Home About us Contact
|
Home About us Contact | ||
|
|
||
Cyclic Trimers (cyclic + trimer)
Selected AbstractsModification of Supramolecular Binding Motifs Induced By Substrate Registry: Formation of Self-Assembled Macrocycles and Chain-Like PatternsCHEMISTRY - A EUROPEAN JOURNAL, Issue 42 2009Leslie-Anne Fendt Abstract The self-assembly properties of two ZnII porphyrin isomers on Cu(111) are studied at different coverage by means of scanning tunneling microscopy (STM). Both isomers are substituted in their meso -positions by two voluminous 3,5-di(tert -butyl)phenyl and two rod-like 4,-cyanobiphenyl groups, respectively. In the trans -isomer, the two 4,-cyanobiphenyl groups are opposite to each other, whereas they are located at right angle in the cis -isomer. For coverage up to one monolayer, the cis- substituted porphyrins self-assemble to form oligomeric macrocycles held together by antiparallel CN,,,CN dipolar interactions and CN,,,H-C(sp2) hydrogen bonding. Cyclic trimers and tetramers occur most frequently but everything from cyclic dimers to hexamers can be observed. Upon annealing of the samples at temperatures >150,°C, dimeric macrocyclic structures are observed, in which the two porphyrins are bridged by Cu atoms, originating from the surface, under formation of two CN,,,Cu,,,NC coordination bonds. The trans -isomer builds up linear chains on Cu(111) at low coverage, whereas for higher coverage the molecules assemble in a periodic, densely packed structure. Both cis - and trans -bis(4,-cyanobiphenyl)-substituted ZnII porphyrins behave very differently on Cu(111) compared to similar porphyrins in literature on less reactive surfaces such as Au(111) and Ag(111). On the latter surfaces, there is no signal visible between molecular orientation and the crystal directions of the substrate, whereas on Cu(111), very strong adsorbate,substrate interactions have a dominating influence on all observed structures. This strong porphyrin,substrate interaction enables a much broader variety of structures, including also less favorable intermolecular bonding motifs and geometries. [source] Ab Initio Study on Interactions in Difluoroamine Clusters from one to four MoleculesPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 6 2002Xue-Hai Ju Ab initio calculations at HF, MP2 levels of theory with 6-311G* basis set in combination with counterpoise procedure for BSSE correction have been performed on difluoroamine clusters consisting of up to four molecules. The dimer, trimer and tetramer were all found to exhibit two minima. There are two types of clusters: cyclic and chain. The corrected bond energies are 9.19, 19.22 and 33.67,kJ/mol at the MP2/6-311G*//HF/6-311G* level for the more stable dimer, trimer and tetramer, respectively. The contribution of cooperative effect to the interaction energy is quite significant in the cyclic clusters, but negligible in the chain ones. There exist H-bonds which involve six and eight F,,,H contacts at ca. 0.23,0.24,nm in cyclic trimer and cyclic tetramer, respectively. The intermolecular interaction is an exothermic process under 400.0,K accompanied by a decrease in the probability of complex formation, and the interactions become weak as temperature increases. [source] Poisson's ratio of simple planar ,isotropic' solids in two dimensionsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2007Konstantin V. Tretiakov Abstract The influence of molecular geometry on the Poisson's ratio has been investigated for a few two-dimensional (2D) hard body systems (hard discs, hard dimers, hard cyclic trimers, and hard cyclic hexamers) forming elastically isotropic solid phases. The Poisson's ratio of the studied periodic and aperiodic solids was determined by Monte Carlo simulations using the analysis of the box fluctuations in the constant pressure ensemble (NpT) with variable box shape. The results obtained for all the studied systems showed that the Poisson's ratio grows when the density is decreased. It has been also found that the Poisson's ratio decreases with increasing number of discs in the molecule and its value for hard trimers is negative near close packing. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A preliminary crystallographic study of recombinant NicX, an Fe2+ -dependent 2,5-dihydroxypyridine dioxygenase from Pseudomonas putida KT2440ACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 5 2010José Ignacio Jiménez NicX from Pseudomonas putida KT2440 is an Fe2+ -dependent dioxygenase that is involved in the aerobic degradation of nicotinic acid. The enzyme converts 2,5-dihydroxypyridine to N -formylmaleamic acid when overexpressed in Escherichia coli. Biophysical characterization of NicX by analytical gel-filtration chromatography revealed that it behaves as an oligomeric assembly in solution, with an apparent molecular weight that is consistent with a hexameric species. NicX was crystallized by the hanging-drop vapour-diffusion method at 291,K. Diffraction data were collected to a resolution of 2.0,Å at the ESRF. The crystals most probably belong to the orthorhombic space group C222 or C2221. The estimated Matthews coefficient was 2.4,Å3,Da,1, corresponding to 50% solvent content, which is consistent with the presence of three protein molecules in the asymmetric unit. Analysis of the crystal data together with chromatographic results supports NicX being a hexameric assembly composed of two cyclic trimers. Currently, crystallization of recombinant selenomethionine-containing NicX is in progress. [source] | ||||||||||