bluuebunny/crossref_metadata_2025_split huggingface dataset:
Transfer Ribonucleic Acids
Authors: Zachau H. G. 
Published: December 2003

AbstractTransfer ribonucleic acids (tRNAs)Abbreviations used according to IUPAC‐IUB convention: tRNA = transfer ribonucleic acid; tRNAyeast = mixture of tRNAs from yeast; tRNAPhe = phenylalanine specific tRNA; Phe‐tRNA = tRNA esterified (“charged”) with Phe; mRNA = messenger RNA; DNA = deoxyribonucleic acid; U = uridine; A = adenosine; C = cytidine; G = guanosine; pA = 5′‐adenylic acid; Ap or A‐ = 3′‐adenylic acid; m2′G = 2′‐O‐methyl guanosine; m7G = 7‐methyl guanosine; mG = N(2)‐dimethyl guanosine; other methylated nucleosides are abbreviated analogously; abbreviations of other odd nucleosides are given with Fig. 2; p or – signifies phosphate; RNase = ribonuclease; DEAE = diethylaminoethyl; fMet = N‐formayl methionine.
occur in all living organisms. In biological protein synthesis they accept activated amino acids which are then transferred to growing peptide chains. With molecular weights lying between 25000 and 30000, tRNAs are easily within the reach of today's physical, chemical, and biochemical methods. The primary structures of several tRNAs as well as some relationships between structure and function have been elucidated. Three‐dimensional structure, specificity, and mechanism of action are the subjects of present research efforts.
Organic Dyes in Laser Technology
Authors: Schäfer F. P. 
Published: December 2003

AbstractLaser technology has been developed to a very high level since 1960. Significant advances have been made possible only by the use of organic dyes as optical shutters for the production of giant pulses. Ultrashort pulses in the picosecond range were first produced in 1966; their measurement was greatly facilitated by the use of organic dyes. Probably the most important recent advance in the laser field is the dye laser, which was first described in 1966, and in which the active medium is a solution of an organic dye.
Chlorosulfenylated Carbonic Acid Derivatives
Authors: Zumach G. , Kühle E. 
Published: December 2003

AbstractThe present article deals first with the synthesis of chlorocarbonylsulfenyl chloride and its imino analogs. The chemical behavior of these bifunctional compounds is illustrated for selected examples. Special attention is given to the syntheses of five‐membered heterocycles containing sulfur, oxygen, and/or nitrogen.
Structure of the Solvation Shell of Dissolved Particles
Authors: Hertz H. G. 
Published: December 2003

AbstractThe first part of the article deals with the structure of the solvation shell of simple ions, particularly those with the inert gas structure, in aqueous and non‐aqueous solution. It is nowadays possible in favorable cases, to determine the solvation number or the coordination number in the first sphere. The solvation shells appear to be “more liquid” than was formerly believed. The hydration of neutral inert particles in aqueous solution is discussed in the second part. Caution is advised in the postulation of particular structural arrangements in the solution.
Magnetic Resonance of Paramagnetic Complexes
Authors: Keller Heimo J. , Schwarzhans Karl E. 
Published: December 2003

AbstractThe line width of the ESR and NMR signals of paramagnetic transition metal complexes is determined mainly by the electron spin‐lattice relaxation time τe. Values of τe greater than 10−9 lead to ESR spectra that are readily resolved, while values smaller than 10−11 give NMR spectra having small line widths. Since fast relaxation processes are effective in nearly all transition metal complexes with several unpaired electrons and in all complexes having an orbitally degenerate ground state, the NMR method has a wider scope. The sign and magnitude of the electron‐nucleus coupling can be determined with great sensitivity from the NMR spectra, whereas only the magnitude of this interaction can be determined from the ESR spectra. Free spin densities can be found very accurately from the NMR shifts, and the method can therefore be advantageously applied to kinetic measurements, e.g. on short‐lived contact complexes.
Structure and Immunogenicity of Synthetic Antigens
Authors: Rüde Erwin 
Published: December 2003

AbstractSynthetic antigens are often valuable tools in immunological research. They are usually linear or branched polypeptides, prepared by polymerization of N‐carboxy‐α‐amino acid anhydrides or by polymerization of defined oligopeptides. Owing to their great variability, these simple antigens allow a systematic study of the chemical and physical parameters involved in immunogenicity, i.e. the ability of a substance to initiate an immune response in higher animals. The study of very simple synthetic antigens made possible the first detailed analysis of genetic factors controlling the immunogenicity of a substance. The results have led to a better understanding of many immunological phenomena, and have helped to explain the complex processes initiated by antigens that lead to a specific immune response.
Development of and Problems in the Chemistry of Mercury‐Nitrogen Compounds
Authors: Breitinger Dietrich , Brodersen Klaus 
Published: December 2003

AbstractThere have been few recent additions to the group of mercury‐nitrogen compounds. Current developments in this field are mainly concerned with the elucidation of the structures of these compounds. The structures of coordination‐polymeric Hg–N compounds may be considered from the point of view of their relationship with silicates. The bond parameters of the Hg–N grouping are compared with the parameters of bonds between mercury and elements adjacent to nitrogen in the periodic system.
TOSAR – A Topological Method for the Representation of Synthetic and Analytical Relations of Concepts
Authors: Fugmann Robert , Nickelsen Herbert , Nickelsen Ingeborg , Winter Jakob H. 
Published: December 2003

AbstractIn mechanized systems used for searching in literature stores there is a steadily growing necessity not only to be able to formulate concepts as a search condition but also the characteristic connections under which these concepts appear in the inquiry. In this way the precision of the mechanized literature search is considerably increased. TOSAR has been developed in order to improve computerized literature searching in this respect.
CCBF – A System for the Computer Processing of Chemical and Biological Facts
Authors: Ohnacker Gerhard , Kalbfleisch Werner 
Published: December 2003

AbstractA computer‐based system for the documentation of company research results in drug research is described. The system stores the numerous results of standardized biological tests and processes chemical formulas by topological techniques. The stored material is processed to provide computer‐printed card indexes and lists on chemical and/or biological topics. The programs enable searches to be made for any desired substructure and for pharmacophoric groups, thus providing highly effective assistance in the search for structure‐activity relationships.
Regulation of Glutamine Synthetase by Enzyme Catalyzed Structural Modification
Authors: Shapiro Bennett M. 
Published: December 2003

AbstractA highly sophisticated mechanism for the regulation of glutamine metabolism in Escherichia coli involves the modification of the enzyme glutamine synthetase by the covalent attachment and release of adenylyl residues. Separate enzymes catalyze the adenylylation and deadenylylation reactions, and these enzymes are reciprocally controlled by the state of nitrogen nutrition of the cell. The resultant structural alterations in glutamine synthetase enable it to change its catalytic potential greatly in response to varying cellular demands, in order to maintain nitrogen homeostasis.
Recent Results in Carbanion Chemistry
Authors: Schöllkopf Ulrich 
Published: December 2003

AbstractThe Wittig rearrangement of ethers metalated in the alpha position with alkali metals is a 1,2‐shift to an atom with a lone pair of electrons. The alkyl shift proceeds with racemization (and partial retention) at the migrating carbon atom. The experimental observations suggest a cleavage‐recombination mechanism involving a radical pair, as must also be assumed for the ylide rearrangements. In the allyl migration, the double bond is displaced within the migrating residue (allyl inversion). This is readily explained by a one‐step S′ mechanism, the transition state of which is allowed on symmetry grounds. In the case of benzyl 2‐butynyl ether metalated on the butynyl residue, the ortho isomerization (corresponding to the Sommelet rearrangement) predominates. – α‐Metalated alkyl isocyanides can react with carbonyl compounds to form olefins and metal cyanate (carbonyl olefination). Isocyanoacetic esters occupy a special place; they react with aldehydes and ketones to form β‐substituted α‐formylaminoacrylates (formylaminoalkoxycarbonylmethylenation), which can enter into a wide range of reactions. β‐Hydroxyalkyl isocyanides or 2‐oxazolines can be isolated as intermediates in the carbonyl olefination. α‐Metalated isocyanides react with acylating agents to form β‐ketoalkyl isocyanides, which cyclize to give oxazoles.
Chemical Lasers
Authors: Kompa K. L. 
Published: December 2003

AbstractChemical lasers are based on the new principle of obtaining energy in the form of coherent radiation from chemical reactions. Since the discovery of the first laser of this type in 1965, laser emission has been investigated in a considerable number of fast gasphase reactions. In physical chemistry such lasers provide a diagnostic tool for the detailed investigation of the energy distribution in elementary reactions. The barrier to technological use has probably also been overcome by the recent continuous operation of a chemical hydrogen fluoride laser. An attempt is made in the present article to trace the development of this still youthful field and to demonstrate the possibilities and limitations of the generation of energy with chemical lasers.
The Ionization (Heterolysis) of Covalent Compounds as a Coordination‐Chemical Phenomenon
Authors: Gutmann Viktor 
Published: December 2003

AbstractThe ionization of a covalent compound into a cation and an anion is a coordination‐chemical phenomenon. The ions are formed with coordination either by nucleophilic attack of an electron pair donor or by electrophilic attack of an electron pair acceptor. The dielectrically controlled component of the ionization affects only the degree to which it occurs. It is shown that the ionization generally increases with increasing donor strength of the donor or with increasing acceptor strength of the acceptor. For a given M, the ease of ionization of the MX bond increases in the order X = fluoride < chloride < bromide < iodide. These general aspects are discussed with the aid of examples.
Modern Industrial Microbiological Fermentations and Their Effects on Technical Developments
Authors: Rehm H.‐J. 
Published: December 2003

AbstractOwing to the development of many new processes and techniques, the production of primary and secondary metabolites with the aid of microorganisms has not only expanded greatly, but has also been simplified in many details, and has therefore become more widely feasible on the industrial scale. The present article surveys the principal fermentation processes and the main types of equipment, which shows a trend toward continuous processes and increasing automation. Some recent developments observed e.g. in the cultivation of animal cells or the production of electrical energy, and particularly in the production of proteins by microorganisms, are also described.
NMR Spectroscopy of Paramagnetic Complexes
Authors: Schwarzhans Karl E. 
Published: December 2003

AbstractServiceable NMR spectra can, with a few exceptions[1,6], be recorded for paramagnetic complexes in solution. These spectra provide information about the structure of the complexes and the distribution of the unpaired electrons, and hence also about reactive centers in the molecule. The elucidation of intermolecular and intramolecular exchange phenomena, e.g. the determination of ligand exchange rate constants, the determination of rotation barriers, and the detection of contact complexes in solution, or even of occupation equilibria of the electrons, is possible in this way. It can be seen, therefore, that NMR studies on paramagnetic complexes can be a rich source of information.
Nuclear Energy Reserves and Long‐Term Energy Requirements
Authors: Zimen K. E. 
Published: December 2003

AbstractIn this article the world's potential energy reserves are summarized on the basis of available estimates, and the future energy consumption is estimated with the aid of a working hypothesis relating to the growth of the world's population (P) and the energy consumption per capita (γ). According to this estimate, the energy requirements in 30 years will be about seven times and in 80 years about fifty times as high as they are today, and all the concentrated energy sources will be exhausted in about 120 to 160 years. – The energy requirements in the steady state (Pγ = const.) could be of the order of 1013 MWh per year and could be met out of magmatic rocks (U,Th) and sea water (U,D). High priority should be given to research and development work on the construction, reliability, and safety of breeder and fusion reactors, the chemical treatment and final disposal of radioactive waste, the extraction of uranium, thorium, and deuterium from the “primitive” raw materials, the relevant problems relating to protection of the environment, and the worldwide control of the excess birthrate.
Some Aspects of Chemical Mutagenesis in Man and in Drosophila
Authors: Obe Günter , Sperling Karl , Belitz Hans J. 
Published: December 2003

AbstractSince mutations produced by chemicals are, like spontaneous mutations, generally transmitted by recessive factors, i. e. as they are only expressed after several generations, depending on the circumstances, it is extremely difficult to trace the true cause of such mutations, which are almost always unfavorable. However, it is possible to study the mutagenic action of chemicals in test systems, e.g. in Drosophila in vivo or in human chromosomes in vitro. Alkylating agents which damage DNA indirectly have a particularly severe mutagenic action. The present progress report contains, among other things, a comprehensive list of substances that produce aberrations in human chromosomes.
Bis(trimethylsilyl)diimine: Preparation, Structure, and Reactivity
Authors: Wiberg Nils 
Published: December 2003

AbstractThe highly reactive compound bis(trimethylsilyl)diimine (BSD), which was first prepared by oxidation of lithium tris(trimethylsilyl)hydrazide, is light blue, sensitive to thermolysis and hydrolysis, and ignites spontaneously in air. On the basis of electron transfer, acid‐base, or free‐radical reactions, it acts in particular as a (preparatively useful) redox system and as an agent for the introduction of azo groups. Redox reactions lead by oxidation or reduction of the other reactant through two oxidation stages to hydrazine derivatives or molecular nitrogen, and in the case of electrochemical reduction, to BSD radical‐anions. Azo‐group transfers, on the other hand, yield new inorganic azo compounds with no change in the oxidation state of the diimine group.
Biological Membranes
Authors: Gross Werner 
Published: December 2003

AbstractMembranes are essential components of living cells. They act as barriers and pumps, as enzyme supports, and also as a means of protection. Their chemical makeup from proteins, lipids, carbohydrates, ribonucleic acids, and water is well known, but there is as yet no generally accepted model for their structure. It is also by no means certain that all membranes have the same structural plan.
Synthesis and Reactions of Ketenimines
Authors: Krow Grant R. 
Published: December 2003

AbstractKetenimines R1R2CCNR3 were first reported in 1919, but the development of the chemistry of these compounds is of relatively recent origin. These heterocumulenes are becoming of increasing interest as a dehydrating agent for peptide syntheses, as a coreagent in dimethyl sulfoxide oxidations, and as a substrate in the synthesis of heterocycles by condensation with polar multiple bonds and dipolar systems. The cycloadditions are of special interest from the viewpoint of orbital symmetry considerations. Some unusual organometallic complexes of ketenimines have also been synthesized. Recently some triaryl ketenimines have been found to have insecticidal and miticidal activity. Polymeric ketenimines have been implicated as molecular precursors of life.
Approaches to Total Synthesis of Heterocyclic Steroidal Systems
Authors: Huisman H. O. 
Published: December 2003

AbstractIn heterocyclic steroids the carbon atoms of the steroid skeleton (positions I to 17) are replaced in part by hetero atoms. The term generally denotes compounds containing one or two nitrogen, oxygen, and/or sulfur atoms. The part of the molecule containing the hetero atom is either introduced as a preformed unit or is constructed by stepwise synthesis. The examples given in the present article contain the hetero atoms in ring A, ring B, between rings A and B, and/or between rings C and D.
Chemical Bonding in Solids
Authors: Hartmann Hermann 
Published: December 2003

AbstractThe most important of the current quantum‐mechanical models of solids can be divided into one‐electron (band) models and incomplete polyelectron theories. There are no complete polyelectron theories at present. However, the interatomic exchange forces become less important in lattices with high coordination numbers; a theory of metallic phases can therefore be developed, in principle, on the basis that the antisymmetrization is only local in atoms or groups of atoms.
The Present Position of Hemoglobin Research
Authors: Buse Gerhard 
Published: December 2003

AbstractUp to the present time hemoglobin has enjoyed pride of place among the objects of study in molecular biology. Investigations on this molecule have culminated in the complete description of its atomic structure in various functional states and point the way for future research on other biopolymers. However, more than just a knowledge of the anatomy of this protein is required before we can make quantitative predictions about its function.
Non‐rigid Molecular Skeletons—Berry Pseudorotation and Turnstile Rotation
Authors: Gillespie Paul , Hoffman Peter , Klusacek Hans , Marquarding Dieter , Pfohl S. , Ramirez Fausto , Tsolis E. A. , Ugi Ivar 
Published: December 2003

AbstractMolecules rearrange by regular and irregular processes. The former occur with preservation of bonds and coordination numbers while the latter involve rupture and reformation of bonds via states possessing different coordination numbers. For molecules with skeletons that carry n ligands both processes can be represented by permutation operators belonging to the symmetrical group Sn. These operators are obtained from the descriptors of the particular molecules in a simple manner; the conceivable reaction mechanisms correspond to the skeletal symmetry subclasses of Sn. Regular rearrangements—e.g. of phosphorane derivatives with a monocentric skeleton, including their analogs, or of ethane derivatives with a dicentric skeleton—all submit themselves to classification based upon skeletal non‐rigidity. Flexible molecular skeletons are representable by a collection of rigid skeletal models which can be interconverted by deformations of bond angles and/or some form of internal rotation. To accomplish mechanistic understanding of these regular processes, it is useful to represent the symmetries of the rigid skeletal models and the existent ligand equivalencies by subgroups of Sn and consider the dependence of the skeletal flexibility upon the given ligand set. This procedure is based on the ideas of Polya and Longuet‐Higgins. The former introduced separation of permutational molecular symmetry into skeletal symmetry and ligand set symmetry, which, combined with the total molecular symmetry concept developed by Longuet‐Higgins for the treatment of non‐rigid symmetric molecules, yield the foundation for the following classification of regular pentacoordinate rearrangements and related processes. It will be shown that with the given quantum chemical properties of the pentacoordinate phosphorus bond system, including participation of the P3d orbital set, not only the BPR‘2’ mechanism but also single and multiple TR[2] processes must be considered in the interpretation of the observed phenomena. Although some experimental observations can be interpreted by either of the two mechanisms, and certain other regular rearrangements can be explained only by invoking the TR mechanism, no regular processes are known, as yet, that can be explained solely by the BPR mechanism.
Active Transport of Na+ and K+ through Animal Cell Membranes
Authors: Schoner Wilhelm 
Published: December 2003

AbstractThe transport of Na+ out of the cell and K+ into the cell against a concentration gradient is catalyzed by a (Na+ + K+)‐activated ATPase. The way in which the cations pass through the cell membrane has not yet been elucidated. Studies on the ATP hydrolysis revealed a Na+‐dependent phosphorylation of the enzyme protein; the conformation of the enzyme also appears to change. The energy required for transport of the cations against their concentration gradients is probably provided by K+‐dependent hydrolysis of the enzyme‐bound phosphate. The enzyme can synthesize ATP from inorganic phosphate and ADP on reversal of the cation concentration gradient. By keeping the enzyme in a particular conformation, the cardiac glycoside ouabain specifically inhibits the Na+ pump.
Ternary Fission
Authors: Brandt Reinhard 
Published: December 2003

AbstractIn this review paper, ternary fission is defined as the break‐up of heavy nuclei into three fragments of approximately equal mass. This relatively new type of nuclear decay has been investigated intensively during the last decade. Muga has studied ternary fission induced by thermal neutrons in uranium. The results of his experiments are discussed in detail. Ternary fission can definitely be observed and studied in the interaction of heavy target elements with 39 MeV alpha particles, 20 GeV protons, and 300—400MeV argon ions. The ratio of binary‐to‐binary fission for uranium is approximately 10−4: 100 in the interaction of 39 MeV alpha particles, approximately 0.1:100 for 20 GeV protons, and 3:100 for 400MeV argon ions. A theoretical model for the interpretation of ternary fission, the “cascade model”, is described. The paper concludes with a discussion of the possible connection of ternary fission with the production and decay of still undiscovered super‐heavy elements (Z = 114).
The Electronic Properties of Diradicals
Authors: Salem Lionel , Rowland Colin 
Published: December 2003

AbstractA review of the various possible definitions of diradicals leads the authors to describe these systems as having two odd electrons in degenerate or nearly‐degenerate molecular orbitals. A study of the wave‐function for the two odd electrons shows that its form depends entirely on whether the diradical is homo‐ or heterosymmetric. Energy schemes are given in these two cases, as well as in the intermediate “non‐symmetric” case. The extent of zwitterionic character in diradical states is also investigated. This is followed by a discussion of intersystem crossing between singlet and triplet diradical states via spin‐orbit coupling and other mechanisms. The electronic matrix elements for spin‐orbit coupling are calculated and evaluated numerically for various model cases. It is then possible to establish general rules for favorable (electronic) intersystem crossing. In 1,3 or 1,4 diradicals its efficiency is estimated to be comparable with that in aromatics. The role of the electron‐nuclear hyperfine interaction in mixing singlet and triplet states, particularly in CIDNP, is explained.Finally the question of whether diradicals actually occur as secondary minima on potential energy surfaces is examined. Recent quantum‐mechanical calculations, in contradiction to some thermochemical and kinetic evidence, lead to flat singlet surfaces without significant minima.