bluuebunny/crossref_metadata_2025_split huggingface dataset:
Biopolymers: Origin, Chemistry, and Biology
Authors: Calvin Melvin 
Published: December 2003

AbstractA brief statement concerning the way in which biopolymers may have originated in the nonbiological world is made, including experimental evidence. This also includes a discussion of such matters as the way in which the code might have originated, that is, the relationship between polypeptides and polynucleotides as well as the secondary and tertiary structure resulting from the primary structure determination. The importance of the interaction of biopolymers with lipids for the formation of limiting membranes, leading to the formation of cells and other self‐organizing cellular type organelles, is discussed. Thus, the second critical physical‐chemical problem for cellular organization, namely, the biopolymer‐lipid interaction, is now coming under scrutiny, both in terms of synthetic systems as well as natural ones.
The Crystalline State of Macromolecular Substances
Authors: Zachmann H. G. 
Published: December 2003

AbstractUnlike low molecular weight substances, high polymers do not crystallize completely. However long the crystallization process is continued, they still consist of a mixture of crystalline and non crystalline regions. In the undrawn material, these regions form larger units which are known as spherulites. Questions that are of special interest concern the arrangement of the chains in the noncrystalline regions, the causes of chain folding, and the imperfections in the crystals. The incomplete crystallization is a consequence of kinetic inhibitions. This can be deduced from the fact that the crystalline fraction increases with rising crystallization temperature. If polymerization is carried out in the solid crystalline state, one obtains a completely crystalline polymeric material.
The Pigments of Reddish Hair and Feathers
Authors: Thomson R. H. 
Published: December 2003

AbstractDespite sporadic investigations over many years the structures of the pigments responsible for most of the yellow, orange, and brown colors of mammalian hair, and of feathers, have remained unknown. However, recent work has shown that the major reddish‐brown pigments, gallophaeomelanins, are polymers containing benzothiazole and tetrahydroisoquinoline units. The yellow‐orange and violet trichochromes are minor pigments based on a new chromophore, Δ2,2′‐bi(1,4‐benzothiazine). Both groups arise in vivo from tyrosine and cysteine by a deviation from the biogenetic pathway which normally leads to the formation of eumelanins, the black pigments.
Photochemistry in Interstellar Space and Reactions Generating Interstellar Molecules
Authors: Breuer H. D. 
Published: December 2003

AbstractOver 20 different molecules and radicals have so far been detected in interstellar space. The number of such molecules and radicals and their frequently complicated structure raise the question of the processes leading to their formation. Diatomic molecules and radicals could arise in gas‐phase reactions. Formation of polyatomic molecules can be explained in terms of reactions proceeding on the surface dust particles and possibly involving interstellar radiation. The author demonstrates, with the aid of a model, that the molecular abundances and distributions within dust clouds in certain regions can be explained by photocatalytic reactions.
Isopeptide Crosslinks—Their Occurrence and Importance in Protein Structure
Authors: Asquith R. S. , Otterburn M. S. , Sinclair W. J. 
Published: December 2003

AbstractIsopeptide links refer to bonds between the ϵ‐amino group of lysine and the side‐chain carboxyl groups of glutamic or aspartic acid. Covalent crosslinks of this kind, which undoubtedly occur in protein feedstuffs and reduce their nutritional value, also participate In the fibrinogen → fibrin transformation and play a part in stabilizing the structure of keratins. Isopeptide links also arise on heating of both fibrous and globular proteins, the amount formed depending upon the severity of heat treatment. The mode of formation and the significance of these crosslinks require further elucidation.
Prescriptions and Ingredients for Controlled CC Bond Formation with Organometallic Reagents. New synthetic methods (5)
Authors: Schlosser Manfred 
Published: December 2003

AbstractMerits and drawbacks of known carbon‐carbon linking procedures are outlined. Two novel methods are discussed in some detail: the copper‐catalyzed alkylation of Grignard reagents and reactions with allylpotassium compounds. Both methods provide a very efficient access to saturated, unsaturated, as well as functionally substituted hydrocarbons and moreover permit an astonishing degree of regio‐ and stereoselective control of olefin synthesis.
Biochemistry of the Peroxisome in the Liver Cell
Authors: Sies Helmut 
Published: December 2003

AbstractThe marker enzyme of the peroxisome—a phylogenetically old yet only recently discovered cell organelle—is catalase, a hemoprotein which decomposes hydrogen peroxide catalatically as well as peroxidatically. In the peroxisomes, catalase is associated with H2O2‐producing oxidases and other enzymes. Also in parenchymal cells such as liver and kidney cells part of the reduction of oxygen occurs via formation of H2O2. A central role in peroxisomal H2O2‐metabolism is played by the active intermediate, catalase‐Fe3+‐H2O2, (Compound I), which is distinguished from free catalase by specific absorption bands. Organ photometry on intact hemoglobin‐free perfused rat liver in order to measure Compound I selectively provides a direct insight into the dynamics of the H2O2 metabolism which takes place in the range of nanomolar concentrations. Endogenously, 1g of liver forms approximately 50 nmol of H2O2 per min. The turnover number, which in the steady state is < 10 min−1 in the cell as compared to > 108 min−1 for the isolated enzyme with an excess of substrate, can be increased to approximately 102 min−1 by intracellular stimulation of the H2O2 production (e.g. by glycolate or urate). The peroxidatic oxidation of hydrogen donors (e.g. methanol and ethanol), favored relative to the catalase pathway at low turnover numbers, is of importance in normal metabolism and in pathological conditions.
Specific Interactions. Intermolecular Forces, Steric Requirements, and Molecular Size
Authors: Simon Zeno 
Published: December 2003

AbstractSpecific interactions or recognition processes are extremely important in many fields of chemistry and biology: typical examples are the enzyme‐substrate and the antigen‐antibody reactions. Quantitative predictions require a knowledge of steric fit, intermolecular forces, and molecular size. These considerations should facilitate design of new drugs.
α‐Metalated Isocyanides in Organic Synthesis. New synthetic methods (6)
Authors: Hoppe Dieter 
Published: December 2003

Abstractα‐Alkali‐metalated isocyanides, which can be obtained from isocyanides and bases, can be used for nucleophilic introduction of (masked) α‐aminoalkyl groups. Intramolecular ring‐closure may then follow if a nucleophile combines at the electron sextet of the isocyanide carbon. Treatment of α‐alkali‐metalated isocyanides with electrophilic agents permits rapid and efficient synthesis of, inter alia, 2‐ and 3‐amino alcohols, straight‐chain, branched, and β‐functional α‐amino acids, olefins, vinyl isocyanides, and of a large number of mainly five‐, but also six‐ and seven‐membered aza‐, diaza‐, oxa‐aza‐, and thia‐aza heterocycles.
The Replacement of Phosphates in Detergents—Possibilities and Limits
Authors: Berth Peter , Jakobi Günter , Schmadel Edmund , Schwuger Milan J. , Krauch Carl Heinrich 
Published: December 2003

AbstractElimination of phosphates from detergents alone, is insufficient to solve the eutrophication problem. Nevertheless, the endeavors of the detergent industry are aimed at finding phosphate subsitutes. Novel approaches for at least a partial replacement of detergent phosphates have been found by using water‐soluble organic complexing agents as well as heterogeneous organic or inorganic ion exchangers. So far, a large‐scale replacement is not possible. It is not justifiable until all technical requirements are met with and safety with regard to man, animals, plants and drinking water has been established.
What Do We Know About Nuclear Fission?
Authors: Aumann Dieter C. 
Published: December 2003

AbstractNuclear fission, although a physical phenomenon, is also of interest to the chemist. It was discovered by chemists, and chemists have made a material contribution to our present knowledge in this field. A comprehensive theory of nuclear fission has not, as yet, been developed, despite the volume of experimental data that has amassed in the 35 years since its discovery. In the past few years discoveries have been made which cannot be explained with conventional theoretical models of nuclear fission. The most important discovery was that of shape isomerism, a novel type of nuclear isomerism. It would appear that an accurate calculation of fission barriers is a prerequsite for precise explanation of many experimental findings. Recent results in this connection and an outline of the present state of our knowledge of nuclear fission constitute the main theme of this paper.
Synthetic Reactions of Metal Atoms at Temperatures of 10 to 273°K
Authors: Timms Peter L. 
Published: December 2003

AbstractMetal atoms, easily formed by vaporizing metals at high temperatures under vacuum, are reacted with inorganic and organic compounds in solution at 80 to 273°K or by co‐condensing the atoms and substrate vapors at 10 to 80°K. Gram quantities of products containing metals in low valency states can be isolated in many cases, and some of the compounds formed are inaccessible by conventional synthetic routes.
The Photochemistry of Organic Solids
Authors: Cohen Mendel D. 
Published: December 2003

AbstractDuring the years that he was at the Weizmann Institute of Science the late G. M. J. Schmidt initiated and developed a very vital research program on the relationship between the structures of organic crystals and the reactions which occur in these crystals. As the program developed the “solid‐state group” became larger and more diversified—X‐ray crystallographers, organic chemists, spectroscopists, and even theoreticans had their contributions to make. Professor Schmidt died in 1971. As one of the members of this group, I here present a review of recent developments in parts of the field of interest to me.Of the reactions of organic crystals which have been studied thus far a large number are induced by light and other radiation. This is because radiation, unlike most chemical reactants, can readily penetrate the surface of the crystal, and thus reach the sites one wishes to activate. It is my aim in this article to point out some of the principles operative in the photochemistry of organic crystals. I shall choose examples to illustrate these principles and will not attempt to cover all the various types of reaction and systems which have been studied.It is appropriate at this stage to pay tribute to the memory of another great scientist who recently passed away—Theodorr Föster. Many of the ideas which I develop here are extensions of Professor Förster's earlier work, as are so many of the recent developments in photochemistry.
Stereochemistry of Reaction Paths as Determined from Crystal Structure Data—A Relationship between Structure and Energy
Authors: Bürgi Hans‐Beat 
Published: December 2003

AbstractThe three‐dimensional structure of frequently occurring molecular fragments has been studied systematically. Molecular subunits which were examined include hydrogen bridges (O—HċO), triiodide anions (I−3), other linear triatomic fragments (Cl—SbċCl, S—SċS, Mo—OċMo), tetrahedral ions (SO2−4, PO3−4, AlCl−4) and related species (MSO3, etc.), molecules containing both keto‐ and amino‐groups (mostly alkaloids), substituted [10]annulenes and cycloheptatrienes, organic five‐membered rings, and five‐coordinated metal and nonmetal atoms. The bond distances and angles describing the structure of a giving fragment cover a range that is many times larger than the range of the experimental standard deviations. The changes of the various structural parameters of a fragment are correlated with each other. The observed mutual dependence (structural correlation) may be described by means of Pauling's equation relating bond length r to bond number n:r = r0 ‐ clogn. The bond numbers n are expressed in terms of bond angles. The sum of bond numbers at a given atom is roughly constant and does not depend on the environment. “Standard bond lengths” of a fragment are determined by a least‐squares procedure based on all available data. They are supplemented by curves that describe the observed distortions.The shape of these correlation curves is reminiscent of the structural changes occurring along the pathways of chemical reactions, e.g. nucleophilic substitution at tetrahedrally coordinated atoms (SN1 and SN2), nucleophilic addition to carbonyl groups, electrocyclic ring closure of polyenes, pseudo‐rotation of five‐membered rings or Berry pseudo‐rotation. For many of these reactions approximate energy hypersurfaces have been obtained from quantum mechanical calculations, model force fields and from spectroscopic information (IR, NMR). Comparison between reaction pathways determined from structural correlations with those obtained from models of the energy surfaces shows fair agreement.
Nonenzymatic Simulation of Nitrogenase Reactions and the Mechanism of Biological Nitrogen Fixation
Authors: Schrauzer G. N. 
Published: December 2003

AbstractThe development of biologically relevant model systems of nitrogenase permitted the simulation of virtually all known reactions of the nitrogen reducing enzymes under nonenzymatic conditions. On the basis of these experiments, a mechanism of biological nitrogen fixation is formulated which is in accord with the available enzymological evidence. The key reactions of the substrates of nitrogenase occur at a molybdenum active site. The non‐heme iron, which is bound to sulfur and protein‐S⊝ groups, mediates the transport of electrons to the molybdenum active site but does not participate directly in the reduction of the substrates. ATP is required for the acceleration of the reduction and activation of the molybdenum site and is hydrolyzed to ADP and inorganic phosphate. Diimine and hydrazine were detected as intermediates in the reduction of molecular nitrogen under nonenzymatic conditions.
Heterocyclic Systems Having Eight π‐Electrons. Synthesis, Properties, and Significance
Authors: Schmidt Richard R. 
Published: December 2003

AbstractThe terms “aromatic” and “antiaromatic” have attracted widespread interest in recent years. After all the discussion about the meaning of words, one fact remains: the energy content of planar systems displaying cyclic electron delocalization depends upon the number of, and the orbital interactions between, the participating electrons. The concept underlying this progress report is that the electronic destabilization of systems having eight π‐electrons should be especially pronounced in the case of six‐membered rings for steric and electronic reasons. Such systems are found primarily in anionic, but also in neutral, heterocycles containing hetero ring members which possess electron pairs capable of delocalizing into the π‐system. The production and reactivity of such systems are investigated and their biological significance discussed.
The Pyruvate Dehydrogenase Multienzyme Complex
Authors: Hucho Ferdinand 
Published: December 2003

AbstractThe three enzymes pyruvate dehydrogenase, dihydrolipoamide transacetylase, and dihydrolipoamide dehydrogenase constitute the pyruvate dehydrogenase multienzyme complex of E. coli; in mammals the complex also contains a kinase and a phosphatase. Multienzyme complexes are structural, functional, and regulatory units enabling the organism to operate more economically than with single enzymes. The pyruvate dehydrogenase multienzyme complex may stand at the switch‐point between energy metabolism and gluconeogenesis.
Tertiary Phosphane/Tetrachloromethane, a Versatile Reagent for Chlorination, Dehydration, and PN Linkage
Authors: Appel Rolf 
Published: December 2003

AbstractIn this review the processes occurring on the treatment of triphenylphosphane with tetrachloromethane are first reported; a key role is played here by (trichloromethyl)phosphonium chloride which reacts with a further amount of the phosphane to give the stable salt [chloro(triphenylphos‐phoranediyl)methyl]triphenylphosphonium chloride by way of dichlorophosphorane and (dichloromethylene)phosphorane that is detectable only as an intermediate. The preparative application of the two‐component system as a chlorinating, dehydrating, and P—N‐linking reagent is ascribable to “phosphorylation”Here and below, “phosphorylation” is used lo mean in general the formation of a phosphorus derivative. In the narrower sense this term denotes the introduction of phosphoryl groups P(O).
of the substrate in question through several reactive species. In these applications the reactions with the substrate compete with reactions of the two‐component system with itself, so that the overall course of the reaction is in nearly all cases much more complex than was previously assumed. Nevertheless. very good results can be achieved by the use of this reagent, high yields and mild reaction conditions being characteristic.
Chemistry and Biochemistry of Unsaturated Fatty Acids
Authors: Kunau Wolf‐H. 
Published: December 2003

AbstractAlthough unsaturated fatty acids have long been known to accompany saturated fatty acids in most lipids, qualitative and quantitative determination of fatty acid patterns only became possible with the advent of modern analytical methods. Present day knowledge of the chemical structure, physical properties, and metabolism of unsaturated fatty acids provides the basis for the development of new concepts of their function. Thus unsaturated fatty acids crucially determine the properties of biological membranes. Moreover, essential fatty acids are precursors of prostaglandins.
Unusually High Barriers to Rotation Involving the Tetrahedral Carbon Atom
Authors: Ōki Michinori 
Published: December 2003

AbstractRotation about CC single bonds in suitably substituted compounds gives rise to rotational isomers (also called rotamers) which can be isolated if the barrier to rotation is high enough. The present article considers derivatives of triptycene and fluorene whose barriers to rotation are of the order of 30 kcal/mol.
Oxidation‐Reduction Condensation
Authors: Mukaiyama Teruaki 
Published: December 2003

AbstractThis review is concerned with syntheses which can be carried out in the absence of acids and bases. In the case of condensations, i.e. the union of two or more reaction partners with elimination of simple molecules such as H2O or NH3, the combination of a weak oxidizing agent with a weak reducing agent has proven useful. A particularly suitable reagent for condensations with elimination of water is triphenylphosphane/di(2‐pyridyl)disulfane. The phosphane takes up the oxygen atom and is converted into the phosphane oxide, while the disulfane reacts with the two hydrogen atoms to form pyridinethione. Oxidation‐reduction condensations have been utilized for the preparation of, inter alia, esters, peptides, nucleotides, and macrolides.
Integrated Dust Analysis by Physical Methods
Authors: Malissa Hanns 
Published: December 2003

AbstractEnvironmental analyses show that the air which we breathe, and which is so essential to life, is in general a mixture of gaseous, liquid, and solid components. The solid airborne particles, whose concentration, homogeneity, chemical composition, size, and shape can vary over wide ranges, and whose origin may be “natural” or “artificial” are referred to as “dust”. Dust particles can act, inter alia, as condensation nuclei, catalysts, and directly as hazardous materials. Unfortunately, we still know far too little about dust. Dust analysis is extremely difficult and challenging, even for modern analytical chemistry; it is still far from being fully automated. The simultaneous determination of as many “dust parameters” as possible, and particularly the synoptic consideration of all available data against a background of physicochemical and technological knowledge on the development, transformation, and effects of dust, are summarized as “integrated dust analysis”.
The Analogy between O and C(CN)2
Authors: Wallenfels Kurt , Friedrich Klaus , Rieser Jürgen , Ertel Werner , Thieme Herbert K. 
Published: December 2003

AbstractThe compounds of oxygen, i.e. OH acids, ethers, quinones, aldehydes, ketones, carboxylic esters and amine oxides, sulfoxides, and phosphane oxides, are compared and contrasted with those compounds which contain a C(CN)2 moiety in place of the characteristic O atom. In their typical properties the pairs of analogs frequently display surprising agreement. The purpose of such analogy studies lies primarily in the estimation of reactivities; their limits are reached where specific properties of the structual components become effective.
The Synthetic Utility of 2‐Oxazolines
Authors: Meyers A. I. , Mihelich E. D. 
Published: December 2003

AbstractThe oxazoline ring system, first prepared in 1884, has only in the last five years been shown to possess considerable utility in the synthesis of a variety of functionalized organic compounds. By appropriate metalation of the 2‐alkyl substituent it is possible to prepare a variety of homologated carboxylic acid derivatives, both chiral and achiral. Various other manipulations of the oxazoline ring lead to aldehydes, ketones, lactones, amino acids, thiiranes, and olefins. Due to the inertness of the oxazoline ring system to various reagents (RMgX, LiAlH4, CrO3, mild acid or alkali) it may be utilized as a protecting group for carboxylic acids during the transformations involving the above reagents. Two major new developments in oxazoline chemistry involve (a) the use of a chiral oxazoline as a reagent for asymmetric synthesis of chiral carboxylic acids, lactones, alcohol, thiiranes, and olefins and (b) a new mode of aromatic substitution in which the aryloxazolines act as nucleophilic or electrophilic reagents. This behavior has led to a variety of polysubstituted benzenes and biphenyls which would be inaccessible by classical routes. A variety of examples illustrating the utility of oxazolines indicated above are presented so that the reader may appreciate the vast potential of this simple ring system in synthesis.
The Chemistry of Simple Phosphorus‐Carbon Compounds
Authors: Harnisch Heinz 
Published: December 2003

AbstractWith the aid of selected examples an overview is given of the development trends in phosphoruscarbon chemistry over the past few years. An attempt is made to demonstrate the relationships between various parameters and properties such as constitution, basicity, substitution by functional groups, reaction behavior etc. of the compounds. In the case of basis compounds containing methylphosphorus groups the state of development of industrially interesting processes is also outlined. In addition, the synthesis of a few bifunctional phosphorus‐carbon compounds which can be employed as comonomers in the production of polymers is described.
Polar Effects in Organic Reactions
Authors: Grob Cyril A. 
Published: December 2003

AbstractAttempts to gauge the effect of polar substituents on organic reaction rates by means of “inductive substituent constants” are based on the assumption that these effects are independent of the type of reaction observed. The measured rate constants of nucleophilic substitution reactions show, however, that this assumption is only partly justified even for saturated molecules. It is invalid if the substituent and the reaction center are an electron donor and an electron acceptor, respectively, which are hyperconjugated by way of σ‐bonds. In extreme cases the resulting polarization can lead to heterolytic fragmentation.