bluuebunny/crossref_metadata_2025 huggingface dataset:
Task prioritisation in multitasking during driving: opportunity to abort a concurrent task does not insulate braking responses from dual‐task slowing
Authors: Levy Jonathan , Pashler Harold
Published: July 2007

In typical dual‐task driving studies, participants concurrently perform pairs of driving‐related and ‐unrelated tasks (e.g. vehicle braking and mental arithmetic). Requiring responses to both may implicitly equate their importance. In real‐life driving, however, the potential for collision dictates that a concurrent task should be assigned far lower priority than driving. To better reflect naturalistic driving conditions, we not only instructed participants to assign maximum priority to braking in a simulated driving task, but also encouraged them to ignore the concurrent task altogether on dual‐task trials. Despite these instructions, responses to the concurrent task often preceded braking, which suffered from dual‐task interference. We also found that redundant signals to the lead vehicle's brake lights resulted in faster braking responses and an increased likelihood that the braking response would occur first. The results are consistent with the Central Bottleneck (CB) model of dual‐task interference and may help guide the design of driver‐assistance systems. Copyright © 2007 John Wiley & Sons, Ltd.
Cytocompatible Hydrogels Based on Photocrosslinkable Methacrylated O‐Carboxymethylchitosan with Tunable Charge: Synthesis and Characterization
Authors: Poon Y. F. , Zhu Y. B. , Shen J. Y. , Chan‐Park M. B. , Ng S. C.
Published: July 2007

Ionic hydrogels are attractive for the protection, delivery and controlled release of charged biomacromolecules such as proteins, growth factors or DNA. We have prepared and characterized a series of photocrosslinked anionic hydrogels based on water soluble methacrylated (MA) O‐carboxymethylchitosan (OCMCS) and polyethylene glycol (PEG) diacrylate. OCMCS samples with varying degree of substitution of carboxymethyl group ranging from 0.69 to 1.86 were prepared by reacting native chitosan with different amounts of monochloroacetic acid. The OCMCS products demonstrated differences in solubility, zeta potential (–52.7 to –12.8 mV) and thermal decomposition temperature (260 to 283 °C). The OCMCS products were then reacted with glycidyl methacrylate to make ultra‐violet (UV) crosslinkable OCMCS‐MAs which were blended with PEG diacrylate, a photoinitiator and water and successfully photocrosslinked to create OCMCS‐MA/PEG hydrogels. Water uptake of the hydrogels varied between 226 % to 358 % and the porous structures of the prepared OCMCS‐MA/PEG hydrogels could be modulated by the degree of methacrylation. All the OCMCS‐MA/PEG hydrogel substrates similarly supported attachment and proliferation of Smooth Muscle Cells (SMCs). The in vitro biodegradation of these hydrogels, in the presence of SMCs, could be controlled by the degree of methacrylation; weight loss after 9‐week was (15±1) % and (19±2) % using OCMCS 4‐MA (12.7 % MA) and OCMCS 1‐MA (4.6 % MA), respectively. In addition, the hydrogel based on the most anionic OCMCS 1 showed higher adsorption of basic TGF‐β1 than similarly modified ‐agarose, ‐dextran, and ‐chondroitin sulfate hydrogels.
Synthesis and characterization of novel poly(o‐toluidine) montmorillonite nanocomposites: Effect of surfactant on intercalation
Authors: Singhal Rahul , Datta Monika
Published: July 2007

The investigation of clay based polymer nanocomposites has opened the door for the development of novel, ecofriendly advanced nano materials that can be safely recycled. Because of their nanometer size dispersion, these nanocomposites often have superior physical and mechanical properties. In this study, novel nanocomposites of poly(o‐toluidine) (POT) and organically modified montmorillonite (MMT) were synthesized using camphor sulfonic acid (CSA), cetyl pyridinum chloride (CPCl), and N‐cetyl‐N,N,N‐trimethyl ammonium bromide (CTAB) to study the role of surfactant modification on the intercalation. The in situ intercalative polymerization of POT within the organically modified MMT layers was analyzed by FTIR, UV–visible, XRD, SEM as well as TEM studies. The average particle size of the nanocomposites was found to be in the range 80–100 nm. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007
Swelling and network parameters of pH‐sensitive poly(acrylamide‐co‐acrylic acid) hydrogels
Authors: Turan Eylem , Çaykara Tuncer
Published: July 2007

Poly(acrylamide‐co‐acrylic acid) [P(AAm‐co‐AAc)] hydrogels were synthesized by the free‐radical crosslinking copolymerization of acrylamide (AAm) monomer at fixed amount, but at changing amount of acrylic acid (AAc) as the ionic comonomer in water. The swelling behavior of the hydrogels thus obtained was analyzed in buffer solutions at various pHs. The polymer‐solvent interaction parameter (χ) and the average molecular mass between crosslinks (Mc) of the hydrogels were calculated from swelling studies in buffer solutions at various pHs and were related to the AAc content. The results indicated that the swelling behavior of the hydrogels at different pHs agreed with the modified Flory–Rehner equation based on the affine network model and the ideal Donnan theory. In addition, from differential curves of dOv/dpH versus pH, the volume phase transition pH (VPT‐pH) was found to be 4.33 ± 0.05, independent of the AAc content in the hydrogel. Scanning electron microscope (SEM) was used to identify the morphological changes in the hydrogel as the concentration of AAc increased. In water at 22°C, an increase of the AAc content further increased the response rate of the hydrogels because of simultaneous increase of both the porosity and hydrophilicity of the network. In buffer solution with a pH increase from 9.0 to 2.0 at 22°C, the hydrogels with more AAc also exhibited a more rapid deswelling rate than that of the hydrogels with less AAc. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007
Synthesis and characterization of β‐CD derivatized bovine serum albumin protein as chiral selector in pressurized capillary electrochromatography
Authors: Dai Rongji , Tang Li , Li Hong , Deng Yulin , Fu Ruonong , Parveen Zahida
Published: July 2007

The synthesis and characterization of β‐cyclodextrin derivatized bovine serum albumin (β‐CD‐BSA) as chiral selector in pressurized capillary electrochromatography were investigated in this article. β‐CD‐BSA was synthesized by crosslinking β‐cyclodextrin and BSA using ethylene glycol diglycidyl ether in boric acid buffer at pH 8.7. The internal surface of the fused silica capillary was coated with amino groups using triethoxylmethylsiloxane and (3‐aminopropyl)trimethoxysiloxane by sol–gel technology. Then the inner surface of the capillary was further changed to aldehyde groups surface after being treated using glutaraldehyde. β‐CD‐BSA aqueous solution was pumped in to the capillary, and β‐CD‐BSA was successfully covalently bonded to the inner surface of the capillary. Chlorpheniramine, phenylalanine, tryptophan, and ibuprofen were separated by β‐CD‐BSA modified capillary. The operation voltage has certain effect on the separation of the enantiomers. The data showed that higher operation voltage resulted in good separation of tryptophane. However, when the other chiral selectors, β‐CD and BSA were employed, the racemic tryptophan was not separated. This indicated that a good chiral separation was obtained when β‐CD‐BSA was used as chiral selector. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007
Parameter Estimation and Model Selection for Neyman‐Scott Point Processes
Authors: Tanaka Ushio , Ogata Yosihiko , Stoyan Dietrich
Published: July 2007

This paper proposes an approximative method for maximum likelihood estimation of parameters of Neyman‐Scott and similar point processes. It is based on the point pattern resulting from forming all difference points of pairs of points in the window of observation. The intensity function of this constructed point process can be expressed in terms of second‐order characteristics of the original process. This opens the way to parameter estimation, if the difference pattern is treated as a non‐homogeneous Poisson process. The computational feasibility and accuracy of this approach is examined by means of simulated data. Furthermore, the method is applied to two biological data sets. For these data, various cluster process models are considered and compared with respect to their goodness‐of‐fit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Phosphorothiolate Analogues of Phosphatidylinositols as Assay Substrates for Phospholipase C
Authors: Liu Yinghui , Mihai Cornelia , Kubiak Robert J. , Rebecchi Mario , Bruzik Karol S.
Published: July 2007

Accurate measurement of phosphatidylinositol‐specific phospholipase C (PI‐PLC) activity is important in view of the key role of this enzyme in signal‐transduction pathways. In this work we synthesized enantiomerically pure phosphorothiolate analogues of all natural PI‐PLC substrates, including those of phosphatidylinositol 4,5‐bisphosphate (PI‐4,5‐P2), 4‐phosphate (PI‐4‐P), 5‐phosphate (PI‐5‐P) and unphosphorylated PI, in both long‐ and short‐chain versions. The enzymatic cleavage of these substrates produces thiol analogues of diacyl glycerol, which can be quantified by UV absorbance after treatment with dipyridyl disulfide. The monodisperse dihexanoyl derivatives are suitable substrates for PI‐PLC assay: they give rise to high enzyme activity, and provide excellent linear kinetic responses. For all substrates, we found a good linear correlation between the reaction rate and the amount of enzyme; this indicated the suitability of this assay for enzyme quantification. The short‐chain substrates enable the enzyme specificity with variously phosphorylated inositol head groups to be established—unobstructed by substrate aggregation, “scooting” kinetics on micelles, or surface dilution effects. The kinetic results indicated allosteric behavior of PLC for all substrates tested. We found that substrates phosphorylated at the inositol 4‐position (phosphorothiolate analogues of PI‐4,5‐P2 and PI‐4‐P) displayed very similar kinetic properties, and were cleaved with ∼20‐ to 30‐fold higher activity than the 4‐nonphosphorylated substrates (analogues of PI‐5‐P and PI). Hence it appears that interactions between the enzyme and the 4‐phosphate group of the substrate, but not its 5‐phosphate group, is important for PI‐PLC catalysis. In addition, the binding affinities of all four substrate types were found to be quite similar; this indicates that the energy of enzyme interaction with the 4‐phosphate group is directed almost entirely to catalysis.
Optimization of Carbon Dioxide Fixation by Chlorella vulgaris Cultivated in a Membrane‐Photobioreactor
Authors: Fan L.‐H. , Zhang Y.‐T. , Cheng L.‐H. , Zhang L. , Tang D.‐S. , Chen H.‐L.
Published: July 2007

In this paper, an enclosed membrane‐photobioreactor was designed to remove CO2 using Chlorella vulgaris. The performances of four reactors, which included the presented novel bioreactor, a draft tube airlift photobioreactor, a bubble column and a membrane contactor, were compared. The effects of the gas flow rate, light intensity, quality of the inner light source, and the characteristics of membrane module on CO2 fixation were investigated. The results showed that the rate of CO2 fixation in the membrane‐photobioreactor was 0.95–5.40 times higher than that in the other three conventional reactors under the optimal operating conditions
Hexaalkyl Terpyrrole: A New Building Block for the Preparation of Expanded Porphyrins
Authors: Sessler Jonathan L. , Weghorn Steven J. , Hiseada Yoshio , Lynch Vincent
Published: July 2007

A new, general synthesis of the first β‐substituted tetra‐ and hexaalkyl terpyrroles is described. Also described are two new classes of expanded porphyrins derived from the hexaalkyl terpyrrole. The key step in the terpyrrole formation is the copper(II)‐mediated oxidative coupling of the LDA‐derived enolates of α‐keto pyrroles. The first new expanded porphyrin reported here, the so‐called “orangarin”, contains five pyrrolic subunits and two bridging carbon atoms, and is formally a 20π‐electron nonaromatic macrocycle. The second new class of expanded porphyrins, the “amethyrins”, are 24π‐electron nonaromatic macrocycles containing six pyrrole units. Both of these new macrocycles, as well as one of the new terpyrrolic precursors have been structurally characterized by single crystal X‐ray diffraction analysis.
5,10,15,20‐Tetraphenylsapphyrin‐Identification of a Pentapyrrolic Expanded Porphyrin in the Rothemund Synthesis
Authors: Chmielewski Piotr J. , Latos‐Grażyński Lechoslaw , Rachlewicz Krystyna
Published: July 2007

The Rothemund‐type condensation of pyrrole and benzaldehyde yields, apart from 5,10,15,20‐tetraphenylporphyrin (TPPH2) and inverted tetraphenylporphyrin 2‐aza‐21‐carba‐5,10,15,20‐tetraphenylporphyrin (CTPPH2), a unique pentapyrrolic macrocyclic molecule with the aromatic nucleus of sapphyrin, namely, 5,10,15,20‐tetraphenylsapphyrin (TPSH3). Its unorthodox structural skeleton with an inverted pyrrole ring lying opposite to the bipyrrole unit accounts for the spectroscopic properties of the novel sapphyrin. The diprotonation of TPSH3 acts as a trigger for a structural transformation involving a flip of the pyrrole units, which relocates the 27‐NH pyrrolic nitrogen from the periphery into the center of the macrocycle. The formation of 5,10,15,20‐tetraphenylsapphyrin proves that the pentapyrrolic product is accessible by the mechanism of the Rothemund synthesis.
Hydrophobic Concave Surfaces and Cavities by Combination of Calix[4]arenes and Resorcin[4]arenes
Authors: Timmerman Peter , Nierop Klaas G. A. , Brinks Erik A. , Verboom Willem , van Veggel Frank C. J. M. , van Hoorn Willem P. , Reinhoudt David N.
Published: July 2007

Coupling reactions of calix[4]arenes and modified resorcin[4]arenes have been investigated. Reaction of mono(chloroacetamido)calix[4]arene 4 with tetrahydroxycavitand 9 gave the 1:1 coupled product 13 in 61% yield. Combination of upper‐rim 1,3‐difunctionalized calix[4]arene 5 with 9 afforded predominantly the 2:1 calix‐resorcinarene 18 in 47% yield. Reaction of 1,2‐difunctionalized calix[4]arene 6 with 9 gave five products, namely, endo 1:1 (19), exo 1:1 (20), endo‐endo 2:1 (21), endo‐exo 2:1 (22), and exo‐exo 2:1 (23) in ratios that depend on the reaction conditions. The stereochemistry of the different products was determined with NOESY experiments. The structures of 21 a and 23 b were calculated by using molecular mechanics, which revealed that intramolecular hydrogen bonds are only present in the former. Reaction of 1,2‐bis(chloroacetamido)calix[4]arene 26, which has two additional nitro groups at the remaining aromatic rings, with 9 yielded three different products, namely, endo 1:1 (28), endo‐endo 2:1 (30), and endo‐exo 2:1 (32) in ratios that depend on the reaction conditions. There is a preference for the endo orientation in the formation of the 1:1 coupled product, probably owing to an interaction of the nitro groups with the cavitand in the transition state. After conversion of the nitro groups in 28 into chloroacetamido moieties, reaction with Cs2CO3 in DMF under high‐dilution conditions afforded holand 33 in 26% yield together with calix[4]arene‐based carceplex 34 with an encapsulated DMF molecule (27% yield). Holand 33 was obtained in 33% yield by reaction of the tetrakis(chloroacetamide) endo‐endo 2:1 isomer 31 with tetrahydroxycavitand 9. Holand 33 contains a cavity of nanosize dimensions. Molecular mechanics simulations indicate that holand 33 adopts a conformation with eight hydrogen bonds and a large, preorganized cavity with two entrances of smaller dimensions. Molecular dynamics simulations of holand 33 in both CHCl3 and THF showed that four solvent molecules can be accommodated in the cavity at well‐defined positions.
Dendrimers of Nanometer Size Based on Metal Complexes: Luminescent and Redox‐Active Polynuclear Metal Complexes Containing up to Twenty‐Two Metal Centers
Authors: Campagna Sebastiano , Denti Gianfranco , Serroni Scolastica , Juris Alberto , Venturi Margherita , Ricevuto Vittorio , Balzani Vincenzo
Published: July 2007

Treelike (dendritic) structures made of RuII‐polypyridine complexes have been prepared by the “complexes‐as‐metals and complexes‐as‐ligands” synthetic strategy. The key building blocks are RuII complexes of the 2,3‐bis‐(2‐pyridyl)pyrazine(2,3‐dpp) bridging ligand, where one of the two chelating sites can be protected by methylation. Dendrimers containing four, ten, and twenty‐two metal ions have been obtained. These dendrimers exhibit strong absorption in the UV/Vis spectral region, a moderately strong red luminescence, and a great number of metal‐based oxidation and ligand‐based reduction processes. The nature of the terminal ligands determines in which subunits of the supramolecular array the HOMO and LUMO orbitals and the luminescent excited state are localized. The reported synthetic strategy is efficient, characterized by a full, step‐by‐step control of the growth process, and can be easily extended to building blocks containing other metals and/or ligands.
The Structural Properties of Uncompressed Crystalline Monolayers of Alcohols CnH2n+1 OH (n = 13–31) on Water and Their Role as Ice Nucleators
Authors: Majewski Jaroslaw , Popovitz‐Biro Ronit , Bouwman Wim G. , Kjaer Kristian , Als‐Nielsen Jens , Lahav Meir , Leiserowitz Leslie
Published: July 2007

A systematic analysis of grazing incidence synchrotron X‐ray diffraction data of uncompressed amphiphilic alcohols CnH2n+1 OH (n = 31, 30, 23, 20, 19, 18, 16, 14, 13) on a water subphase at 5°C is presented. Pronounced structural changes were observed on reduction of chain length from n = 31 to 13. The relative amount of two dimensional (2‐D) crystalline material formed fell drastically; shorter crystalline coherence lengths were also observed. For n–31–18 the molecules are arranged in a rectangular cell (a ≈ 5 Å, b increases from ca. 7.4 to ca. 8.2 Å) with plane symmetry p1 g1. For n < 18 a tilted free‐rotator phase is probably adopted. The two glide‐related molecules in the unit cell form a herringbone arrangement in which the chain axes are parallel and separated by [(a + b)/2]. The molecular chains are tilted from the vertical in the b direction; the tilt angle increases from ca. 7° to ca. 21° over the range n = 31 to 19, and then drops to 12° for n = 13. There is a continuous increase in molecular cross‐sectional area from 18.4 to 20.3 Å2 and in the atomic displacement parameter parallel to the water surface; it increases from 0.1 Å2 for n = 31 to 0.30 Å2 for n = 19. We explain the preference for chain tilt along the b axis, rather than a, in terms of hydrogen bonding to the water subphase. The various structural properties of the CnH2n+2OH (n = 31–13) monolayer series, such as degree of crystallinity and coherence length, lattice dimensions, chain orientation, and molecular motion, may be correlated with the ice‐nucleating efficiency of these alcohol monolayers as a function of n.
New Coordinatively Unsaturated Lutetium Mono‐ and Bis(alkyl) Complexes with a Bis(ortho)‐Chelating Aryldiamine Ligand—Crystal Structures of [LuCl2{2,6‐(Me2NCH2)2C6H3}(μ‐Cl)(μ‐Li(thf)2)]2 and [Lu(μ‐Cl){2,6‐(Me2NCH2)2C6H3}(CH2SiMe3)]2
Authors: Hogerheide Marinus P. , Grove David M. , Boersma Jaap , Jastrzebski Johann T. B. H. , Kooijman Huub , Spek Anthony L. , van Koten Gerard
Published: July 2007

New complexes of lutetium and yttrium containing the monoanionic, terdentate ligand [2,6‐(Me2NCH2)2C6H3]− (NCN) have been synthesized by substitution reactions starting from MCl3 (M = Lu, Y). Reaction of MCl3 (M = Y, Lu) with one equivalent of (NCN)Li affords the ate complexes [(NCN)MCl2(μ‐Cl)(μ‐Li(thf)2)]2) (M = Lu (1a), Y (1b)) in which the terdentate ligand is bound in mer fashion and all three chloride atoms are retained in the product. Crystals of 1a are monoclinic (space group P21/n, a =10.4559(4), b = 21.6150(9), c=12.1700(7) Å, β = 105.294(4)°, Z = 2, final R = 0.039 for 3695 observed reflections [I>2.50σ(I)]). Attempted substitution of chloride in the yttrium complex 1b by Me3SiCH leads to decomposition. However, reaction of 1a with Me3SiCH2Li gives the monoalkyl complex [(NCN)Lu‐(μ‐Cl)(CH2SiMe3)]2 2, 30% yield), in which the terdentate ligand is bound in a pseudo‐facial manner. Crystals of 2 are triclinic (space group P1, a = 9.8575(7), b = 10.0171(7), c = 11.1460(14) Å, α = 75.096(8). β = 78.092(8), γ =77.474(6)°, Z = 1, final R1 = 0.11 for 1361 reflections [I 2σ(I)]). Substitution of the chloride ions in 2 by Me3SiCH is possible and affords quantitatively the bisalkyl complex [(NCN)Lu(CH2SiMe3)2] (3). The lutetium complexes 2 and 3 are formally coordinatively unsaturated complexes, which are moisture‐sensitive and thermally stable for several weeks when dissolved in aromatic solvents. However, they decompose rapidly in aliphatic solvents such as hexane, and a decomposition route involving the formation of carbene species is proposed.
Metal‐Mediated Synthesis of Multidomain Ligands—A New Strategy for Metallosupramolecular Chemistry
Authors: Constable Edwin C. , Thompson Alexander M. W. Cargill , Harveson Peter , Macko Ludwig , Zehnder Margareta
Published: July 2007

The bridging ligand bis{4′‐(2,2′:6′,2″‐terpyridinyl)}ether (1) can be prepared in 69% yield from the reaction of 4′‐chloro‐2,2′:6′,2″‐terpyridine (3) with 2,2′:6′,2″‐terpyridin‐4′(1′H)‐one (2) in Me2NCHO in the presence of KOH. More conveniently, complexes of 1 can be prepared in situ by the reaction of 2 with a ruthenium(II) complex of 3 in the presence of K2CO3. This methodology has been developed for the synthesis of a range of mono‐, di‐, tri‐ and hexanuclear complexes with a variety of Xtpy (Xtpy = 4′‐substituted 2,2′:6′,2″‐terpyridine) terminator ligands. The molecular structure of 1 (a = 9.623(2), b = 11.241(1), c = 11.828(1) Å; space group P1; α = 93.064(9), β = 107.072(14), γ = 99.088(14)°; Z = 2, R = 0.0450, Rw = 0.0577) has been determined. The generality of the methodology may ultimately be limited by the sensitivity of the ether‐linkage in 1 to attack by nucleophiles.
Potential Linear‐Chain Organic Ferromagnets
Authors: Yoshizawa Kazunari , Hoffmann Roald
Published: July 2007

The electronic structures of conjugated polymers containing methyl radicals, carbenes, and nitrogen‐based radicals coupled in various ways through benzene rings are examined, employing band‐structure calculations in the extended Hückel approximation. The structural and electronic properties of polymers with a para‐phenylene or meta‐phenylene coupling unit are compared. In the polymer with methyl radicals coupled through a para‐phenylene unit, a pairing or Peierls distortion occurs to remove the degeneracy at the Fermi level. The resulting bandgap is nevertheless relatively small; we conclude that such polymers are likely to exhibit high electrical conductivity upon doping, very much like polyacetylene. On the other hand, in the polymers with a meta‐phenylene coupling unit, striking symmetry‐determined, halfoccupied narrow bands appear at the Fermi level and contribute to the stability of the ferromagnetic state. The relation of a potential ferromagnetic state to metallic, CDW, and SDW states is discussed from the viewpoint of orbital interactions in extended systems. We suggest novel 3‐ and 4‐fold helical structures for the meta‐phenylene‐coupled polymers.
An Enantioselective Synthesis of cis‐4‐tert‐Butoxycarbamoyl‐1‐methoxycarbonyl‐2‐cyclopentene—A Useful, General Building Block
Authors: Trost Barry M. , Stenkamp Dirk , Pulley Shon R.
Published: July 2007

The amino acid derivative in the title represents an important building block for the synthesis of a number of biologically important targets such as the antiviral carbanucleosides and amidinomycin. By using asymmetric palladium‐catalyzed desymmetrization of meso‐2‐ene‐1,4‐diols, cis‐1,4‐dibenzoyloxy‐2‐cyclopentene can be converted to the enantiomerically pure title compound in only four steps. Chemoselective ester reduction allows entry into the domain of carbanucleosides, whereas double‐bond reduction provides the precursor for amidinomycin. In an ancillary study, a facile diastereoselective cis‐hydroxylation provides aminocyclopentitols, compounds that have proven to be potent glycosidase inhibitors.
Intramolecular Long‐Range Exchange Coupling in Dinuclear Copper(II) Complexes with Cu…︁Cu Separations Greater than 10Å
Authors: Bürger Kai‐Silke , Chaudhuri Phalguni , Wieghardt Karl , Nuber Bernhard
Published: July 2007

Magnetic long‐range exchange coupling between two unpaired electrons of two copper(II) ions in a square‐pyramidal ligand environment (d)1, which are rigidly separated by > 10 Å by an organic spacer ligand (e.g., a μ‐dicarboxylato ligand) has been systematically studied. The following complexes have been synthesized: [{LCu(OH2)}2(μ‐terephthalato)](ClO4)2 (I), [LCuII(OH2)(benzoato)](CIO4) (2), [{LCuII(NCS)}2(μ‐terephthalato)]‐2 CH3OH (3), [{LCuII(NCS)}2 (μ‐4,4′‐oxybisbenzoato)] · 3 CH3OH (4), [{LCuII(OH2)}2 (μ‐tetrafluoroterephthalato)](CIO4)2 (5), [{LCuII(OH2)}2{LCuII}2 (μ‐tetrafluorobenzoato)3](CIO4)2 · 6 H2O (6), [{LCuII(OH2)}2 (μ‐biphenyl‐4,4′‐dicarboxylato)](CIO4)2 (7), [LCuII(μ‐biphenyl‐4,4′‐dicarboxylato)]n (HZO)2n (8), [{LCuII(NCS)}2 (μ‐biphenyl‐4.4′‐dicarboxylato)] (9), [{LCu(OH2)}2, (μ‐trans,trans‐2,4‐hexadien1,6‐dicarboxylato)] (CIO4)2 (10) (L = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane). Complexes 1, 2, 3, 4, and 6 have been characterized by X‐ray crystallography; the structure of 1 has been described previously.[15] Temperature‐dependent magnetic susceptibility data for all complexes have been measured over the range 2.0–298 K. In complexes 2, 3, 4, 6, 8, and 9 only very weak, if any, intermolecular antiferromagnetic or, in the case of 6, ferromagnetic coupling has been observed (|J|⩽2cm−1; H= −2JS1 S2 S1 = S2 = 1/2). In contrast, in complexes 5 (2J=−80cm−1), 7(2J=−62cm−1), and 10 (2J = −100 cm−1) relatively strong intramolecular antiferromagnetic exchange coupling has been detected. The intramolecular Cu…︁Cu separation is 11.3Å in 5, 15.6 Å in 7, and approximately 12.0Å in 10. These data demonstrate that intramolecular antiferromagnetic exchange coupling of considerable magnitude is possible over distances > 15 Å, provided that the effective magnetic orbitals are favorably aligned.
Isolation of Selected Exchange Processes in Nuclear Magnetic Resonance
Authors: Zwahlen Catherine , Vincent Sébastien J. F. , Schwager Martina , Bodenhausen Geoffrey
Published: July 2007

We have shown that it is possible to inhibit the transfer of magnetization in a system with several exchanging sites in dynamic equilibrium, as in a mixture of cis‐ and trans‐ZrCl4L2 with excess free ligand L (L = (CH3O)3PO). The forward and backward reaction rates involving two selected sites can be studied while the effect of competing exchange processes is “quenched”. This can be achieved either by selective inversion of the magnetization of the two chosen sites in the course of the reaction interval, or alternatively by inversion of the magnetization of all other sites in the exchange network. The rate of exchange from the free to the cis site was determined to be kcis←free = 0.018 s−1. In the usual methods, this process would tend to be overshadowed by the almost two hundred times faster competing exchange process from the cis to the trans site (Ktrans←cis = 3.32 s−1).
Alkenyl Fischer Carbene Complexes and α,β‐Unsaturated Imine Derivatives: Synthesis of Azepines and Mechanistic NMR Studies
Authors: Barluenga José , Tomás Miguel , Ballesteros Alfredo , Santamaría Javier , Carbajo Rodrigo J. , López‐Ortiz Fernando , García‐Granda Santiago , Pertierra Pilar
Published: July 2007

4‐Amino‐1‐azadienes 1 react with α,β‐unsaturated Fischer carbene complexes at −40°C to give stereoselectively a variety of substituted 3H‐4,5‐dihydroazepines 3; similarly, 1‐hydroxy‐1‐azadienes (α,β‐unsaturated oximes) 6 afforded the corresponding azepine derivatives 7. Chiral, nonracemic carbene complexes 11 gave azepines 12–13 (d.e. = 40–44%) upon reaction with oxime 6a; the major isomers were obtained in a diastereomerically and enantiomerically pure form (45–50% overall yield) after crystallization. An X‐ray structure of 12a allowed assignment of the absolute stereochemistry. The acid hydrolysis of azepines synthesized provided racemic and enantiomerically pure 1,6‐dicarbonyl compounds (±)‐5, (±)‐9, and (–)‐14, as well as diol (–)‐15. The mechanism of the reaction of 1 and 2 was investigated by multinuclear (1H, 13C, 15N, and 183W) NMR characterization of four intermediates (A, B, C, and D) at low temperature. The experimental sequence of events involves: i) 1,2‐nucleophilic addition of the unsubstituted imine nitrogen of 1 to the metal carbene function (zwitterion A, −60°C), ii) cyclization to the seven‐membered ring with 1,2‐migration of the pentacarbonyl metal (zwitterion B, −40°C), iii) reductive elimination and coordination of the metal to the amine nitrogen (intermediate C, −40°C), and iv) thermal decomplexation and tautomerization (intermediate D and compound 3, above −20°C).
Resolution of the Absorbance and CD Spectra and Formation Constants of the Complexes between Human Serum Albumin and Methyl Orange
Authors: Ambrosetti R. , Bianchini R. , Fisichella S. , Fichera M. , Zandomeneghi M.
Published: July 2007

Difference absorbance and circular dichroism techniques show that two complexes are formed between human serum albumin (HSA) and Methyl Orange (MO). The stoichiometries of the two HSA‐MO complexes (1:1 (C1) and 1:2 (C2)), their association constants (K1, 1 = 2.32 (0.18) × 105 M−1 and K1, 2 = 1.12 (0.15) × 1011 M−2), and both absorbance and dichroic spectra have been determined by a computational approach. Nearly 900 experimental points, consisting of absorbance and CD measurements registered in the 340–550 nm interval and over a wide range of concentrations of protein and ligand, have been included in a unique fitting procedure. The Scatchard plot indicates the existence of a unique binding site which can accommodate up to two molecules of MO in a positive co‐operative process. Calculation of the CD spectrum for the C2 complex according to the DeVoe method reproduces the fitted dichroic spectrum for the same complex. The shapes of the fitted absorbance and dichroic spectra, as well as the influence of concentrated NaCl or ethylene glycol on the absorbances of both free MO and HSA‐MO mixtures are consistent with the presence of dominant electrostatic interactions in C1. The C2 complex can be envisaged as a unique chromophore, consisting of two MO units associated in a stacking process into the same binding site of HSA, leading to a well‐defined chirality. The general validity of this multitechnique, multiwavelength approach in the investigation of protein‐ligand complexes is discussed.
In Situ 13C Solid‐State NMR and Ex Situ GC–MS Analysis of the Products of tert‐Butyl Alcohol Dehydration on H‐ZSM‐5 Zeolite Catalyst
Authors: Stepanov Alexander G. , Sidelnikov Vladimir N. , Zamaraev Kirill I.
Published: July 2007

The hydrocarbon products that are formed upon dehydration at 296–673 K of tert‐butyl alcohol (tBuOH), adsorbed on H‐ZSM‐5 zeolite in concentrations equal to that to active AlOHSi sites in the catalyst, have been analyzed by 13C solid‐state MAS NMR and GC–MS. To facilitate 13C NMR analysis, the alcohol selectively labeled with 13C isotope in the COH group was used. It was found that tBuOH transforms to the adsorbed C8 butene dimers plus a trace amount of alkanes at 296 K. Butene dimers exist inside HZSM‐5 pores in the form of interconverting adsorbed octene, octyl silyl ether, and octyl carbenium ion; octyl silyl ether is the main adsorption state. Fluxionality of the carbenium ion form provides a pathway for isomerization of the highly branched hydrocarbon skeleton of the intial alcohol to the predominantly linear one in the adsorbed butene dimer. The driving force for the isomerization into the linear structure is the shape selectivity induced by the small size of the zeolite channels. At 373 K the adsorbed butene dimers further crack into species that contain an average of about 6.5 carbon atoms, in addition to further alkanes. At 448 K the adsorbed C3–C7+ paraffins become the predominant hydrocarbon products observed with both in situ 13C NMR and ex situ GC–MS. Simultaneously, a mixture of adsorbed polyenes is formed. According to 13C CP/MAS NMR, polyenes exist in the zeolite pores in the form of rather stable cyclopentenyl cations. At 573–673 K adsorbed cyclopentenyl cations further transform into a mixture of condensed and simple aromatics and then into xylenes and toluene. Simultaneously, paraffins crack further to give mainly C3–C4 paraffinic species at 573 K and propane at 673 K.
Effect of the Position and Number of Chiral Carbons on Ferroelectric Liquid Crystals from Multichain Mononuclear ortho‐Palladated Complexes
Authors: Thompson Neil J. , Serrano José Luis , Baena Maria Jesús , Espinet Pablo
Published: July 2007

A series of twelve mononuclear ortho‐palladated complexes incorporating a Schiff base and a β‐diketone ligand have been synthesised. These compounds have four nonequivalent terminal chain positions and differ in the number and position(s) of chiral chains [(R)‐2‐methylheptyl] in the structure. A study of the ferroelectric properties of the compounds has been performed. It is found that the spontaneous polarisation is highly dependent not only on the number of chiral carbons present, but also on the position within the molecular core.
Polysaccharides of Intracrystalline Glycoproteins Modulate Calcite Crystal Growth In Vitro
Authors: Albeck Shira , Weiner Steve , Addadi Lia
Published: July 2007

Assemblies of glycoproteins from within the mineralized tissues of sea urchins and mollusks both interact in vitro in a similar manner with growing calcite crystals. A protein‐rich fraction, a polysaccharide‐rich fraction, and a fraction composed of densely glycosylated peptide cores were obtained by chemical and enzymatic treatment of the glycoproteins from sea‐urchin spines. Each fraction was partially purified and characterized (amino acid composition, FTIR and NMR spectroscopy). A comparison of the interactions of these fractions with growing calcite crystals in vitro shows that the polysaccharide moieties of these glycoproteins are intimately involved in the interaction with growing calcite crystals on planes approximately parallel to the c crystallographic axis. Presumably the polysaccharides in the mollusk‐shell glycoproteins are likewise responsible for the similar interactions of these macromolecules with calcite. We suggest that structured polysaccharide moieties of glycoproteins are important in controlling aspects of crystal growth in vivo as well.
Mechanisms of Water Exchange between Lanthanide(III) Aqua Ions [Ln(H2O)n]3+ and bulk water: A Molecular Dynamics Simulation Approach Including High‐Pressure Effects
Authors: Kowall Thomas , Foglia François , Helm Lothar , Merbach André E.
Published: July 2007

We studied the microscopic mechanisms of the water exchange reaction between the hydration shells of lanthanide(III) ions (Ln = Nd, Sm, Yb) and bulk water by means of molecular dynamics simulations. In contrast to the residence time of a water molecule in the first hydration shell (τres (1st shell) = 1577, 170 and 410 ps for Nd3+, Sm3+ and Yb3+, respectively), that in the second hydration shell is nearly independent of the type of the cation and amounts to 12–18 ps. Along the lanthanide series a change in the coordination number from 9 to 8 is coupled to a changeover in the water exchange mechanism. The observed water exchange events on the [Nd(H2O)9]3+ aqua ion follow a dissociatively activated Id mechanism via an eightfold‐coordinated transition state of square antiprismatic geometry. The lifetime of the transitory square antiprism varies between virtually 0 and 10 ps. The assignment of an Id mechanism (instead of a limiting D mechanism) is supported by the existence of a preferential arrangement between the exchanging water molecules (1800) and by the fact that the calculated average activation volume ΔV≠ = + 4.5 cm3 mol−1 is clearly smaller than the estimated activation volume ΔV≠lim ≈ΔV0 = + 7.2 cm3 mol−1 for a limiting D process. In the case of Sm3+ a ninth water molecule exchanges frequently between the first hydration shell and the bulk and maintains the coordination equilibrium between a [Sm(H2O)8]3+ and a [Sm(H2O)9]3+ aqua ion. The resulting trajectory pattern of incoming and leaving water molecules is an alternation of elimination and addition reactions and cannot be classified into the scheme of D, I or A mechanisms for substitution processes. The reaction volume ΔV0 for the coordination equilibrium [Sm(H2O)8]3+ + H2O → [Sm(H2O)9]3+ can be evaluated consistently both by a thermodynamic and a geometric approach. The observed exchange events for [Yb(H2O)8]3+ exhibit the characteristics of an Ia mechanism. The water exchange takes place via a transition‐state geometry close to that of a tricapped trigonal prism and involves a slightly negative activation volume.
Vanadium(II)‐ and Niobium(III)‐Induced, Diastereoselective Pinacol Coupling of Peptide Aldehydes to Give a C2‐Symmetrical HIV Protease Inhibitor
Authors: Kammermeier Bernhard , Beck Gerhard , Holla Wolfgang , Jacobi Detlev , Napierski Bernd , Jendralla Heiner
Published: July 2007

Peptide aldehydes 15 a–c are prepared without epimerization from enantiomerically pure (S)‐α‐amino acids (Scheme 3). Reductive pinacol homocoupling of 15 a–c, induced by vanadium complex 11 or niobium complex 16 in refluxing THF, yields C2‐symmetrical (S,R,R,S)‐configurated 6a, 6b and 2, respectively, with moderate to high stereoselectivity (Scheme 4). In a novel protocol for the preparation and utilization of THF solutions of 11, the isolation of air‐sensitive intermediates can be avoided and the potent HIV protease inhibitor 2 prepared in enantio‐ and diastereomerically pure form on a kilogram scale without chromatographic purification. The (S,R,R,S) selectivity of the pinacol homocouplings is confirmed by means of an independent, stereochemically unequivocal synthesis of 6 a and 2 from D‐mannitol 4 (Scheme 1).
Structural Principles of the Coordination Number Eight: WF, ReF, and XeF
Authors: Adam Steffi , Ellern Arkady , Seppelt Konrad
Published: July 2007

WF6, ReFn (n = 6 and 7), and XeF6 combined with NOF to give (NO+2) WF, (NO+)2ReF, and (NO+)2XeF2–8, respectively. Also NO2F reacted with ReF6 to form (No2+)ReF2–8. Cs2XeF8·4BrF5 crystallized from a solution of Cs2XeF8 in BrF5. These five compounds were subjected to X‐ray structure determinations. The structure of [NO(NOF)2]+IF–8 was used as a standard for comparison. All anions exhibit square‐antiprismatic geometry, independent of their electronic configurations. Bond lenghts in ReF2–8 and XeF2–8 are larger than in WF2–8 and IF–8, owing to the presence of nonbonding electrons. Deviations from the ideal structure in XeF2–8 are attributed to cation‐anion interactions.
Stabilization of Octahedral Thorium Clusters by Interstitial Hydrogen
Authors: Braun Thomas P. , Simon Arndt
Published: July 2007

The synthesis, structure, and characterization of the cluster compounds Th6HxBr15 (x = 5 and 7) are reported. The compounds can be prepared by a reversible hydrogenation/dehydrogenation process; their structures have been determined by X‐ray and neutron diffraction methods. Both compounds form a body‐centered cubic structure of octahedral cluster units with five or seven hydrogen atoms in eight disordered positions above the faces of the octahedron. The chemical bonding in these diamagnetic compounds has been analyzed: the simple counting rules for octahedral cluster compounds are fulfilled, and MO and band structure calculations confirm the hydridic nature of the bonding.
Synthetic Cyclic Oligosaccharides–Syntheses and Structural Properties of a Cyclo[(1 → 4)‐α‐L‐rhamnopyranosyl‐(1 → 4)‐α‐D‐mannopyranosyl]trioside and ‐tetraoside
Authors: Ashton Peter R. , Brown Christopher L. , Menzer Stephan , Nepogodiev Sergey A. , Stoddart J. Fraser , Williams David J.
Published: July 2007

An efficient polycondensation–cyclization approach to the synthesis of cyclodextrin analogues is demonstrated by the preparation of cyclohexaoside 1 and cyclooctaoside 2. The key intermediate, disaccharide 3, bearing the cyanoethylidene group as a glycosyl donor function and the trityloxy group as a glycosyl acceptor function was prepared in 15 steps starting from L‐rhamnose and D‐mannose. The crucial cyclooligomerization of the disaccharide monomer 3 was carried out in the presence of TrClO4 as a promoter with the use of ultra‐dry conditions at normal concentrations. This reaction led to formation of the cyclic oligosaccharides 28 and 29 (in 34 and 31% yield, respectively), which were deprotected to afford 1 and 2, respectively. The X‐ray crystal structural analysis of the cyclooctaoside 2 reveals a cylindrical shape for the cyclic oligosaccharide with C4 symmetry. Individual molecules of 2 are arranged perfectly in stacks that form nanotubes in the solid state.
Me3TTF–PO3H2, a Redox Phosphonic Acid and Its Monoanilinium Salt [PhNH+3][Me3TTF–PO(OH)O‐], the Electrocrystallized Neutral (Zwitterionic) π Radical [Me3TTF–PO(OH)O‐]·+, and Their Associated Lamellar Constructions in the Solid State
Authors: Dolbecq Anne , Fourmigué Marc , Krebs Frederik C. , Batail Patrick , Canadell Enric , Clérac Rodolphe , Coulon Claude
Published: July 2007

The reaction of Me3TTFLi (TTF = tetrathiafulvalene) with ClP(O)‐(OEt)2followed by hydrolysis with Me3SiBr affords the novel π‐donor molecule trimethyltetrathiafulvalenylphosphonic acid (Me3TTF–PO3H2) in a partially oxidized form. Subsequent reduction and neutralization with aniline gives the corresponding phosphonate monoanilinium salt. A unique hydrogen‐bonded hexagonal net is identified within the lamellar structure of [PhNH+3][Me3TTF–PO(OH)O‐], which is described by analogy with the anti‐CaSi2structure type. Electrocrystallization of the former salt yields single crystals of a neutral (zwitterionic) π radical, formulated as [Me3TTF–PO(OH)O‐]·+. Their structure reveals the presence of hydrogen‐bonded molecular ribbons whose association creates a novel layered architecture similar to that obtained within radical cation salts of π‐donor molecules of larger spatial extension. The analysis of the calculated HO‐MO–HOMO intermolecular interaction energies demonstrates that these slabs contain strong π‐π intermolecular interactions despite the nonexistence of any 2D network of short S ⃛S contacts. The spin susceptibility of [Me3TTF–PO‐(OH)O‐]·+, determined by single‐crystal ESR measurements, is characteristic of triplet excitons, the origin of which may be understood from the electronic structure of the compound.
Kinetic Analysis of the Stepwise Platination of Single‐ and Double‐Stranded GG Oligonucleotides with Cisplatin and cis‐[PtCl(H2O)(NH3)2]+
Authors: Sadler Peter J. , Barnham Kevin J. , Berners‐Price Susan J. , Frey Urban
Published: July 2007

We report the first direct comparison of the kinetics of platination of defined single‐ and double‐stranded DNA with the anticancer drug cisplatin. The courses of the reactions of the 14‐mer duplex d(A‐T‐A‐C‐A‐T‐G‐G‐T‐A‐C‐A‐T‐A)·d(T‐A‐T‐G‐T‐A‐C‐C‐A‐T‐G‐T‐A‐T) with [15N]cisplatin and cis‐[PtCl(H2O)‐(15NH3)2]+ and of each of the single strands with [15N]cisplatin have been studied at 298 K, pH 6, by [1H, 15N] NMR spectroscopy. As expected the reactions of cisplatin proceed via cis‐[PtCl(H2O)(NH3)2]+, and lead to two monofunctional adducts on the duplex and two on the GG single strand. In both the GG single strand and the duplex, one of the two G's is platinated faster than the other (by a factor of ca. 4). Remarkably, ring closure on the duplex to form the GG chelate occurs about an order of magnitude faster for one monofunctional adduct than for the other. The latter monofunctional adduct has distinctive 1H and 15N NMR chemical shifts for Pt‐NH3, and is very long‐lived (persists for >5 d). The Pt‐Cl bond in this monofunctional adduct is protected from hydrolysis by the duplex. In contrast, the two monofunctional adducts on the GG single strand undergo ring closure at about the same rate. Equilibria between kinked and distorted forms of the GG platinated duplex, the platination of G's on the complementary strand, and the potential biological significance of long‐lived monofunctional adducts of platinum drugs with DNA are discussed.
[Cu24S12(PMeiPr2)12], [Cu28S14(PtBu2Me)12], [Cu50S25(PtBu2 Me)16], [Cu70Se35(PtBu2Me)21], [Cu31Se15(SeSiMe3)(PtBu2Me)12] and [Cu48Se24(PMe2Ph)20]: New Sulfur‐ and Selenium‐Bridged Copper Clusters
Authors: Dehnen Stefanie , Fenske Dieter
Published: July 2007

Reaction of copper(I) acetate with bis(trimethylsilyl)chalcogenides E‐(SiMe3)2 (E = S, Se) in the presence of tertiary phosphines in organic solvents leads to the formation of chalcogenbridged copper clusters in crystalline form. In recent investigations, the following structures were isolated and characterised by X‐ray crystallography: [Cu24S12‐(PMeiPr2)12] (1), [Cu28S14(PtBu2Me)12] (2), [Cu50S25(PtBu2Me)16] (3), [Cu70Se35‐(PtBu2Me)21] (4), [Cu31Se15(SeSiMe3)‐(PtBu2Me)12] (5) and [Cu48Se24(PMe2‐Ph)20] (6). The cluster cores of 1‐6 have new structures or new structural characteristics and go some way towards providing an insight into the mechanisms of cluster formation reactions. With the help of the aforementioned compounds a number of structure principles that govern sulfur‐and selenium‐bridged copper clusters can be formulated.
Oxygen and Nitrogen in Competitive Situations: Which is the Hydrogen‐Bond Acceptor?
Authors: Böhm Hans‐Joachim , Brode Stefan , Hesse Ute , Klebe Gerhard
Published: July 2007

In the design of novel protein ligands one of the major challenges is the replacement of functional groups to modify and improve the binding characteristics. Often nitrogen‐ and oxygen‐containing groups are exchanged, or both atoms occur in a competitive situation. We have investigated the hydrogen‐bonding abilities of oxygen atoms covalently bound to two non‐hydrogen atoms of which at least one is formally assigned to an sp2‐type hybridization. In particular, examples in which such oxygen atoms compete with nitrogen atoms in the same molecular segment have been studied. Based on interaction energies obtained from ab initio calculations for complexes of these molecules with water, the oxygen atoms can be classified as rather weak hydrogen‐bond acceptors; nitrogen atoms present in the same fragment exhibit much stronger interaction energies. The ab initio results are confirmed by the relative frequencies with which oxygen and nitrogen atoms are found to be involved in hydrogen bonding in the crystal structures of organic molecules containing the fragments of interest.
Kinetic and thermodynamic study of the mechanism of reversible intermolecular electron‐transfer reactions between horse heart cytochrome c and a series of cobalt imine complexes
Authors: Meier Martin , van Eldik Rudi
Published: July 2007

The Kinetics and Thermodynamics of the Reversible Outer‐Sphere Electron‐Transfer Reactions between Horse Heart Cytochrome cII/III and [Co(phen)3]3+/2+ and [Co(bpy)3]3+/2+ were studied in detail, in particular as a function of temperature and pressure. It was possible to construct a volume profile for both reactions from the pressure data. The transition state was found to be halfway between the reactant and product states on a volume basis in all studied systems. This is in agreement with the λ≠ parameter estimated from the Marcus theory. For all the systems investigated, the differences in the activation volumes are in good agreement with the reaction volumes determined from spectrophotometric and electrochemical measurements at elevated pressure, and from the difference in the partial molar volumes of the metal complexes. The activation and reaction volumes of the bipyridine system are significantly smaller than those of the corresponding phenanthroline and terpyridine systems. A detailed mechanistic analysis is presented. The results show that the different kinetic and thermodynamic techniques employed complement one another.