Ethylene Bridge (ethylene + bridge)

Distribution by Scientific Domains

Selected Abstracts

A Single ,-Helical Turn Stabilized by Replacement of an Internal Hydrogen Bond with a Covalent Ethylene Bridge,

Durch eine Kohlenstoffbrücke stabilisiert: ,-Helix-Mimetika wurden hergestellt, in denen eine Ethylenbrücke die (i,i+4)-Wasserstoffbrücke ersetzt (siehe Struktur). Das kleinste denkbare Beispiel, ein modifiziertes Pentapeptid, zeigt die Charakteristika einer Helix (siehe CD-Spektrum). Der Einbau der Kohlenstoffbrücke stellt den ersten Austausch einer internen Wasserstoffbrücke in einem Peptidrückgrat dar. [source]

Tuning of Copper(I),Dioxygen Reactivity by Bis(guanidine) Ligands,

Sonja Herres-Pawlis
Abstract A series of bis(guanidine) ligands designed for use in biomimetic coordination chemistry, namely bis(tetramethylguanidino)-, bis(dipiperidinoguanidino)-, and bis(dimethylpropyleno)propane (btmgp, DPipG2p and DMPG2p, respectively), has been extended to include bis(dimethylethyleneguanidino)propane (DMEG2p), which has both Namine atoms of each guanidine functionality connected by a short ethylene bridge, as a member. From this series, a family of novel bis(guanidine)copper(I) compounds , [Cu2(btmgp)2][PF6]2 (1), [Cu2(DPipG2p)2][PF6]2 (2), [Cu2(DMPG2p)2][PF6]2 (3), and [Cu2(DMEG2p)2][PF6]2·2MeCN (4) , has been synthesised. Single-crystal X-ray analysis of 1,4 demonstrated that these compounds contain dinuclear complex cations that contain twelve-membered heterocyclic Cu2N4C6 rings with the Cu atoms being more than 4 Å apart. Each copper atom is surrounded by a set of two N-donor functions from different ligands, resulting in linear N,Cu,N coordination sites. Depending on their individual substitution patterns, the guanidine moieties deviate from planarity by characteristic propeller-like twists of the amino groups around their N,Cimine bonds. The influence of these groups on the reactivity of the corresponding complexes 1,4 with dioxygen was investigated at low temperatures by means of UV/Vis spectroscopy. The reaction products can be classified into ,-,2:,2 -peroxodicopper(II) or bis(,-oxo)dicopper(III) complex cations that contain the {Cu2O2}2+ core portion as different isomers. The electronic properties of the specific bis(guanidine) ligands are discussed from the viewpoint of their ,-donor and ,-acceptor capabilities, and it is shown that ,-,2:,2 -peroxodicopper(II) complexes are stabilised relative to the bis(,-oxo)dicopper(III) ones if , conjugation within the guanidine moieties is optimised. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

Influence of differently ionized species on fragmentation pathways and energetics of a potential adenosine receptor antagonist using a triple quadrupole and a multistage LTQ-OrbitrapÔ FTMS instrument

Wendy Zhong
A systematic study was conducted to investigate the influence of differently ionized species on the fragmentation pathways and energetics of a piperazine-containing adenosine by using different cations or anions. Very different fragmentation mechanisms were observed in protonated- versus sodiated-molecules, which indicated that the proton is mobilized to promote the charge-direct fragmentation, whereas Na+ cation was fixed at the heterotricyclic ring structure provoking charge-remote fragment ions. This finding was also supported by the results observed in the fragmentation behaviors in the deprotonated-molecule. The energetics of these fragment ions were also explored by using the breakdown curves obtained from the triple quadrupole and LTQ-OrbitrapÔ instrument. The data indicated that the lowest energy pathways in the protonated-molecule [M+H]+ involve breaking a CN bond connecting an ethylene bridge and heterotricyclic ring structure. The lowest energy pathway is the cleavage of a CO bond connecting the methoxy ethyl group and phenolic oxygen to form a distonic radical ion for a sodiated-molecule [M+Na+]and a deprotonated-molecule [M-H],. The data suggest that by choosing the differently ionized species, one can probe different fragmentation channels that can provide additional structure information for an unknown impurity and possibly degradation product identification. In addition, by comparing the data obtained from triple quadrupole and LTQ-Orbitrap instruments, one can develop further understanding of the differences in the fragmentation behaviors due to the variations in the collision activation-dissociation process. From the side-by-side comparison with the breakdown curves obtained for both instruments, the difference in fragmentation behaviors caused by the difference in dissociation processes that occur in these two types of instruments can be probed. J. Heterocyclic Chem., (2009). [source]

Oligomeric carbon and siloxane series observed by matrix-assisted laser desorption/ionisation and laser desorption/ionisation mass spectrometry during the analysis of soot formed in fuel-rich flames

Barbara Apicella
Oligomeric carbon and siloxane series have been observed by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS), during the analysis of the dichloromethane (DCM)-soluble fractions of condensable material recovered from fuel-rich flames. Laser desorption (LD) spectra showed a pattern of oligomeric dimethyl-siloxane structures with a spacing of 74,u. The siloxane series appears to have originated as contamination of samples by silicone oil used to lubricate connections of polymer tubing. This was confirmed by extracting silicone tubing and silicone grease with DCM followed by MALDI-MS analysis. A series of peaks with a mass spacing of 24,u was also observed, superimposed on the continuum of unresolved organic ions. This oligomeric series appears to correspond to polycyclic aromatics separated by (mainly) ethylene bridges. Thus LD-MS appears to have revealed a series of soot precursors, intermediate between polycyclic aromatics and particulate soot, which was not detected by MALDI-MS. More detailed work is necessary to define these species with precision. Copyright © 2004 John Wiley & Sons, Ltd. [source]