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Alternating Arrangement (alternating + arrangement)
Selected AbstractsSelf-assembling properties of ionic-complementary peptides,JOURNAL OF PEPTIDE SCIENCE, Issue 3 2009Gabriella D'Auria Abstract Self-complementary synthetic peptides, composed by 8 and 16 residues, were analyzed by CD, NMR and small angle neutron scattering (SANS) techniques in order to investigate the relevance of charge and hydrophobic interactions in determining their self-assembling properties. All the sequences are potentially able to form fibrils and membranes as they share, with the prototype EAK16, a strictly alternating arrangement of polar and nonpolar residues. We find that 16-mer peptides show higher self-assembling propensities than the 8-mer analogs and that the aggregation processes are favored by salts and neutral pH. Peptide hydrophobic character appears as the most relevant factor in determining self-assembling. Solution conformational analysis, diffusion and SANS measurements all together show that the sequences with a higher self-assemble propensity are distributed, in mild conditions, between light and heavy forms. For some of the systems, the light form is mostly constituted by monomers in a random conformation, while the heavy one is constituted by ,-aggregates. In our study we also verified that sequences designed to adopt extended conformation, when dissolved in alcohol-water mixtures, can easily fold in helix structures. In that media, the prototype of the series appears distributed between helical monomers and ,-aggregates. It is worth noticing that the structural conversion from helical monomer to ,-aggregates, mimics ,-amyloid peptide aggregation mechanisms. Copyright © 2008 European Peptide Society and John Wiley & Sons, Ltd. [source] Random discontinuous carbon fiber preforms: Experimental permeability characterization and local modelingPOLYMER COMPOSITES, Issue 4 2010A. Endruweit Injection experiments indicate that for random discontinuous carbon fiber preforms, increasingly uneven flow fronts develop with increasing fiber bundle length and filament count. While at high propensity for fiber bundle splitting, the preform permeability increases continuously with increasing fiber length, no trend can be identified at low propensity. No clear influence of the virgin bundle filament count on the preform permeability was observed. Types of sizing used on the fibers and bundle cross-sectional shapes may vary and affect the intrinsic filamentization behavior, thus dominating the preform permeability. In a model for local preform permeability, interbundle voids, distributed randomly across the preform thickness, are approximated via a regular void structure. Simulated filling patterns are qualitatively similar to those observed experimentally, showing more pronounced features than those derived from a model based on local through-thickness homogenization of the filament distribution. A model based on an alternating arrangement of fiber bundles and voids allows prediction of global preform permeability values from series of injection simulations, showing quantitatively better agreement with corresponding experimental results than the homogenization model. For global permeability, agreement between simulated and experimental mean values improves with increasing fiber volume fraction, whereas calculated coefficients of variation show no strong dependence on the fiber volume fraction. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers [source] The hydrated and anhydrous gold(III) tetrachloride salts of l -ecgonine, an important forensic toxicology marker for cocaineACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2010Matthew R. Wood The structure of the hydrated gold(III) tetrachloride salt of l -ecgonine {hydronium tetrakis[(1R,2R,3S,5S,8S)-3-hydroxy-8-methyl-8-azoniabicyclo[3.2.1]octane-2-carboxylate pentakis[tetrachloridoaurate(III)] hexahydrate}, (C9H16NO3)4(H3O)[AuCl4]5·6H2O, demonstrates an unprecedented stoichiometric relationship between the cations and anions in the unit cell. The previous tropane alkaloid structures, including the related hydrochloride salts, all have a cation,anion ratio of 1:1, as does the anhydrous salt described here, namely (1R,2R,3S,5S,8S)-3-hydroxy-8-methyl-8-azoniabicyclo[3.2.1]octane-2-carboxylate tetrachloridoaurate(III), (C9H16NO3)[AuCl4]. The hydrated salt, however, consists of four monopositive N-protonated units of the alkaloid and five [AuCl4], counter-ions, plus seven solvent water molecules. The H atom required for change balance has been assigned to a water molecule. In addition, the hydrate has a novel arrangement, with all seven of the water molecules and all of the O atoms in the cations participating in an alternating arrangement of interleaved sheets of the anionic species. Both the hydrate and the anhydrous salt of the same toxicologically important marker for cocaine show that the cation and anion are in close proximity to each other, as was found in the gold(III) tetrachloride salt of l -cocaine. [source] Two polymorphs of N -(2,6-difluorophenyl)formamideACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2009Bernard Omondi The structures of two distinct polymorphic forms of N -(2,6-difluorophenyl)formamide, C7H5F2NO, have been studied using single crystals obtained under different crystallizing conditions. The two forms crystallize in different space groups, viz. form (Ia) in the orthorhombic Pbca and form (Ib) in the monoclinic P21 space group. Each polymorph crystallizes with one complete molecule in the asymmetric unit and they have a similar molecular geometry, showing a trans conformation with the formamide group being out of the plane of the aromatic ring. The packing arrangements of the two polymorphs are quite different, with form (Ia) having molecules that are stacked in an alternating arrangement, linked into chains of N,H...O hydrogen bonds along the crystallographic a direction, while form (Ib) has its N,H...O hydrogen-bonded molecules stacked in a linear fashion. A theoretical study of the two structures allows information to be gained regarding other contributing interactions, such as ,,, and weak C,H...F, in their crystal structures. [source] Self-Assembly of Bis(merocyanine) Tweezers into Discrete Bimolecular ,-StacksCHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2009Andreas Lohr Dr. Abstract Dipolar aggregation: Examples of a novel class of tweezer molecules have been constructed through the tethering of two dipolar merocyanine chromophores. The electrostatic-interaction-directed self-assembly of these tweezers affords centrosymmetric bimolecular complexes with a unique aggregate geometry of four ,-stacked chromophores (see picture) with an alternating arrangement of their dipole moments and very high dimerization constants, even in the good solvating solvent chloroform. Examples of a novel class of tweezer molecules have been constructed through the tethering of two dipolar merocyanine chromophores by a naphthalenedimethylene or dimethylenediphenylmethane spacer. The electrostatic-interaction-directed self-assembly of these bis(merocyanine) tweezers affords centrosymmetric bimolecular complexes with very high dimerization constants of up to >109,M,1, even in the good solvating solvent chloroform. This pronounced self-association of the bis(merocyanine) tweezers is attributed to the strongly dipolar nature of the merocyanine chromophores and a unique aggregate geometry of four ,-stacked chromophores with an alternating arrangement of their dipole moments. The structural assignment of the tetrachromophoric ,-stack has been accomplished by MALDI-TOF mass spectrometry and ROESY NMR spectroscopy. Furthermore, molecular modeling studies have accounted for the relationships between the dimerization constants and optical properties of the bimolecular complexes of the present bis(merocyanine) dyes and the structure of the spacer and the positions at which the merocyanine chromophores are attached. [source] | ||||||||||