bluuebunny/crossref_metadata_2025_split huggingface dataset:
Distorted Molecules: Perturbation Design, Preparation and Structures
Authors: Bock Hans , Ruppert Klaus , Näther Christian , Havlas Zdenek , Herrmann Hans‐Friedrich , Arad Claudia , Göbel Ilka , John Andreas , Meuret Jochen , Nick Sabine , Rauschenbach Andreas , Seitz Wolfgang , Vaupel Torsten , Solouki Bahman 
Published: December 2003

AbstractThe structure of a molecule can change considerably as its energy and thus its electron distribution within the time‐domain of dynamic relaxation varies. Based on comparison of approriate measured data of related compounds and supported by quantum chemical calculations, therefore, charge‐perturbed and/or sterically overcrowded molecules can be designed. Their preparation, handling, and structural characterization, frequently under extreme and especially largely aprotic conditions, provides some surprises. New structural principles become evident and old‐fashioned ones are confirmed. Thus the contact‐ion aggregates that form on ultrasonically supported reduction of unsaturated hydrocarbons with sodium metal partly contain dibenzene sodium sandwiches. Vicinal dimethylamino substituents or isoelectronic isopropyl groups cause steric overcrowding and facilitate oxidation to molecular cations by energetically favorable delocalization of the generated positive charge. Molecules and molecular ions in which an even number of π electrons are distributed over a σ skeleton containing an odd number of centers preferentially form cyanine subunits. This is demonstrated by the novel ethene dication and dianion salts with central CC single bonds and molecular halves twisted relative to each other. Altogether in two years well over 50 structures have been determined. Much has been learned from them, especially about electron transfer and contact ion‐pair formation in aprotic solvents. Nevertheless, we had to realize that answers to many questions, above all “what crystallizes, how, and why”, are still out of reach.
Synthetic Bilayer Membranes: Molecular Design, Self‐Organization, and Application
Authors: Kunitake Toyoki 
Published: December 2003

AbstractLipid bilayers are a most central building block of the biological molecular organization. Their two‐dimensional self‐assembly is essential to the generation of biological shapes and sizes on the molecular level. The observation that a totally synthetic amphiphile in water is spontaneously assembled to a bilayer structure suggested that bilayer formation is a general physicochemical phenomenon that is not restricted to particular structures of biolipid molecules. Bilayer formation is now observed for a large variety of synthetic amphiphiles which contain one, two, three, or four alkyl tails. The flexible alkyl tail may be replaced by perfluoroalkyl chains. The supramolecular structures obtained therefrom can be related to the component's molecular structure in many cases. The structural variety and the ease of molecular design make the synthetic bilayer an attractive vehicle for organizing covalently bound functional units and guest molecules. In addition, stable monolayers on water, planar lipid membranes (BLM), and free‐standing cast films are obtainable because of the self‐assembling property of bilayer‐forming compounds. These molecular organizations display common supramolecular features. The use of the cast film as a molecular template provides exciting potential for the production of novel two‐dimensional materials.
On Quantifying Chirality
Authors: Buda Andrzej B. , der Heyde Thomas Auf , Mislow Kurt 
Published: December 2003

AbstractSince Pasteur's epochal discoveries a century and a half ago, the concept of chirality has continued to play a central role in chemistry and biochemistry. Can chirality be measured? It has long been known that molecular chirality can be given a quantitative meaning through functions specifically parametrized to match the magnitude of pseudoscalar observables. However, chirality is a property that is independent of its physical and chemical manifestations : for a system to be chiral, all that is required is the absence of improper rotations in the symmetry group of the system. This being the case, how can chirality be measured if the “system” is an abstract geometric figure, for example, a scalene triangle in the plane or an asymmetric tetrahedron in three‐dimensional space? How does chirality vary as a function of pure shape? In this review we describe recent efforts designed to answer these and related questions.
Oxoniobates Containing Metal Clusters
Authors: Köhler Jürgen , Svensson Gunnar , Simon Arndt 
Published: December 2003

AbstractMetal clusters, discrete or condensed, are characteristic of the structures of many compounds which contain transition metals in low oxidation states. The highly reduced oxoniobates support the concept of condensed clusters. They contain Nb6O12 clusters which are either isolated or linked at the apices of the Nb6 octahedra to form oligomeric chains or networks. The analysis of the bonding relationships allows the identification of different types of Nb atoms and thus the quantitative prediction of valence electron concentrations for finite or infinite structures composed of these condensed M6X12 clusters.
Mercury, a Structural Building Block and Source of Localized Reactivity in Extended Metal–Metal Bonded Systems
Authors: Gade Lutz H. 
Published: December 2003

AbstractTransition metal–mercury complexes were among the first compounds of study for the concept of direct metal–metal bonding which was established more than three decades ago. Since then, a large number of such systems have been synthesized and studied. The fact that mercury is readily attached to a large variety of main group or transition metals has stimulated its use as a general building block in the systematic synthesis of mixed‐metal clusters. The past decade has witnessed a rapid expansion of bimetallic cluster chemistry in which species containing mercury have played a prominent role, and which has led to the discovery of many unprecedented cluster structures and reactions. In particular, the ability of mercury to form multicenter metal–metal bonds with polynuclear cluster fragments has substantially extended its coordination chemistry which was thus far dominated by simple linear structural arrangements. Although certain structural motifs are found to be common to many of the transition metal–mercury clusters investigated to date and thus enable a relatively systematic synthetic approach, the multitude of surprising discoveries has kept the interest in the chemistry of the element itself alive. The recent discovery of the redox and photochemical reactivity of some of these systems has opened up an exciting and promising area of cluster research. Its significance for the synthetic methodology lies in the fact that the increasing redox activity of molecular carbonyl clusters on going to higher nuclearities appears to set a limit on the size of metal frameworks attainable by the standard preparative methods. On the other hand, their potential use as photochromes or redox mediaters in coupled electron‐transfer reactions provides an additional stimulus for future studies in this field.
Transmembrane Signaling of T Lymphocytes by Ligand‐Induced Receptor Complex Assembly
Authors: Eichmann Klaus 
Published: December 2003

AbstractT lymphocytes (T cells) are the central cell type initiating all immune responses. They are able to recognize other cells in the body that have been invaded by foreign living or nonliving matter. In such cells, foreign peptides generated by intracellular breakdown are complexed with molecules of the major histocompatibility complex (MHC) specially designed for peptide binding. Peptide‐loaded MHC molecules appear on the surface of these cells and alert the immune system. The molecular complex which T cells use for recognition of peptide‐loaded MHC molecules is among the most sophisticated and versatile receptor systems in biology. It consists of specific and nonspecific transmembrane components which assemble to a functional signal transduction unit as the result of ligand binding. Correct assembly leads to activation and relocation of enzymes including membrane‐associated, tyrosin‐specific protein kinases and phosphatases. Transmembrane signaling in T cells depends on the correct assembly and cooperation among multiple molecular components. This may be related to a multitude of different cellular responses of T cells at different stages of differentiation, all elicited through the T cell receptor complex.
Sequential Transformations in Organic Chemistry: A Synthetic Strategy with a Future
Authors: Tietze Lutz F. , Beifuss Uwe 
Published: December 2003

AbstractOrganic‐chemical synthesis has always fascinated chemists and will not lose its importance in the future. It is a truism that all chemists—and others too—are dependent on the synthesis of those compounds with which they want to work. As a result, organic‐chemical synthesis today is more than ever before the cutting edge of organic chemistry, biology, biochemistry, medicine, physics, and material science. Synthesis is also the basis of the chemical industry. For the passionate synthetic chemist, however, synthesis is much more than just a method for obtaining compounds; it is the expression of his creativity, intelligence, ability, and also his perseverance.
Chemistry of and with Highly Reactive Metals
Authors: Fürstner Alois 
Published: December 2003

AbstractToday, the synthetic chemist has a large repertoire of metal activation methods at his disposal. After a first breakthrough was made at the beginning of the seventies with the introduction of the Rieke metals, a series of further, in part more efficient methods were describes, based on which not only classical metal‐induced reactions could be substantially improved but also completely new reactions could be discovered. In this article the individual activation methods are discussed and compared as far as is possible using the currently available data. Especially noteworthy are the metal–graphite combinations because of their unsurpassed reactivity and concomitant easy preparation and manipulation. As shown by numerous applications of these reagents on polyfunctional substrates, particularly natural products, high reactivity of the metal and excellent selectivity are by no means precluded. Besides the purely preparative aspects also insights gained so far into the general principles and limits of metal activation are outlined, and attempts at determining the morphology of highly dispersed systems are reported.
Sequence‐Specific Recognition and Modification of Double‐Helical DNA by Oligonucleotides
Authors: Thuong Nguyen T. , Hélène Claude 
Published: December 2003

AbstractNucleic acids can be selectively recognized by a large number of natural and synthetic ligands. Oligonucleotides provide the highest specificity of recognition. They can bind to a complementary single‐stranded sequence by forming Watson–Crick hydrogen bonds. They can also recognize the major groove of double‐helical DNA at specific sequences by forming Hoogsteen or reverse Hoogsteen hydrogen bonds with purine bases of the Watson‐Crick base pairs, resulting in a triple helix. Triple‐helix formation through oligonucleotide binding to DNA is a sequence–specific interaction involving primarily homopurine·homopyrimidine sequences in the double‐helical target. Extending the range of recognition sequences remains a challenge to chemists. Both thermodynamic and kinetic parameters for triplex formation have been determined. These parameters indicate, for example, that triple‐helix formation is a much slower process than duplex formation. Nuclease‐resistant oligonucleotides synthesized with the anomers of nucleosides (instead of the natural β‐anomers) also form triple helices with double–stranded DNA. Triple‐helix‐forming oligonucleotides can be modified, for example, by attaching DNA intercalating agents to enhance their binding affinity. They may also be modified with reagents that induce irreversible reactions in their target sequence upon chemical or photochemical activation. Thus, artificial nucleases can be developed with very high sequence specificity on megabase‐size DNA. Furthermore, triple‐helix‐forming oligonucleotides can be used to selectively control gene expression. When bound to the regulatory region(s) of specific genes they may prevent activation (or repression) of transcription. When binding occurs near or downstream from the transcription initiation site, elongation of the transcript may be inhibited. Therefore, the potential exists for developing new gene‐blocking agents with therapeutic applications in the treatment of gene disorders.
Syntheses of Polycyclic Natural Products Employing the Intramolecular Double Michael Reaction
Authors: Ihara Masataka , Fukumoto Keiichiro 
Published: December 2003

AbstractThe one‐pot intramolecular double Michael reaction of compounds having two different α, β‐unsaturated carbonyl groups to form polycyclic compounds can be carried out by three different methods: in the first the substrate is treated with lithium hexamethyldisilazide, in the second with chlorotrimethylsilane, triethylamine, and zinc chloride at an elevated temperature, and in the third with tert‐butyldimethylsilyl trifluoromethanesulfonate and triethylamine. The reaction proceeds with complete regioselectivity and high stereoselectivity following a stepwise mechanism. Spiro‐fused bicyclo[2.2.2]octane derivatives can be constructed with high stereoselectivity by the intromolecular double Michael reaction by using the first method. Enantiomerically pure atisine and the enantiomer of atisirene were synthesized stereoselectively by application of this methodology. The syntheses of steroids and angular triquinane‐type sesquiterpenoids were possible with the second method. Heterocyclic compounds with a bridgehead nitrogen atom were obtained by the reaction of α, β‐unsaturated amides following the second and third methods. The asymmetric synthesis of tylophorine with diastereofacial control was achieved by the intramolecular reaction according to the third method. The sulfur‐mediated intramolecular double Michael reaction utilizing the third method produced trans‐hydroindane derivatives. Furthermore, the intramolecular Michael‐aldol reaction can be employed in synthesizing polycyclic systems with cyclobutane units by treatment with tert‐butyldimethylsilyl trifluoromethanesulfonate in the presence of triethylamine. The intermolecular double Michael reaction and related reactions will also be described.
Chemistry and Biology of Taxol
Authors: Nicolaou Kyriacos Costa , Dai Wei‐Min , Guy Rodney Kiplin 
Published: December 2003

AbstractOne can view plants as a reference library of compounds waiting to be searched by a chemist who is looking for a particular property. Taxol, a complex polyoxygenated diterpene isolated from the Pacific Yew, Taxus brevifolia, was discovered during extensive screening of plant materials for antineoplastic agents during the late 1960s. Over the last two decades, interest in and research related to taxol has slowly grown to the point that the popular press now seems poised to scoop each new development. What was once an obscure compound, of interest only to the most masochistic of synthetic chemists and an equally small number of cellular biologists, has become one of the few organic compounds, which, like benzene and aspirin, is recognizable by name to the average citizen. In parallel, the scientific study of taxol has blossomed. Physicians are currently studying its effects on nearly every known neoplasm. Biologists are using taxol to study the mechanisms of cell function by observing the effects of its interactions with the cellular skeletal systems. Synthetic chemists, absorbed by the molecule's unique and sensitive structure and functionality, are exploring seemingly every available pathway for its synthesis. Indeed, the demand for taxol has risen so in the last five years that alternative sources to the extraction of T. brevifolia are being vigorously pursued. Because of the rapidly expanding scope of research in the multifaceted study of taxol, those who are interested in the field may find acquisition of a reasonable base of knowledge an arduous task. For this reason, this account attempts to bring together, for the first time, in a cogent overview the chemistry and biology of this unique molecule.
Chemical Analysis of Inorganic and Organic Surfaces and Thin Films by Static Time‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐SIMS)
Authors: Benninghoven Alfred 
Published: December 2003

AbstractBy using mass spectrometry to analyze the atomic and molecular secondary ions that are emitted from a solid surface when bombarded with ions, one obtains detailed information about the chemical composition of the surface. A time‐of‐flight mass spectrometer is especially suitable for the analysis of secondary ions because of its high transmission, high mass resolution, and ability to detect ions of different masses simultaneously. By using a finely focused primary ion beam it is also possible to analyze microareas and generate surface images with a lateral resolution of 0.1 μm or less. Static time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) allows monolayer imaging and local analysis of monolayers with high sensitivity, a wide mass range, high mass resolution, and high lateral resolution. Besides information on elements and isotopes, the technique yields direct information on the molecular level and can also be used to analyze surface species of high molecular mass that are thermally unstable and cannot be vaporized. The method can be applied to practically all types of materials and sample forms, including insulators in particular. In this article the basic principles of TOF‐SIMS are explained, and its analytical capabilities for both large area and imaging applications are illustrated by examples. These include silicon surfaces (both uniform and structured), thermally unstable organic molecules on surfaces, synthetic polymers, and synthetically prepared molecular surface films, particles, and fibers. Emitted neutral particles can also be analyzed by postionization with a laser, and the possibilities of this technique are discussed.
Conformational Requirements for Sweet‐Tasting Peptides and Peptidomimetics
Authors: Yamazaki Toshimasa , Benedetti Ettore , Kent Darin , Goodman Murray 
Published: December 2003

AbstractThe molecular basis o taste has been extensively studied over many years. The research carried out in our laboratories is focused on the elucidation of detailed structure–taste relationships of peptides and peptidomimetics using an “integrated” approach employing synthesis, analysis of NMR spectra, computer simulations, and X‐ray crystallography. Various peptidomimetic residues have been incorporated to introduce predictable structural constraints into taste ligands. These constraints eliminate some of the molecular flexibility and allow us to develop structure–activity relationships. We describe here the topochemical requirements of the sweet and bitter taste receptor(s) and develop detailed structure‐taste relationships with considerable predictive power for peptides and related molecules.
Hydrogenation of acrylonitrile–butadiene rubber latexes
Authors: Zhou Shuqin , Bai Huadong , Wang Jianguo 
Published: December 2003

AbstractThe hydrogenation of acrylonitrile–butadiene rubber (NBR) latex was carried out by a system consisting of hydrazine hydrate and hydrogen peroxide, with boric acid as catalyst. Highly saturated hydrogenated NBR (HNBR) latex was obtained through the optimization of the reaction conditions. The dried HNBR was found to be heavily gelled. The cause for the crosslink of dried hydrogenated NBR products was investigated. With the improvement of the hydrogenation system, that is, by adding gel inhibitor to the system, the crosslinking was controlled to a large extent, and dried HNBR with gel content of about 3% was prepared by the improved system. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2072–2078, 2004
Continuous tubular reactors for latex production: Conventional emulsion and miniemulsion polymerizations
Authors: Ouzineb Keltoum , Graillat Christian , McKenna Timothy 
Published: December 2003

AbstractAn investigation of the use of continuous tubular reactors for the production of polymeric latices is presented. In an initial step, it is demonstrated that tubular reactors can be used to produce latices by conventional emulsion polymerization at solids contents of up to 30% by volume. Under these conditions, the kinetics are identical to those found in batch reactors (as expected), and stable operating conditions are obtained, usually after one to two residence times. The use of a miniemulsion in a tubular reactor was also investigated, and it is demonstrated that in this case, solids contents of at least 60% can be obtained under stable operating conditions. It is therefore proposed that difficulties in operating tubular reactors with conventional emulsion polymerization arise essentially from the presence of monomer droplets in the early stages of the reaction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2195–2207, 2004
Hot compaction of woven poly(ethylene terephthalate) multifilaments
Authors: Hine P. J. , Ward I. M. 
Published: December 2003

AbstractHere we describe the development of a process, and the resulting mechanical properties, for hot‐compacted sheets of woven poly(ethylene terephthalate) (PET) multifilaments. Investigation of the various processing parameters showed that a key aspect was the time spent at the compaction temperature, termed the dwell time. Molecular weight measurements, using intrinsic viscosity, showed that hydrolytic degradation occurred rapidly at the temperatures required for successful compaction, leading to embrittlement of the resulting materials with increasing dwell time. A dwell time of 2 min was found to be optimum because this gave the required percentage of melted material to bind the structure together, while giving only a small decrease in molecular weight. A combination of techniques, including mechanical tests, differential scanning calorimetry, and scanning electron microscopy, was used to examine the mechanical properties and morphology of the optimum compacted sheets. These tests reinforced the view from previous studies on hot‐compacted polypropylene, of hot‐compacted sheets as self‐reinforced composites, whose behavior is a combination of the properties of the two components, that is, the original oriented multifilaments and the melted and recrystallized matrix. Other key findings from the research included a confirmation of the importance of obtaining high ductility in the melted and recrystallized phases, promoted by using a high molecular weight or by suppressing crystallinity during processing, and the proportionately high‐impact performance of hot‐compacted sheets, compared with that of other materials. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2223–2233, 2004
Swelling and drug‐release behavior of the poly(AA‐co‐N‐vinyl pyrrolidone)/chitosan interpenetrating polymer network hydrogels
Authors: Lee Wen‐Fu , Chiang Wen‐Hsuan 
Published: December 2003

AbstractA series of novel hydrogels were prepared from acrylic acid (AA), N‐vinyl pyrrolidone (NVP), and chitosan by photopolymerization. The swelling behavior, gel strength, and drug release behavior of the poly(AA/NVP) copolymeric hydrogels and corresponding interpenetrating polymer network hydrogels were investigated. Results showed that the swelling ratios for the present hydrogels decreased with an increase of NVP content in the gel, but the gel strength increased with an increase of NVP content in the gel. Results also showed that the drug‐release behavior for the gels is related to the ionicity of drug and the swelling ratio of the gel. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2135–2142, 2004