bluuebunny/crossref_metadata_2025_split huggingface dataset:
Manipulation of Electric Charge on Vesicles by Means of Ionic Surfactants: Effects of Charge on Vesicle Mobility, Integrity, and Lipid Dynamics
Authors: Yaroslavov Alexander A. , Udalyk Oleg Y. , Kabanov Viktor A. , Menger Fredric M. 
Published: July 2007

AbstractA combination of electrophoresis, dynamic light scattering, conductometry, and fluorescence spectroscopy was applied to investigate vesicles (both in the “solid” and “liquid” states) that had been imparted with electric charge through the incorporation of ionic amphiphiles. These amphiphilic compounds comprised cardiolipin (with two negative charges), sodium dodecyl sulfate (with one negative charge), and cetylpyridinium bromide (with one positive charge). By this means it was discovered that negative vesicles could be converted into neutral vesicles, and then into positive vesicles, by the addition of a cationic surfactant. The amount of cationic surfactant required for the conversion depended upon the mobility of the surfactant within the bilayer. Vesicles were found to be capable of absorbing large amounts of surfactant, both cationic and anionic, before ultimately disintegrating and releasing their contents. Mixtures of cationic and anionic vesicles were able to exchange surfactant, and thereby neutralize each other's charges, without any concurrent vesicle fusion. This phenomenon is reliable only if the vesicles are in the liquid state. Finally, a biphasic exchange process was observed in which a surfactant rapidly departs from one bilayer and then enters another, while a fluorescently labeled lipid travels the reverse path only slowly.
A Packing Model for Interpenetrated Diamondoid Structures—an Interpretation Based on the Constructive Interference of Supramolecular Networks
Authors: Hirsch Keith A. , Wilson Scott R. , Moore Jeffrey S. 
Published: July 2007

AbstractThe ability to predict and subsequently control solid‐state structure has been identified as a major challenge in the field of crystal engineering. Here we suggest the concept of constitutive models as a tool for understanding crystal packings and for designing new solid‐state structures. Such models are intended to relate molecular interactions and their geometrical constraints with solid‐state organization. These models will most likely be of greatest use for crystals consisting of supramolecular networks, that is, infinite assemblies of small molecules associating through strong, directional, and selective noncovalent interactions. The concept of the constitutive packing model is illustrated for interpenetrated diamondoid coordination networks based on crystalline adducts of 4,4′‐biphenyldicarbonitrile with silver(I) salts. Observed structural deformations induced by counterions of varying size may be understood in terms of the interference of two supramolecular networks within this system: the diamondoid metal–ligand coordination network and face‐to‐face aromatic stacks of the organic ligand. The constitutive model developed here has been applied to other diamondoid coordination networks in the literature and is found to be general.
A Density Functional Study of [Fe(PR3)4H3]+ Isomers: Comparing Model Compounds (r = H) with Real Molecules (R = CH3)
Authors: Jacobsen Heiko , Berke Heinz 
Published: July 2007

AbstractThe structures, energetics, and dynamics of various isomers of [Fe‐(PR3)4H3]+ (R = H, Me) were studied by density functional theory. The conformations considered were derived from a square planar (P), tetrahedral (T) and C2v‐butterfly (C) arrangement of the phosphine ligands. For PH3, the stability ranking PC>T was obtained, whereas in the PMe3 case the P isomer was least stable, T>C>P. Mechanisms for the hydrogen exchange in the C and T isomers are discussed. For the latter, this process might be described as a tetrahedral jump, but could also be viewed as a dodecahedral distortion of a cubic arrangement. The theoretical findings are compared with the results of experimental studies of [Fe(PR3)4H3]+ (R = Me, Et).
Enantioselective Hydrogenation of Imines with Chiral (Phosphanodihydrooxazole)iridium Catalysts
Authors: Schnider Patrick , Koch Guido , Prétôt Roger , Wang Guozhi , Bohnen Frank Michael , Krüger Carl , Pfaltz Andreas 
Published: July 2007

AbstractCationic iridium(I) complexes of chiral phosphanodihydrooxazoles were used as catalysts for the enantioselective hydrogenation of prochiral N‐alkyl and N‐aryl imines. The complexes are air‐stable crystalline solids that can be readily prepared and are easy to handle. The structures of two complexes were determined by X‐ray analysis. For N‐alkyl imines of acetophenone, enantiomeric excesses of up to 79% were obtained. Dialkyl ketimines and cyclic imines showed lower reactivity and selectivity. A remarkable dilution effect was observed for the hydrogenation of the N‐phenyl imine of acetophenone: decreasing the substrate and catalyst concentration led to a significant improvement of the enantioselectivity. Thus, up to 89% ee could be achieved using 0.1 mol% of catalyst. The highest enantioselectivities were obtained in weakly coordinating solvents such as CH2Cl2. Additives such as halides, imides, or amines were found to poison the catalyst. Hydrogen pressures of 100 bar were usually employed, but in some cases identical results were achieved with only 1 bar H2.
Control of the Helical Chirality in Octahedral Complexes by a Chiral Macrobicyclic Cavity Possessing Six Convergent Hydroxyl Groups
Authors: Baret Paul , Beaujolais Virginie , Gaude Didier , Coulombeau Christian , Pierre Jean‐Louis 
Published: July 2007

AbstractThe (aR, aR, aR) and (aS, aS, aS) enantiomers of a chiral macrobicyclic ligand with a bicapped tris(binaphthol) structure were synthesized. Complexation of gallium(III), chromium(III) and iron(III) centres in the chiral cavities of these two ligands yielded exclusively one octahedral configuration in each case: the (aR, aR, aR) enantiomer gave a A complex and the (aS, aS, aS) enantiomer a complex. These assignments were established by CD spectroscopy for chromium and iron centres and by molecular modelling.
Twelve‐Membered O‐Bridged Cyclic Ethers of Red Seaweeds in the Genus Laurencia Exist in Solution as Slowly Interconverting Conformers
Authors: Guella Graziano , Chiasera Giuseppe , Mancini Ines , Öztunç Aysel , Pietra Francesco 
Published: July 2007

AbstractThe twelve‐membered O‐bridged cyclic ether obtusallene IV (1), a new isolate from the red seaweed Laurencia obtusa from Kaş in the Turkish Mediterranean, revealed temperature‐dependent NMR signals attributable to a major conformer in equilibrium with a minor conformer by 180° flipping of the trans olefinic bond. This prompted us to reexamine the known congeners obtusallene I (2), 10‐bromoobtusallene I (3), obtusallene II (4), and obtusallene III (5), isolated both from the same and taxonomically related seaweeds, as well as their semisynthetic derivative peracetylobtusallene III (6). Two conformers could in fact be directly observed at room temperature for 2–3 and at low temperature for 4. Marked cross‐saturation‐transfer effects between the couples of conformers confirmed these observations. Activation energies for processes involving 10‐membered subunit rings (2–3) are higher than for 11‐membered (4–5) analogues, where faster conformational motion occurs too resulting in mediated vicinal J couplings. 1,3‐Dihydroxy substituents in 5 form an intramolecular hydrogen bond in low‐polarity, non‐H‐bonding solvents; this results in the existence of two further conformers, giving more complex NMR spectra. Descriptions in the literature of single conformers for obtusallenes 2–5 must have resulted from overlooking minor NMR signals or attributing them to impurities.
New Strategies for the Generation of Large Heteropolymetalate Clusters: The β‐B‐SbW9 Fragment as a Multifunctional Unit
Authors: Bösing Michael , Loose Ina , Pohlmann Heinrich , Krebs Bernt 
Published: July 2007

AbstractProceeding from the α‐B‐SbW9O33 fragment 1, various novel large heteropolyanions ([Na2Sb8W36O132(H2O)4]22– (2), [Sb2W22O74(OH)2]12− (3) and [Sb2W20M2O70(H2O)6](14−2n)− (4); Mn+ = Fe3+, Co2+, Mn2+, Ni2+) have been obtained in crystalline form by systematic synthesis. Their unexpected structures have been studied by X‐ray diffraction: 2, 3 and 4 contain β‐SbW9 units connected to each other by Sb3+ ions, WO2/WO2OH groups, and WO2/Mn+ (H2O)3 groups, respectively. Structural details and especially the systematic synthesis strategy for the formation of these interesting polytungstates are discussed in order to explain the unusual behaviour of 1 in aqueous solution under different reaction conditions.
Self‐Assembly of Bisurea Compounds in Organic Solvents and on Solid Substrates
Authors: van Esch Jan , De Feyter Steven , Kellogg Richard M. , De Schryver Frans , Feringa Ben L. 
Published: July 2007

AbstractNew low molecular weight gelators based on the structure RNHCONHXNHCONHR have been synthesized and tested for their ability to cause gelation of organic solvents. Compounds 2 (R = n‐dodecyl, X = ‐(CH2)9‐), 3 (R = n‐dodecyl, X = ‐(CH2)12‐), 4 (R = n‐dodecyl, X = 4, 4′‐biphenyl), and 5 (R = benzyl, X = ‐(CH2)9‐) form thermoreversible gels with a wide range of organic solvents, at concentrations well below 10 mgmL−1. Depending on the nature of the R and X groups, the solvents that undergo gelation include hexadecane, p‐xylene, 1‐octanol, n‐butyl actetate, cyclohexanone, and tetralin. The gels are stable up to temperatures well above 100°C, but are easily disrupted by mechanical agitation. Light microscopic investigations revealed that compounds 2–5 spontaneously aggregate to form thin flat fibers, which can be several hundreds of micrometers long and only 2–10 μm wide. Depending on the solvent, multiple twists in the fibers are observed. In the gels, these fibers form an extended three‐dimensional network, which is stabilized by multiple mechanical contacts between the fibers. Electron microscopy and X‐ray powder diffraction revealed that the fibers consist of stacks of sheets. The thickness of the sheets is 3.65 and 3.85 nm for 2 and 3, respectively. Scanning tunneling microscopic investigations of 2 absorbed on graphite showed that 2 forms long ribbons with a width of 5.0 nm. In the ribbons the molecules have a parallel arrangement, with the long molecular axis perpendicular to the long ribbon axis. The two urea groups within a given molecule are each part of mutually parallel extended chains of hydrogen bonds. Based on these observations a model is proposed for the arrangement of the molecules in the fibers. In this model the bisurea molecules aggregate through hydrogen‐bond formation into long ribbons, which assemble into sheets. In these sheets the ribbons are tilted. Finally, the sheets stack to form long thin fibers. This model is supported by molecular dynamics simulations.
New Evidence for Conformational Flexibility in Cyclodextrins from Vibrational Raman Optical Activity
Authors: Bell Alasdair F. , Hecht Lutz , Barron Laurence D. 
Published: July 2007

AbstractConformational flexibility in cyclodextrins (CDs) as a function of methylation, solvent interaction and the extent of inclusion complex formation has been studied by using vibrational Raman optical activity (ROA). The work exploited the sensitivity of ROA to skeletal mobility by comparing the intensity of the glycosidic ROA couplet between about 850 and 970 cm−1in maltoheptose (MH), β‐cyclodextrin (β‐CD), heptakis(2,6‐di‐O‐methyl)‐β‐cyclodextrin (DM‐β‐CD) and heptakis(2,3,6‐tri‐β‐methyl)‐β‐cyclodextrin (TM‐β‐CD) in buffered aqueous solution, in DMSO and with sodium benzoate and benzoic acid as guests in buffered aqueous solution. Increases in couplet signal strength were interpreted in terms of a reduction in conformational flexibility of the CD ring. In buffered aqueous solution the ROA intensity order MH < TM‐β‐CD < β‐CD < DM‐β‐CD was observed. The linear molecule MH is expected to be the most flexible of the four oligosaccharides studied, while the changes registered for the three CD macrocycles may be related to the degree of intramolecular hydrogen bond formation and its influence on conformational flexibility. In DMSO, the same ROA intensity order is observed, but with an approximately constant increase relative to the values obtained in aqueous solution. This can be explained by the tighter binding of DMSO in the CD cavities compared with H2O. For the inclusion complexes, our results indicate that the tighter the guest is bound, the larger is the reduction in the conformational flexibility of the CD macrocycle. The residual mobility sensed by ROA in CDs is similar to that sensed in proteins; this provides further insight into their analogous ligand‐binding and catalytic properties.
Tris(trimethylsilyl)silanides of the Heavier Alkali Metals—A Structural Study
Authors: Klinkhammer Karl W. 
Published: July 2007

AbstractTransmetalation reactions between bis(hypersilyl)zinc Zn[Si(SiMe3)3]2 and alkali metals have already been established as a facile route to powders of the solvent‐free potassium, rubidium, and cesium derivatives of tris(trimethylsilyl)‐silane (hypersilane, (Me3Si)3SiH).[1,2] By the use of boiling n‐heptane as the solvent, the hitherto unknown NaSi(SiMe3)3 (1) along with the previously synthesized KSi(SiMe3)3 (2) have now been obtained as colorless crystalline materials. Information from NMR and Raman spectra in conjunction with the acute Si‐Si‐Si angles found in their molecular structures indicate mainly ionic bonding. This was verified by population analyses of suitable model systems. Both hypersilanides[2] consist of cyclic dimers [MSi(SiMe3)3]2 (1a, M = Na; 2a, M = K) with almost planar M2Si2 rings (Na‐Si = 299 pm (av); K‐Si = 339 pm (av)), which are linked up into coordination polymers. In a similar manner to the related rubidium and cesium derivatives, a pentane suspension of the potassium compound afforded a yellow solution on addition of benzene, from which the crystalline, bright yellow tris(benzene) solvate 2·(benzene)3 (2b) was isolated. Complex 2b consists of monomers containing short K‐Si bonds (332–334 pm) and three η6‐bonded benzene molecules. No solvate of 1 was obtained under these conditions. However, on crystallization from pure benzene, crystals of (1)2·benzene (1b) were isolated (Na‐Si = 302 pm (av)). Benzene was found to be intercalated between rods of coordination polymers of (1)2. In addition, the influence of π or s̀ donors on the molecular and crystal structures of hypersilylrubidium (3) and cesium (4) was investigated. The structures of the tetrahydrofuran solvate (4)2·THF (4c), the biphenylene/pentane complex (4)2·biphen·(pentane)0.5 (4b) along with the known toluene solvates (3)27dot;toluene (3a) and (4)2·(toluene)3 (4a)[1a] are compared. Finally, an example is presented of how the alkali metal hypersilanides could be utilized in preparing extremely bulky stannanide anions.
Reactivity of Coordinatively Unsaturated Trivalent Chromium Complexes with Sulfur: Preparation of Novel Sulfide‐bridged Dinuclear CrIV Derivatives
Authors: Reardon Damien , Kovacs Istvan , Ruppa Kamalesh B. P. , Feghali Khalil , Gambarotta Sandro , Petersen Jeffrey 
Published: July 2007

AbstractThe homoleptic CrIII amide complexes [(RR′N)3Cr] [R = R′ = cyclohexyl (1a); R = adamantyl, R′ = 3,5‐Me2Ph (1 b)] were prepared and characterized. Reactions with elemental sulfur produced the corresponding dinuclear complexes [{(RR′N)2Cr}2(μ‐S)2] (2) through loss of one amide group. These two new dinuclear CrIV species are paramagnetic with magnetic moments lower than expected for the d2 electronic configuration of CrIV. Magnetic measurements and ab initio calculations were carried out to elucidate the nature of magnetic interactions in these dinuclear systems. The molecular structures of 1a and 2a were elucidated by X‐ray crystal structures.
Electrospray Mass Spectrometry of Poly(propylene imine) Dendrimers—The Issue of Dendritic Purity or Polydispersity
Authors: Hummelen Jan C. , Van Dongen Joost L. J. , Meijer E. W. 
Published: July 2007

AbstractElectrospray mass spectrometry (ESI‐MS) is used to analyze in detail the structure of the first five generations of the poly(propylene imine) dendrimers with 4–64 end groups. Although the reaction sequence to prepare these dendrimers is highly optimized, an accumulation of statistical defects is observed at higher generations. A simple statistical simulation allows the assignment of all peaks in the deconvoluted spectrum and the determination of yields for each side reaction in the formation of each generation. The fifth generation of the poly(propylene imine) dendrimer containing amine end groups, which is formed after 248 consecutive reaction steps, possesses a dendritic purity of approx. 20%, which corresponds to a polydispersity of approximately 1.002. The defects in the divergent growth patterns of dendrimers are compared to those observed in the Merrifield synthesis of polypeptides.
Reaction of „Jordan′s Cation”︁ [Cp2ZrCH3(thf)]+ with Amino Acid‐Derived Isocyanates—A First Synthetic Route to Oligopeptide‐Modified Group 4 Bent Metallocene Cation Systems
Authors: Oberhoff Markus , Erker Gerhard , Fröhlich Roland 
Published: July 2007

AbstractThe organometallic cation [Cp2ZrCH3(thf)]+, employed as the tetraphenylborate salt (1), reacts cleanly in 1:1 stoichiometry with the isocyanates 2 derived from valine methyl ester or valylvaline methyl ester, respectively. In each case addition of the Zr–CH3 group to the isocyanate sp‐carbon center is observed with formation of a functionalized zirconocene cation derivative containing a chelating N‐metallated N‐acetylvaline methyl ester (3a) or N‐acetylvalylvaline methyl ester (3b) moiety, respectively, coordinated in the bent metallocene s̀‐ligand plane. The spectroscopic data of 3, supported by an X‐ray crystal structure analysis of the zirconated dipeptide derivative 3b, have revealed the presence of chelating (η1‐O:η1‐N)‐coordination of the terminal N‐acetyl groups in addition to a Zr–O=C interaction with the adjacent valyl amido group.
Complex Formation of the Antiviral 9‐[2‐(Phosphonomethoxy)Ethyl]Adenine (PMEA) and of Its N 1, N 3, and N 7 Deaza Derivatives with Copper(II) in Aqueous Solution
Authors: Blindauer Claudia A. , Emwas Abdul H. , Holý Antonín , Dvořáková Hana , Sletten Einar , Sigel Helmut 
Published: July 2007

AbstractThe stability constants of the 1:1 complexes formed between Cu2+ and the anions of the N 1, N 3, and N 7 deaza derivatives of 9‐[2‐(phosphonomethoxy)‐ethyl]adenine (PA2‐), Cu(H;PA)+ and Cu(PA), were determined by potentiometric pH titration in aqueous solution (25 °C; I = 0.1 M, NaNO3) and compared with previous results for 9‐[2‐(phosphonomethoxy)ethyl]adenine (PMEA2‐) and (phosphonomethoxy)ethane (PME2‐). A microconstant scheme reveals that in Cu(H;PA)+ Cu2+ is coordinated to the nucleobase, H+ being at the phosphonate group, in about 90% of the Cu(H;PMEA)+ and Cu(H;1‐deaza‐PMEA)+ species, but only in about 37% and 12% of the corresponding complexes with H(3‐deaza‐PMEA)‐ and H(7‐deaza‐PMEA)‐, respectively. Straight‐line plots of log KCuCu(R–PO3) versus pKHH(R–PO3) for simple phosph(on)ate ligands show that all the Cu(PA) complexes, including those with PMEA2‐ and PME2‐, are more stable than expected simply from the basicity of the ‐PO2‐ group; to some extent five‐membered chelates (Cu(PA)cl/O) involving the ether oxygen of the ‐CH2–O–CH2–PO2‐3 chain are formed, and in all complexes an additional nucleobase–metal‐ion interaction occurs. Based on 1H NMR line‐broadening measurements and structural considerations it is concluded that in Cu(3‐deaza‐PMEA) the interaction occurs with N7 whereas in Cu(7‐deaza‐PMEA), Cu(1‐deaza‐PMEA), and Cu(PMEA) it occurs with N3. The proof of a metal ion–N3 interaction is important (and also of relevance regarding DNA) because so far this interaction has received little attention. In all Cu(PA) systems three major isomeric species are in equilibrium; for example, 17(∓3)% of Cu(PMEA) exists as an isomer with a sole Cu2+–phosphonate coordination, 34(∓10)% as Cu(PMEA)cl/O, and in 49(∓10)% the Cu2+ is bound to the phosphonate group, the ether O, and N3. In contrast, 54(∓8)% of Cu(5′‐AMP) occurs as an isomer with sole Cu2+–phosphate coordination and 46(∓8)% as a macrochelate involving N7 too.
Chirality in Dendritic Architectures
Authors: Peerlings H. W. I. , Meijer E. W. 
Published: July 2007

AbstractAt first glance the topic of chiral dendrimers seems to be a contradiction in terms. However, recent studies reveal that both the building blocks of the dendrimer and the overall dendritic architecture can be chiral and that chirality can be introduced at various levels. The expression of optical activity in these enantiomerically pure dendrimers as a result of conformational (dis)order has proven to be of special interest. In this Concepts article we present the different approaches to introducing chirality in dendritic architectures, organized through their possible impact in fields such as biocompatibility, catalysis, molecular recognition, and surface chemistry. Also, the relation between molecular chirality of core or building block and the macroscopic chirality of dendritic objects is discussed.
Development of Tools for the Design of Selectin Antagonists
Authors: Kolb Hartmuth C. , Ernst Beat 
Published: July 2007

AbstractA molecular modeling tool for the rational design of E‐selectin antagonists based on the lead structure sialyl Lewisx has been developed. The binding affinity to the receptor is considerably influenced by the entropy and consequently by the antagonist's ability to place its pharmacophores in an optimal spatial arrangement, i.e., by its preorganization for binding. The computational model assesses the preorganization of a potential selectin antagonist with the aid of Monte Carlo (jumping between wells)/stochastic dynamics [MC(JBW)/SD] simulations. The model has been validated by correlating preorganization and bioactivity of several selectin antagonists. The results suggest that only preorganized compounds are likely to bind to E‐selectin.