ISSN:0570-0833    

DOI:10.1002/anie.198800013

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198800021

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198800031

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractFollowing the chemical synthesis of vitamin B12, a search was begun for a potentially biomimetic “dark” variant of the photochemical A/D‐secocorrin → corrin cycloisomerization, the central ring‐closure step in one of the two cobyric acid syntheses. Significantly, not just one but a whole family of such variants was discovered. According to what is currently known, one of these variants can indeed be regarded as a chemical model for the reaction path followed by Nature in the biosynthetic construction of the corrin ring. These chemical studies of vitamin B12biosynthesis had revealed that the A/D‐ring junction, regarded as the main obstacle to a chemical vitamin B12synthesis at the outset, is in fact a structural element that is formed readily and in a variety of ways from structurally appropriate precursors. More recent investigations have shown that the same holds for other specific structural elements of the vitamin B12molecule, including the characteristic arrangement of double bonds in the corrin chromophore, the special dimension of the macrocyclic ring of the corrin ligand, the specific attachment of the nucleotide loop to the propionic acid side chain of ring D, and the characteristic constitutional arrangement of the side chains around the ligand periphery (which vitamin B12shares with all uroporphinoid cofactors). All these outwardly complex structural elements are found to “self‐assemble” with surprising ease under structurally appropriate preconditions; the amount of “external instruction” required for their formation turns out to be surprisingly small in view of the complexity and specificity of these structural elements. We view these findings as steps on the way toward a chemical rationalization of the vitamin B12structure. The goal is to arrive experimentally at a perception of the biomolecule's intrinsic potential for structural self‐assembly. This potential, together with the specific type of reactivity related to the biological function, is considered to be responsible for the biomolecule having been chosen by natural selection. The chemical rationalization of the structure of biomolecules is an objective of organic natural product chemistry. The field of natural product synthesis provides appropriate conceptual and methodological tools to approach this o

ISSN:0570-0833    

DOI:10.1002/anie.198800051

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractSince last century, the supply of raw materials for the chemical industry has undergone a radical change. Whereas at the beginning of the nineteenth century the demand for basic chemicals was satisfied entirely by renewable raw materials, from about 1850 the chemical industry came to rely increasingly upon coal. In the 1940's, mineral oil started to become increasingly important, and during the past thirty years it has remained by far as the most important source of raw materials. Renewable raw materials are likely to become important again in the future, as the choice of raw materials is now of great significance not only for economic reasons by influencing competitiveness, but also because this choice largely determines the properties of the derivatives produced and their ecological effects. Following the two oil crises of the 1970's, there is also now a growing awareness of the limits of raw material resources, and against the continuing background of agricultural surpluses, chemists are again showing an increasing interest in renewable raw materials. Since the end of the seventies, the Bundesministerium für Forschung und Technologie (Ministry of Research and Technology of the Federal Republic of Germany) has supported research projects on renewable raw materials in universities and in industry. For the chemical industry the use of natural products as raw materials opens up a wider spectrum of synthetic methods and finished products, some of which are not accessible by petrochemical routes

ISSN:0570-0833    

DOI:10.1002/anie.198800411

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractSpectroscopic analysis techniques have become particularly important in the study of interfacial electrochemical processes since it has become possible to increase their sensitivity to the extent where changes occurring in the absorption of light by fractions of monolayers can be detected. At the same time the development of new techniques in surface physics, though most of these can only be used in ultra‐high vacuum, has also had a considerable influence on electrochemistry since it has enabled studies to be carried out on electrodes after transfer to UHV. The availability of well‐defined semiconductors in which photocurrents can be generated by the absorption of light has led to the development of photoelectrochemical methods that can be used for the study of electrode reactions at semiconductors or at metal surfaces which are covered with a semiconducting film. Finally, it has been observed during the study of semiconductor electrodes that redox reactions at an electrode can result in the generation of luminescence. This, in turn, can lead to important information on the mechanism of the charge transfer process. These recent developments are reviewed in the present arti

ISSN:0570-0833    

DOI:10.1002/anie.198800631

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractProteins may be rigid or flexible to various degrees as required for optimal function. Flexibility of large parts of a protein, which rearrange or move, is particularly interesting and will be discussed in this article. We differentiate between several categories, although the boundaries between them are diffuse: flexibility of peptide segments, order–disorder transitions of spatially contiguous regions, and domain motions. The domains may be flexibly linked to allow rather unrestricted motions or the motions may be constrained to certain modes. The various categories of large‐scale flexibility will be illustrated with the following examples: (1) Small protein proteinase inhibitors are rather rigid molecules which provide binding surfaces complementary to their cognate proteases but show also limited segmental flexibility and adaptation. (2) Large plasma proteinase inhibitors exhibit large conformational changes after interaction with proteases probably for regulatory purposes. (3) Pancreatic serine proteases employ a disorder–order transition of their activation domain as a means to regulate enzymic activity. (4) Immunoglobulins show rather unrestricted and also hinged domain motions in different parts of the molecule probably to allow binding to antigens in different arrangements. (5) Citrate synthase adopts open and closed forms by a hinged domain motion to bind substrates and release products and to perform the catalytic condensation reaction, respectively. (6) Riboflavin synthase, a bifunctional multienzyme complex, catalyzes two consecutive reactions by means of two subunits, α and β. The β‐subunits form a shell, in which the α‐subunits are enclosed. Diffusional motion of the catalytic intermediates is therefore restricted. In addition, rearrangement of theN‐terminal segment occurs during the assembly of the β‐subunit. In contrast, rigidity is dominant in the structures of the light‐harvesting complexes and the photosynthetic reaction centers involved in photosynthetic light reactions. These are large protein–pigment complexes in which the proteins serve as matrices to hold the pigments in the appropriate conformation and relative arrangement. Since motion would contribute to deactivation of the photoexcited states of the pigments and diminish the efficiency of light‐energy and electron transfer, the functional role of rigidity is easy to ration

ISSN:0570-0833    

DOI:10.1002/anie.198800791

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractSupramolecular chemistry is the chemistry of the intermolecular bond, covering the structures and functions of the entities formed by association of two or more chemical species. Molecular recognition in the supermolecules formed by receptor‐substrate binding rests on the principles of molecular complementarity, as found in spherical and tetrahedral recognition, linear recognition by coreceptors, metalloreceptors, amphiphilic receptors, and anion coordination. Supramolecular catalysis by receptors bearing reactive groups effects bond cleavage reactions as well as synthetic bond formation via cocatalysis. Lipophilic receptor molecules act as selective carriers for various substrates and make it possible to set up coupled transport processes linked to electron and proton gradients or to light. Whereas endoreceptors bind substrates in molecular cavities by convergent interactions, exoreceptors rely on interactions between the surfaces of the receptor and the substrate; thus new types of receptors, such as the metallonucleates, may be designed. In combination with polymolecular assemblies, receptors, carriers, and catalysts may lead to molecular and supramolecular devices, defined as structurally organized and functionally integrated chemical systems built on supramolecular architectures. Their recognition, transfer, and transformation features are analyzed specifically from the point of view of molecular devices that would operate via photons, electrons, or ions, thus defining fields of molecular photonics, electronics, and ionics. Introduction of photosensitive groups yields photoactive receptors for the design of light‐conversion and charge‐separation centers. Redox‐active polyolefinic chains represent molecular wires for electron transfer through membranes. Tubular mesophases formed by stacking of suitable macrocyclic receptors may lead to ion channels. Molecular self‐assembling occurs with acyclic ligands that form complexes of double‐helical structure. Such developments in molecular and supramolecular design and engineering open perspectives towards the realization of molecular photonic, electronic, and ionic devices that would perform highly selective recognition, reaction, and transfer operations for signal and information processing at the mole

ISSN:0570-0833    

DOI:10.1002/anie.198800891

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe Part and the Whole. The principle of self‐organization for the creation of functional units is not an invention of modern natural sciences. It was already a basic idea of the ancient philosophies in Asia and Europe: only the mutuality of the parts creates the whole and its ability to function. Translated into the language of chemistry this means: the self‐organization of molecules leads to supramolecular systems and is responsible for their functions. Thermotropic and lyotropic liquid crystals are such functional units, formed by self‐organization. As highly oriented systems, they exhibit new properties. The importance of lyotropic liquid crystals for the life sciences has been known for a long time. They are a prerequisite for the development of life and the ability of cells to function. In materials sciences this concept offunction through organizationled to the development of new liquid‐crystalline materials. From the point of view of macromolecular chemistry, this review tries to combine these two different fields and especially hopes to stimulate their interaction and joint treatment. To exemplify this, the molecular architecture of polymeric organized systems will be discussed. Polymeric liquid crystals combine the ability to undergo spontaneous self‐organization–typical of liquid‐crystalline phases–with the polymer‐specific property of stabilizing these ordered states. As new materials, polymeric liquid crystals have already been investigated intensively. As model systems for biomembranes as well as for the simulation of biomembrane processes, they so far have been little discussed. The intention of this review article is to show that polymer science is able to contribute to the simulation of cellular processes such as the stabilization of biomembranes, specific surface recognition, or even the “uncorking” of cells. Polymer science, having an old tradition as an inter‐disciplinary field, can no longer restrict itself to common plastics. Attempts to reach new horizons have already begun. The borderland between liquid crystals and cells will certainly play an important role. Basic requirements to work in this frontier area between organic chemistry, membrane biology, life science, and materials science will be the delight in scientific adventures as well as the courage to go ahead. The most important prerequisite will be the willingness to cooperate with disciplines which so far have not really accepted each other. From this point of view, this review does not aim at giving defined answers. It wants instead to encourage the scientific venture: too often we cling to painfully acquired knowle

ISSN:0570-0833    

DOI:10.1002/anie.198801131

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractMetals in low oxidation states are capable of forming metal–metal bonds. An attempt has been made to classify the numerous phases and structures occurring in such metal‐rich systems of valence electron poor metals in some sort of order from a rather general point of view. With this purpose in mind, clusters of these elements, their different types of interconnections, and their condensation via shared metal atoms, which finally leads to extended MM bonded structures, are described. Interstitial atoms play an important role in stabilizing electron deficient clusters, and can actually lead to the loss of all MM bonds. Surprising similarities emerge between apparently very different systems as the metal‐rich oxides of alkali metals, the oxides, halides, and chalcogenides of d transition metals, and the halides and carbide halides of the la

ISSN:0570-0833    

DOI:10.1002/anie.198801591

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY