uncertainties deriving from the effect of the strange and of the antiquark parton sea.     

A combined Raman, DFT and MD study of the solvation dynamics and the adsorption process of pyridine in silver hydrosols

The adsorption of pyridine onto silver colloids has been investigated by Raman spectroscopy experiments and by ab initio DFT and MP2 calculations. The solvation dynamics of the pyridine in water has been studied by a molecular dynamics simulation. The results are compared with the latest available experimental and theoretical data. It is found that the pyridine is essentially hydrogen bonded to one solvent molecule. Calculations based on pyridine-water and pyridine-Ag(+) complexes allow the reproduction of the experimentally observed Raman features and explain the adsorption process of the ligand in silver hydrosols.