bluuebunny/crossref_metadata_2025_split huggingface dataset:
Calorimetric Studies of Biopolymers and Aggregates of Phospholipids
Authors: Ackermann Theodor 
Published: December 2003

AbstractThe structural transformations that biopolymers undergo in aqueous solution are complex processes, whose mechanisms can only be explained by the coupling of various partial processes. A spectroscopic analysis of the states of the system often requires considerable effort. In many cases, calorimetric studies have proven to be sufficient for a general characterization of the behavior of the system. These measurements provide information on the stability of the initial state and on the cooperativity of the total process. From the thermodynamic parameters of the system, knowledge on the structure‐determining influence of the various types of inter‐ and intramolecular interactions is obtained. This not only applies to solutions of biopolymers and analogous model substances, but also to solutions of biopolymer complexes with low‐molecular‐weight ligands and to aqueous suspensions of self‐aggregating phospholipids. Possibilities and limitations of the calorimetric methods of measurement are demonstrated for typical examples from the numerous polypeptide and polynucleotide systems and the phospholipid bilayer systems that have been studied. In addition, the special problems involved in carrying out measurements on dilute solutions are pointed out. Here, the latest advances made in measuring techniques are readily apparent.
Covalent versus Dative Bonds to Main Group Metals, a Useful Distinction
Authors: Haaland Arne 
Published: December 2003

AbstractTextbooks of inorganic chemistry describe the formation of adducts by coordination of an electron donor to an electron acceptor, often using the amine‐boranes, X3N → BY3, as examples. In the Lewis (electron dot) formulas of the compounds, the dative bond in H3N → BH3 and the covalent bond in H3CCH3 are both represented by a shared electron pair. In the simple molecular orbital or valence bond models the wave functions of both electron pairs would be constructed in the same manner from the appropriate sp3 type atomic orbitals on the bonded atoms; the difference between the covalent and the dative bond becomes apparent only after the orbital coefficients have been analyzed. This may be the reason why many structural chemists seem reluctant to distinguish between the two types of bonds. The object of this article is to remind the reader that the physiocochemical properties of covalent and dative bonds may be – and often are – quite different, and to show that a distinction between the two provides a basis for understanding the structures of a wide range of main group metal compounds.
The Crystal Chemistry of High‐Temperature Oxide Superconductors and Materials with Related Structures
Authors: Müller‐Buschbaum Hanskarl 
Published: December 2003

AbstractNever before in human history has a chemical substance been the object of collaborative research by so many scientists of different disciplines as in the case of the first high‐temperature (HTc) oxide superconductor La2−xSrxCuO4. If the importance of a class of materials is measured by the number of papers published in a given period, the new superconductors hold the all‐time record; moreover, this flood of publications also reflects the high expectations that are held worldwide for this class of substances. Every day seems to bring further surprises. From the two phases of HTc oxide superconductors that were recognized at the outset there have developed within a period of scarcely two years a series of new materials which all show superconductivity at temperatures above the boiling point of liquid nitrogen; this property is undoubtedly closely related to the crystal chemistry of the individual phases and compounds. This article will report and discuss the resemblances and differences between the HTc superconductors and materials with related structures.
Polysilanes—A New Look at Some Old Materials
Authors: Miller Robert D. 
Published: December 2003

AbstractIn spite of the fact that the first polysilane derivatives were probably prepared in the mid 1920's, there was little scientific interest until recently. The synthesis of the first soluble homo and copolymers about 10 years ago has stimulated an explosive development of this class of materials. This brief review traces the historical development of the polysilanes and focuses on recent studies of polymer structure, electronic properties, photochemical reactions and mechanisms and finally on new applications.
Replication Experiments with Nucleotide Base Analogues
Authors: Tamm Christoph , Strazewski Peter 
Published: December 2003

AbstractThe principles governing the replication fidelity of genomes are not fully understood yet. Watson and Crick's base‐pairing principle for matched deoxyribonucleotide (DNA) bases can explain why the guanine–cytosine and adenine‐thymine base pairs are approximately one hundred times more stable thermodynamically than mismatched combinations. In vitro, DNA polymerases reduce the number of mismatched base pairs to about 10−6 per Watson–Crick base pair. Replication fidelity can further be enhanced to a mutation probability of 10−10or less in vivo if optimal conditions for DNA synthesis are provided by polymerase–assisting proteins and DNA‐repairing enzymes. The precise reasons for the formation of mismatched base pairs (mispairs), which are responsible for a substantial part of DNA mutations, are still in debate. Although it is agreed that a template‐directed “reading” of the hydrogen‐substitution pattern in the heterocyclic bases is crucial for proper base pairing during DNA synthesis, it is not clear which type of “misreading” leads to mispairs. Misreading may be due to a non‐Watson–Crick base pairing as well as to a change in the hydrogen‐substitution pattern, leading to Watson‐Crick‐like mispairs. The surprising discovery of the selective and quantitative DNA‐polymerase‐catalyzed formation of a pyridine‐pyrimidine base pair (involving a nucleotide base analogue) indicated that rare tautomeric forms in template DNA strands can lead to Watson‐Crick‐like mispairings that are hardly recognized by the polymerase's proofreading activity. This reveals new pathways for substitution mutations (replication‐dependent DNA point mutations) and suggests a new type of mutagen in vivo.
Boron‐Phosphorus Compounds and Multiple Bonding
Authors: Power Philip P. 
Published: December 2003

AbstractBoron‐phosphorus compounds have not been as thoroughly studied as their boron‐nitrogen counterparts. Until recently many classes of B‐P compounds that had been well established in B‐N chemistry were either unknown or poorly characterized. This statement is particularly true for compounds involving possible multiple bonding between boron and phosphorus. For example, detailed structural information on simple monomeric phosphino‐boranes, R2BPR′2, did not become available until 1986 even though the isoelectronic SiC double bonded species, the silenes, had already been reported. However, new work has shown that it is possible to prepare and characterize several novel types of boron‐phosphorus compounds with varying degrees of multiple B—P bonding. These include not only monomeric phosphinoboranes but also phosphanediylborates (borylphosphides), three‐ and four‐membered rings (diphosphadi‐boretanes), boron phosphorus analogues of borazine, B‐P skeletal analogues of allyl cations and anions, butadiene and cage compounds. Structural, spectroscopic (mainly NMR) and theoretical studies reveal some important differences between B‐P and B‐N compounds which in many cases can be traced to the presence of a high inversion barrier at phosphorus that reduces the π interaction. This usually causes compounds such as R2BPR′2 to associate through σ bonding between B and P. Supporting evidence for this view comes from species that involve phosphorus and nitrogen in competitive π bonding with a boron p orbital in which the dative interaction between B and N is dominant and the phosphorus center remains pyramidal. Recently published work has shown that steric and electronic factors can be used to favor π bonding and give an approximately planar system. Furthermore, theoretical studies reveal that pp π overlap in a planar B‐P system is of similar efficiency to its B‐N analogue. Good examples are seen in the phosphanediyl borates, the boron‐phosphorus analogues of borazine and the π‐allyl cations, whose molecular configurations and B—P bond lengths support strong boron—phosphorus π bonding.
The Chemistry of Vitamin A and Vision
Authors: Rando Robert R. 
Published: December 2003

AbstractVision is initiated in the retina by the photochemical isomerization of an 11‐cis‐retinal Schiff base to its all‐transcounterpart in the photosensitive protein rhodopsin. All‐trans‐retinal is released, reduced to form all‐trans‐retinol (vitamin A), and finally esterified. In order for vision to proceed an 11‐cis‐retinoid must be regenerated in the eye.Kühneobserved in the 1870s that the pigment epithelium must be in place behind the retina in order for rhodopsin to form. And indeed it was in the pigment epithelium that an isomerase system was recently discovered that can produce 11‐cis‐retinol from added vitamin A.Paulingpredicted in the 1930s that certaincisdouble bonds in carotenoids and retinoids, including 11‐cis‐retinoids, should be quite unstable with respect to their all‐transcongeners. This prediction is borne out by experiment, but leaves open the crucial question as to where the energy comes from to drive the isomerization process in the eye. This question was answered by the demonstration that the energy is found in the membrane phospholipids. The fact that membranes may serve as an energy source reveals a previously unsuspected biological function of membranes. The chemistry of the novel energy‐linked isomerization process is explored in this review.
New Aspects of the Maillard Reaction in Foods and in the Human Body
Authors: Ledl Franz , Schleicher Erwin 
Published: December 2003

AbstractThe reactions that occur during the cooking, baking, and preservation of foods of all kinds are of great importance for the production of aroma, taste, and color. However, more recently it has been shown that these reactions may be accompanied by a reduction in nutritive value and the formation of toxic compounds. For these reasons, the very complex reactions between reducing sugars and the free amino groups of amino acids or proteins, known as non‐enzymatic browning or the Maillard reaction, have again caught the interest of chemists. The Maillard reaction came to be seen in a new light as it was realized that it actually occurred in the human body. As a general rule, the longer the half‐life of a protein, the larger the amount of its Maillard products found, i.e., important factors are the ‘age’ or persistence of the protein in the body and the glucose concentration, particularly in diabetics. Many of the symptoms developed by diabetics resemble those of premature aging, which leads to the possibility that glucose, because of its reactivity towards proteins, is fundamentally involved in the normally slow progress of aging.
The Coordination Chemistry and Organometallic Chemistry of Tridentate Oxygen Ligands with π‐Donor Properties
Authors: Kläui Wolfgang 
Published: December 2003

AbstractMetal complexes are capable of accomplishing almost anything, provided they contain the proper metal/ligand combinations. A host of essential biochemical transformations—but also a great many industrially significant reactions—occur within the coordination spheres of metal ions. For instance, the particular arrangement of ligands in the zinc‐containing enzyme carboanhydrase is responsible for an acceleration in the hydration of CO2 by a factor of 109. It is the ligands that determine whether an iron atom will transfer molecular oxygen, as in the case of hemoglobin, or electrons, as with the cytochromes. By varying the ligands it is possible to establish in advance whether a metal ion in the presence of synthesis gas will cause an olefin to be hydrogenated or hydroformylated. Stated more generally, it is the ligands that stabilize the particular oxidation states of a metal and determine how substrate molecules will be coordinated and undergo reaction. The synthesis of new ligands that confer specific reactivity on metal ions is thus an important challenge for the coordination chemist. The following article describes organometallic compounds of the type [CpCo{P(O)R′R″}3]⊖, which have developed from an extremely unreactive laboratory curiosity into versatile oxygen‐containing ligands whose steric and electronic properties promise a series of interesting applications.
Syntheses, Conformations and X‐Ray Structure Analyses of the Saccharide Chains from the Core Regions of Glycoproteins
Authors: Paulsen Hans 
Published: December 2003

AbstractThe covalently bound carbohydrate moiety in glycoproteins can stabilize the protein molecule intramolecularly, or it may have an intermolecular function as receptor in biological recognition. The discovery of these biological phenomena has led to a renaissance of the chemistry and biochemistry of carbohydrates. Both N‐glycoproteins as well as O‐glycoproteins contain special, invariant oligosaccharide chains in the protein‐binding region, which occur again in all glycoproteins, and are described as the “core regions.” This review describes the various methods of oligosaccharide synthesis that may be used to arrive at the basic core structures by chemical means. Methods of oligosaccharide synthesis have improved so much that it is possible to synthesize complex lactosamine‐type structures, and “bisected”‐type structures up to nona‐ and undecasaccharides respectively. Oligosaccharide chains are considerably less flexible than peptide chains. Using modern methods of NMR spectroscopy, their preferred solution conformation can readily be determined. In the case of one branched octasaccharide, a comparison of the conformations in solution and in the crystal is possible. Oligosaccharides may be linked to the amide group of an asparagine, or to the hydroxyl groups of serine or threonine. By using suitable protecting groups, the glycosyl amino acids obtained can be extended with further amino acids at the N‐ or C‐terminus, thus arriving at the desired glycopeptide sequences. In the linkage region, glycopeptides prefer certain conformations. Future research into glycoprotein functions may involve the synthesis and biochemical study of modified glycoprotein segments.
The Myelin Membrane of the Central Nervous System—Essential Macromolecular Structure and Function
Authors: Stoffel Wilhelm 
Published: December 2003

AbstractThe neural sheaths that surround the nerve fibers (axons) are composed of myelin‐specific complex lipids and are assembled during the myelination phase either by the oligodendrocytes in the central nervous system (CNS) or by the Schwann cells in the peripheral nervous system. These multilayered myelin membranes insulate the axons and permit a rapid, saltatory conduction of excitation and a reduced axon diameter in comparison with noninsulated axons. Myelination was hence the decisive evolutionary event in miniaturization of the central nervous system (brain and spinal cord). The morphology of the myelin membrane has been studied in detail mainly by electron microscopy. Most of its biochemistry has been elucidated in recent years by molecular‐level analysis of both the lipid components (cholesterol, phospholipids and sphingolipids) and the constituent proteins. The multilamellar system is distinguished by a characteristic periodicity due to the 5‐nm‐thick bilayer formed by the myelin‐specific lipids. The bilayer interacts with the myelin basic protein (MBP) on the cytosolic side of the plasma membrane process, while the integral membrane protein proteolipid protein (PLP) has hydrophilic domains exposed on both the cytosolic and extracytosolic faces of the bilayer. Numerous protein‐chemical and ‐immunotopochemical findings have been summarized in a model of the myelin membrane. Through molecular biological studies, the genetic structure and chromosomal location of the myelin proteins have been determined. By employing techniques of molecular and cell biology together, it is now possible to analyze the process of myelinogenesis, the time‐ and location‐specific expression of myelin‐specific genes in the brain. Gene‐technological methods have been used to define the mutations in the models jimpy mouse and myelin‐deficient rat. These are animal models that correspond to genetically determined myelin defects (dysmyelinoses) in humans. Using them, it will be possible to study the cell death of oligodendrocytes on a molecular level; this process is the result of expression of mutant myelin proteins and is incompatible with life. Oligodendrocytes and the myelin structures they synthesize are the target structures of cytotoxic lymphocytes (Tc). In the course of the demyelination process in multiple sclerosis, these cause the breakdown of the myelin sheaths, in gradually appearing inflammations. Tc lymphocytes recognize myelin structures as epitopes and destroy them. The picture of the myelin membrane's molecular composition, which we are now perfecting, will also lead to a better understanding of demyelination on a molecular level, and hence to new therapeutic possibilities.
Synthesis of D‐ and L‐2‐Amino‐2‐deoxyarabinose and 1,4‐Dideoxy‐1,4‐iminolyxitol by (C2 + C3)‐Nitroaldol Addition with 2‐O‐Benzylglyceraldehyde
Authors: Wehner Volkmar , Jäger Volker 
Published: December 2003

AbstractNo Absract.
Cyclization of N‐Protected l‐Amino‐4‐pentene‐2,3‐diols to lxyo‐Configurated Deoxyimino Sugars (cis‐Dihydroxypyrrolidines); Synthesis of Potential Glycosidase Inhibitors
Authors: Jäger Volker , Hümmer Walter 
Published: December 2003

AbstractNo Absract.
What Will Chemistry Do in the Next Twenty Years?
Authors: Whitesides George 
Published: December 2003

AbstractThe path of chemistry in the future will be determined both by its participation in solving large‐scale societal problems and by its generation of new ideas through basic research. This article sketches four of the areas of societal “pull” in which chemistry will play a role in solving applied problems—national security, health care, the environment, and energy—and four areas in which basic research will be especially fruitful—materials chemistry, biological chemistry, computational chemistry, and chemistry exploring the limits of size and speed in chemical phenomena.
Molecular Thermodynamics for Chemical Process Design
Authors: Prausnitz John M. 
Published: December 2003

AbstractThermodynamic properties are essential for quantitative process design to produce chemical products. Caloric properties are required for heat balances, but these properties are usually available or estimated easily. More important—and often much more difficult to estimate—are the chemical potentials of components in mixtures; it is these potentials which determine phase equilibria, as required for separation operations, and chemical equilibria, as required for chemical reactors and for separation operations based on chemical reactions. Molecular thermodynamics is an engineering‐oriented science for calculating the desired chemical potentials from a minimum of experimental data. This applied science, based on classical and statistical thermodynamics, yields chemical potentials through models that are based on molecular physics and physical chemistry. Selected examples are cited to illustrate the applicability of molecular thermodynamics: group‐contribution methods for obtaining chemical potentials in highly nonideal mixtures as required for distillation‐column and process‐safety design; equation of state for precipitation of uniform‐sized crystals from supercritical fluids; molecular‐orbital calculations to guide process development for alternatives to environmentally dangerous chlorofluorohydrocarbons; molecular‐simulation calculations for separation of gas mixtures with porous adsorbents; equilibria in two‐phase aqueous systems for separation of protein mixtures; and, finally, extended polymer‐solution thermodynamics to guide synthesis of hydrogels suitable for protein recovery from soybeans and for novel drug‐delivery devices.
Perspectives in the Development of High‐Temperature Polymers
Authors: Hergenrother Paul M. 
Published: December 2003

AbstractResearch on high‐temperature organic polymers was initiated in the late 1950s primarily to meet the needs of the aerospace and electronics industry. Since then, many different heat‐resistant polymer systems have been reported, of which several are now commercially available. These polymers are used in many diverse applications such as circuitry in microelectronic components, coatings on cookware, binders in brake systems, sealants for fuel tanks in high‐speed aircraft, gears in copying machines, structural components in high‐speed aircraft, and space vehicles, films and wire coatings for electrical insulation. Worldwide use for high‐temperature polymers in 1988 was estimated at 90 million kilograms with a value of $ 2.3 billion. This market is expected to double by the end of this decade. The major polymer classes discussed in the present paper are polyimides and poly(aryl ethers).
Catalytic Antibodies: A New Class of Transition‐State Analogues Used to Elicit Hydrolytic Antibodies
Authors: Shokat Kevan M. , Ko Marcia K. , Scanlan Thomas S. , Kochersperger Lynn , Yonkovich Shirlee , Thaisrivongs Suvit , Schultz Peter G. 
Published: December 2003

AbstractThe design and generation of selective catalysts is an important aim of chemists and biologists. A number of successful strategies have emerged, including the synthesis and derivatization of synthetic hosts, the chemical modification and site‐directed mutagenesis of enzymes, and the attenuation of natural enzyme activities in organic solvents. Since 1986 several laboratories have exploited the immune system to generate selective catalysts capable of catalyzing a wide range of chemical transformations. These include acyl transfer, β‐elimination, carbon—carbon bond‐forming, carbon—carbon bond‐cleaving, porphyrin metalation, peroxidation, and redox reactions. The variety and number of transformations catalyzed by antibodies in this short period of time is testament to the versatility and power of the method in generating selective catalysts for applications in chemistry, biology, and medicine. Here we report the use of a new class of uncharged transition‐state analogues for generating antibodies capable of catalyzing ester and carbonate hydrolysis. These antibodies are compared to those raised against tetrahedral phosphate and phosphonate transition‐state analogues.