bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metallomesogens: Metal Complexes in Organized Fluid Phases
Authors: Giroud‐Godquin Anne‐Marie , Maitlis Peter M. 
Published: December 2003

AbstractMetallomesogens, metal complexes of organic ligands which exhibit liquid crystalline (mesomorphic) character, combine the variety and range of metal‐based coordination chemistry with the extraordinary physical properties exhibited by liquid crystals. Thermotropic metallomesogens have been made incorporating many metals, including representatives of s‐, p‐, d‐and even f‐block elements. Both rodlike (calamitic) and disklike (discotic) thermotropic metallomesogens are known, and examples of all the main mesophase types are found. Many different varieties of ligand can be used: monodentate (4‐substituted pyridines), bidentate (β‐diketonates, dithiolenes, carboxylates, cyclometalated aromatic amines), or polydentate (phthalocyanines, porphyrins). As with organic mesogens, molecular shape and intermolecular forces play an important role, i.e. the ligands are important in determining mesophase character. The chief requirement for a metallomesogen is a rigid core, usually unsaturated and either rod‐ or disklike in shape, bearing several long hydrocarbon tails. The metal atom is usually at or near the center of gravity of the molecule. In some cases the ligands are themselves mesogenic, but this is not a requirement. The presence of one or more metals opens many exciting possibilities: new shapes, not easily generated by organic compounds, and hence new properties are then accessible. The incorporation of d‐block metals brings with it features such as color and paramagnetism. Profound effects arise from the large and polarizable concentration of electron density that every metal atom possesses, since the molecular polarizability is a key factor in determining whether a molecule will form liquid crystals. Enhanced physical properties (e.g. high birefringencies), as well as new and unexpected ones, will result. A major requirement for metallomesogens to find applications in new device technology is that the metal–ligand bonds are strong and inert and the complexes stable; this can be accomplished with, for example, chelating ligands and the 5d metals.
Chemical Characterization of Aerosols: On‐Line and In Situ
Authors: Nießner Reinhard 
Published: December 2003

AbstractWhereas aerosols were formerly considered to be important in meteorological processes but in little else, today they play a significant role not only in environmental protection but also in “new technologies” such as the formation of high performance ceramics, sinter metals, optical waveguides, and superconductors. Analytical procedures which are capable of a continuous qualitative and quantitative description of processes in which aerosols are formed are, however, still almost unknown. The present review will discuss initial developments (which will often appear exotic in nature) such as the use of the condensation properties of ultrafine particles or time‐resolved laser‐induced aerosol fluorescence spectroscopy. The progress of analytical chemistry in the last few years makes it clear that advances in miniaturization, data processing, and optronics will probably be capable of solving many problems which are presently regarded as unsolvable.
Chemical Sensors and Biosensors—Principles and Applications
Authors: Cammann Karl , Lemke Udo , Rohen Anja , Sander Jürgen , Wilken Hildegard , Winter Babette 
Published: December 2003

AbstractChemical and environmental engineering and biotechnology are among the fields now being transformed by continually increasing levels of automation. Whereas the objective in other sectors of industry is simply to increase efficiency, here considerations of system theory or safety demand a high level of automation. Either the processes are too complex and require multifunctional control with feedback, or an analysis of the safety requirements shows the necessity for a certain degree of redundancy in the safety measures, and for elimination of human error as a risk factor. With regard to quality control, cost‐benefit analyses lead to striking conclusions which again indicate the need for highly automated, and above all reliable, systems to eliminate rejects. The crux of any automated system is the measurement and control technology; of central importance is the rapid, reliable, and in some cases continuous, measurement and interpretation of key processes or control variables. For this purpose a wide variety of recording instruments and sensors are used to give as accurate a picture as possible of the state of the system. It is obvious from this that the performance of the control system is critically dependent on the sensors. Errors in the measured quantities can become amplified in the control variables or, in dynamic systems, can lead to undesirable operating conditions. Moreover, as a consequence of great advances in microelectronics, “intelligent sensors” which can calibrate and control themselves will be one of the key technologies of the nineties. Unless fast and immediate information on the true current status of a system is available, microprocessors as control devices react blindly and unpredictably to errors in input information. New discoveries in the fields of electronic, electrochemical, and optical transducers are now being applied in heterogeneous catalysis and surface physics, and in biochemistry (enzymology and immunology); in these fields new chemical sensor principles are being tested, which could revolutionize instrumental methods of molecular analysis in particular, owing to their very favorable cost‐performance relationship. This article aims to give an up‐to‐date overview of the current state of the art in these developments, with emphasis on their importance for analysis and their significance in relation to the chemist's interest in mechanisms for identifying substances.
Chemically Modified Oligonucleotides as Probes and Inhibitors
Authors: Englisch Uwe , Gauss Dieter H. 
Published: December 2003

AbstractOligonucleotides bind specifically to single‐stranded nucleic acids to form a double helix if there is a complementary antiparallel nucleotide sequence. In addition, certain oligonucleotides bind specifically to a variety of proteins. Therefore, biological processes involving these nucleic acids or proteins can be modulated (normally inhibited) by addition of the respective oligonucleotides. The effects of these oligonucleotides and the fields of potential application can be broadened by the introduction of chemically modified nucleotides. For instance, replacing oligonucleotide phosphate groups by methylphosphonates results in the loss of one negative charge per nucleotide and the oligonucleotide becomes more lipophilic. An oligonucleotide carrying a reactive group can modify its binding partner. An oligonucleotide covalently linked to a dye can be localized in a biological specimen. Oligonucleotides attached to an enzyme can be detected in very small amounts since the enzyme can catalyze the formation of large amounts of the substance assayed (e.g., a fluorescent product). An important biochemical application is the detection and localization of a messenger RNA or its gene. Medical applications include the detection of bacterial or viral sequences. There is also great interest in inhibiting the translation of messenger RNA and the transcription and replication of DNA. So‐called antisense oligonucleotides are currently being used for the inhibition of protein biosynthesis and of reverse transcription of retroviruses.
The Crystal Chemistry of Copper Oxometalates
Authors: Müller‐Buschbaum Hanskarl 
Published: December 2003

AbstractUp to three or four years ago oxocuprates were a class of chemical compounds no more remarkable than countless others. Then, as a result of the euphoria surrounding the high‐temperature oxide superconductors and the discovery that copper plays an essential role in these materials, there was a world‐wide awakening of interest in copper‐containing oxide compounds. The number of scientists engaged in the synthesis and structure determination of copper oxometalates has grown from just a few to very many. As superconductivity is inseparably connected with the structure of the solid state, there has been a spontaneous awakening of interest in the role of copper in the crystal chemistry of solid materials, leading to new research projects on this aspect of oxocuprates. Most of the work published so far on the interesting chemistry of this group of compounds has related to individual compounds. New aids in describing complex solid state structures are now available through the development of methods that give three‐dimensional displays of crystal structures using computer graphics terminals, as this review illustrates for the example of the oxocuprates.
Complex Transition Metal Hydrides
Authors: Bronger Welf 
Published: December 2003

AbstractCompounds having structural units formed by transition metals and hydrogen have only become known fairly recently. Only the synthesis and structural characterization of ternary metal hydrides AxMyHz in which A is an alkali or alkaline‐earth metal and M is a transition metal, led to the discovery of anionic complex groups of the form [MHz]. Although there are obvious structural similarities to the corresponding halides and oxides, the dynamic properties of the hydride ligand are somewhat unusual and are responsible for numerous phase transformations. Transitions throughout the range from saltlike to metallic behavior make us expect interesting physical properties and applications.
Chirality and Crop Protection
Authors: Tombo Gerardo M. Ramos , Belluš Daniel 
Published: December 2003

AbstractThe modern agrochemical industry is searching, more intensively than ever, for new substances to combat pests (weeds, deleterious insects, plant pathogens, etc.). In the complex and costly selection and optimization process, state‐of‐the‐art scientific methods are always needed. The aims of the interdisciplinary optimization are mainly the reduction of the rate of application of the new substance, an increase in the selectivity against the target organism, and the optimal ecological profile. If a promising crop protection compound is a racemate or a diastereoisomeric mixture, the chemist has a unique opportunity to contribute to this optimization process through the synthesis of enantiomerically pure isomers for testing purposes. If the single isomer proves to be biologically superior to the racemate, the development of an economical and ecologically sound process for the production of the single isomer presents an even greater challenge. The average price of a crop protection compound is much lower than that for a pharmaceutical product, and this fact imposes a severe limitation upon the flexibility of the chemist who is concerned with the synthesis and production of a stereochemically pure agrochemical. This forces the crop protection chemist to make full use of both his scientific and creative capabilities. Fortunately, parallel to the development of the above optimization aims of a modern and ecologically sound crop protection research, there has been a continuous and worldwide advance in the area of asymmetric synthesis. Due to the interplay of these two parallel efforts there has been a great accumulation of chemical, biological, and agronomical knowledge in recent years, which should have implications beyond merely the synthesis of enantiomerically pure agrochemicals.
The Elucidation of Peroxyl Radical Reactions in Aqueous Solution with the Help of Radiation‐Chemical Methods
Authors: von Sonntag Clemens , Schuchmann Heinz‐Peter 
Published: December 2003

AbstractWhenever free radicals are formed, independent of whether this occurs thermally, is induced by UV or ionizing irradiation, or takes place in redox reactions, they are converted rapidly into the corresponding peroxyl radicals in the presence of oxygen. Peroxyl radical reactions in aqueous environments are observed not only in aquatic systems (e.g., rivers, lakes and oceans) but also in the living cell and to a considerable degree even in the atmosphere (in water droplets). The peroxyl radical chemistry occurring in this medium is often very different from that observed in the gas phase or in organic solvents. In spite of the great importance of these reactions in medicine (for example in anti‐cancer irradiation therapy and ischaemia) there have been comparatively few studies of peroxyl reactions in aqueous media. Radiation‐chemical techniques such as pulse radiolysis offer the best means for carrying out such studies, so that it is not surprising that the majority of the information available in this area has been obtained with the help of radiation‐chemical methods. The radiation chemistry of water can be con trolled in such a manner that the main products are ˙OH radicals (90 % yield), which react with substrate molecules to give substrate radicals and in the presence of oxygen to give substrate peroxyl radicals. The experimental conditions can also be varied in such a way that HO/O radicals can be formed in 100 % yield and caused to react with substrates. We therefore have a simple access to these intermediates, which are extremely important in biological systems. A detailed product analysis, supported by kinetic studies carried out with the help of pulse radiolysis, has been used to clarify the chemistry of a series of peroxyl radicals, so that sufficient material is now available to justify a review of the variety of the peroxyl radical reactions studied by means of radiation‐chemical methods. A more general survey of the physical properties of the peroxyl radicals and their unimolecular and bimolecular reactions will be followed by a discussion of selected examples of various classes of substance. Because of the great biological importance of radical‐induced DNA damage this area will also be treated briefly.
Single‐Crystal X‐Ray Diffraction Studies of Oligonucleotides and Oligonucleotide–Drug Complexes
Authors: Kennard Olga , Hunter William N. 
Published: December 2003

AbstractNucleic acids constitute the library of genetic information for all living organisms. They also play a regulatory role in many biological events concerned with the utilization of genetic information. The double‐helical model of DNA, proposed by Watson and Crick in 1953, suggested the structural basis for its biological role, but this insight into nucleic acid structures seems to have generated as many questions as it has provided answers. Experimental studies, in particular fiber diffraction work, yielded a wealth of information on the conformational flexibility of nucleic acids and on the importance of interactions with water and cations. Major advances in synthetic organic chemistry, with implications for molecular biology, propelled nucleic acid research forward in the late 1970s. The availability of milligram quantities of synthetic oligonucleotides of defined sequence and high purity paved the way for detailed and accurate structural analysis using single‐crystal X‐ray diffraction methods and, in more recent times, NMR spectroscopy. This article is a detailed survey of the structural results generated by crystallographic techniques as applied to DNA, RNA, and nucleic acid–drug complexes over the period 1979–1990. The appendix lists important definitions used in the characterization of oligonucleotide structures.
Medicinal Chemistry in the Golden Age of Biology: Lessons from Steroid and Peptide Research
Authors: Hirschmann Ralph 
Published: December 2003

AbstractCollaboration between biologists and chemists has generated medicines of enormous benefit to society. The dynamics of this synergistic interaction combines two strengths: the rigor of chemistry and the ability of biologists to construct unifying concepts from diverse fields of investigation. The resulting contributions to the practice of medicine have reduced morbidity and mortality while increasing the quality of life. Building on the work of chemists such asRobinson,Prelog,Woodward, andPauling, medicinal chemists have advanced rational drug discovery, especially of enzyme inhibitors, to a degree unthinkable even twenty years ago. Some of the major contributions of medicinal chemistry to human and animal health stem directly from the creative application of structural and electronic reasoning, from advances in instrumentation, and from spectacular progress in biology. However, medicinal chemists have only begun to scratch the surface of the opportunities made possible by the impact of the “double‐helix revolution” of the 1950s.
Mechanisms of Molecular Recognition : Investigations of Organic Host–Guest Complexes
Authors: Schneider Hans‐Jörg 
Published: December 2003

AbstractNoncovalent interactions constitute the basis for information transfer between molecules in living systems as well as in synthetic supramolecular structures. Despite ever more accurate descriptions of biological systems and a dramatically increasing diversity of synthetic host guest complexes, a systematic and general understanding of the underlying intermolecular forces is still in its infancy. Organic chemistry makes possible the targeted construction of infinitely variable host compounds with multiple centers interacting in complementary ways. This in turn opens the door to systematic experimental analyses based on binding increments, which can be shown to have a high degree of additivity. Even in the case of relatively simple ion pairs, several mechanisms are at work simultaneously, and either entropy or enthalpy contributions may prove to be decisive. Nevertheless, it is possible to identify individual interactions and then use them to practical advantage in ways analogous to the free‐energy relationships of physical organic chemistry. Contributions due to solvophobic effects, to interactions with permanent and induced dipoles, and to hydrogen bonds lend themselves to a high degree of empirical quantification on the basis of classical physical‐chemical principles. Water as a solvent plays a special role due to its low polarizability, apart from hydrophobic effects. Quantification of pairwise interactions and ultimate control over them through manipulation of the microenvironment should prove useful not only in the design of new synthetic host–guest complexes but also, for example, in the rational development of biologically active substances, molecular switches (e.g., for sensor technology), and catalysts that function analogously to enzymes. The results should also assist in the planning and analysis of biochemical experiments including techniques of genetic engineering.
Volume Effect or Paddle‐Wheel Mechanism—Fast Alkali‐Metal Ionic Conduction in Solids with Rotationally Disordered Complex Anions
Authors: Jansen Martin 
Published: December 2003

AbstractTransport phenomena in the solid state are of equal importance in basic research and practice. In the past two decades particular interest has been directed towards so‐called “fast” or “super” ionic conductors because of their attractive potential applications. We have synthesized highly conductive alkali‐metal ionic conductors based on ionic crystals in which, on the one hand, high concentrations of the charge carriers can be realized by doping (point defects in the cation substructure) and, on the other, the activation energy of the interchange of sites is decreased by translationally fixed but rotationally mobile complex anions. Mixed crystals with Na3PO4 or Na3AlF6 structures have proven especially suitable in this connection. With the object of establishing a broader experimental foundation for clarifying the controversially discussed question of whether the higher free transport volume or the rotational motion of the anions is responsible for the high cation mobility in these rotary phases we have systematically varied the type of anions and concentration of defects and monitored the resulting changes in conductivity. Although the macroscopical characteristics investigated are not suitable for explaining mechanisms in detail at the atomic level, the results afford clear support for the assumption of a “paddle wheel mechanism”; but also effects of the enlarged transport volume are not to be disputed. Both these effects enhancing the cationic conductivity are concomitantly operative in amounts varying from system to system; they cannot be totally separated from each other. Seen in this light, the alternative, “volume effect” or “paddle wheel mechanism,” is not as sharply defined as was previously discussed.
Chemistry and Crystal Growth
Authors: Hulliger Jürg 
Published: December 2003

AbstractSingle‐crystal materials, along with other forms of condensed matter (ceramics, polymers, liquid crystals, etc.) are fundamental to modern technology. The basic research and production of new materials with “tailored” solid‐state physical properties therefore necessitate not only chemical synthesis but also the production of single crystals of a particular morphology (either bulk or thin layer crystals) and well‐defined crystal defects (doping). In this review, an attempt is made to broaden the traditional synthetic concept of chemistry to the process of single‐crystal synthesis. The methods of the resulting approach, which takes into account the specific properties of solid materials, are discussed and illustrated by experimental set‐ups for the solution of a range of problems in chemical crystallization. Also included is recent work on the growing of single crystals of high‐temperature superconductors, organic non‐linear optical compounds, and proteins.
Enantioselective Reactions in a Static Magnetic Field
Authors: Zadel Guido , Eisenbraun Catia , Wolff Gerd‐Joachim , Breitmaier Eberhard 
Published: December 2003

AbstractShortly after publication, E. Breitmaier, C. Eisenbraun, and G. J. Wolff declared their disassociation from this article owing to apparent manipulation of results by G. Zadel.Also see DOI: 10.1002/anie.199412011, 10.1002/anie.199414571, 10.1002/anie.199414581, 10.1002/anie.199414591.
Turning Points in Catalysis
Authors: Thomas John Meurig 
Published: December 2003

AbstractThis personal journey through the enlarging landscape of heterogeneous catalysis, pure and applied, starts with the discovery in 1823 of a dramatic example of the synthesis of water that was to serve as the basis of the first commercial exploitation of catalysis, Dobereiners lighter (tinder box). The quickening pace of successive industrial innovations and of academic insight during the intervening 170 years are summarized and the turning points, both major and minor, identified. Although new concepts and new ideas emerged in relative profusion, few exhibited the longevity predicted for them at birth by their proponents. Some concepts, like broken milestones on a vanished road, have ceased to retain their significance. Some have survived, even flourished. But in catalysis, as in most other branches of natural science, new tools and new techniques, rather than new concepts, tend to hold sway. And just as experimental advances in enzymology and immunology have led to the design of remarkable new biocatalysts so also, but not yet at the same level of delicate control, have the tools and techniques of solid‐state chemistry (including novel preparative and computational procedures), generated many powerful rationally designed inorganic catalysts.