bluuebunny/crossref_metadata_2025 huggingface dataset:
The Meaning and Use of the Arrhenius Activation Energy
Authors: Menzinger M. , Wolfgang R.
Published: December 2003

The nature of the Arrhenius activation energy and frequency factor is reexamined in terms of information now becoming available on the microscopic aspects of collisional reactions. It is pointed out that the activation energy is not generally equal to the threshold for reaction, and its correct conceptual meaning is discussed. The temperature dependence of this quantity and its relation to the threshold energy is developed for a number of representative forms of the energy dependence of the reaction cross‐section (excitation function). The uses and limitations of the activation energy as a means of evaluating thresholds, excitation functions, and the presence of tunneling processes are discussed.
Hetero Analogs of Sesquifulvalene
Authors: Seitz G.
Published: December 2003

Heterosesquifulvalenes containing sulfur or oxygen as hetero atom may be prepared, e.g., by condensation of cyclopentadienes with thiopyrones or pyrones in acetic anhydride. Heterosesquifulvalenes containing nitrogen can be obtained from the oxygen compounds by O,N‐exchange. The ground state of all these compounds is determined mainly by the nonpolar resonance formula.
Chemical Investigations of the Molecular Origin of Biological Radiation Damage
Authors: Fahr E.
Published: December 2003

The radiation damage observed when UV and ionizing radiations react on biological objects is caused in many cases by changes in the nucleic acids. Exposure of these compounds to UV radiation in vitro and in vivo leads, inter alia, to dimerization of the pyrimidine bases with formation of cyclobutane derivatives, and to addition of water to the 5,6‐double bond of the pyrimidine bases to form derivatives of the 6‐hydroxyhydropyrimidine system. The structure of the irradiation products has been established. The dimerization prevents the reduplication of the DNA, and the addition of water appears to be the cause of UV mutations. Ionizing radiation in aqueous solution results e.g. in addition of H and/or HO radicals to the 5,6‐double bond of the pyrimidine bases and cleavage of the imidazole ring of the purine bases. The mutations caused by ionizing radiation are probably also due, in part, to the formation of 6‐hydroxydihydropyrimidine derivatives.
Vapor Pressure Isotope Effects
Authors: Höpfner A.
Published: December 2003

The vapor pressure isotope effect is a reflection of the thermodynamic relations in the condensed phase. It is possible to explain the observed effects on the basis of intermolecular vibrations, e.g. hydrogen bond vibrations, and the frequency shifts of the charateristic vibrations on condensation, though other effects must also be discussed. Information about the thermodynamics of liquids, e.g. about the degree of association or the expected positions of the lattice frequencies, can be obtained from the vapor pressure isotope effect.
Modern Analysis
Authors: Kienitz H.
Published: December 2003

The important advances being made in modern analytical methods are indicative of the fundamental changes that are occuring in the theory and practice of “analytical chemistry”. “Information optimization” demands a new approach in teaching and research, and calls for the intergration of chemistry with other scientific and technical disciplines.
Small Charged Heterocycles
Authors: Crist D. R. , Leonard N. J.
Published: December 2003

The synthesis and important reactions of stable aziridinium salts are described in this article. The more stable azetidinium salts, which undergo similar reactions, have also been studied. Polar and dipolar cycloadditions produce heterocycles by ring expansions that can be represented as
equation image
.
Reactions with Addition Compounds Containing Activated Formic Acid
Authors: Wagner K.
Published: December 2003

Addition compounds of formic acid with tertiary organic bases in ratios higher than that required for salt formation (3:1 and 2:1 adducts) contain formic acid in a highly activated form. In particular, the 3:1 adducts of formic acid with trimethylamine and triethylamine are liquid reducing agents that are convenient to handle and are suitable for use in many selective reductions. These addition compounds have a surprisingly strong reducing action on sulfur dioxide, which is rapidly converted, even at low concentrations, into crystalline rhombic sulfur. Sulfones and polysulfones can be prepared in good yields by three‐component reactions from activated formic acid, sulfur dioxide, and polarized vinyl compounds.
Automatic Analysis of Amino‐Acid Sequences in Proteins and Peptides
Authors: von. Wilm M.
Published: December 2003

The development of the widely used automatic method from Edman's manual sequence analysis of proteins and peptides has resulted from modification of the procedure and from the use of a physically novel principle. All reaction, centrifugation, extraction, and drying operations are carried out in a rotating vessel. At a minimal cost in time and material, for example, sixty amino acids have been split off consecutively from a single sample of myoglobin.
Macromolecular Chemistry and Medicine
Authors: Hastings Garth W.
Published: December 2003

Macromolecular substances are used in medicine both as materials and as reagents. In the former category polymers serve as replacements for soft tissue, as cardiovascular and orthopedic implants, and as adhesives. When employed as reagents, macromolecules interact with living tissue and play an active part in bodily repair processes.
Pyramidal Inversion
Authors: Rauk Arvi , Allen Leland C. , Mislow Kurt
Published: December 2003

Pyramidal inversion is discussed from the point of view of recent theoretical and experimental investigations in an attempt to provide a unified description of this process. Quantum mechanical studies of pyramidal molecules indicate that the origin of the inversion barrier may be dependent on the degree of angular constraint. Effects due to the electronegativity of substituents on the inversion center, to the presence of adjacent lone pairs, and to inclusion of d‐type functions in the basis set are discussed. The utility and limitations of molecular orbital calculations, vibrational spectroscopy, microwave spectroscopy, direct kinetic measurements, and dynamic nuclear magnetic resonance (DNMR) spectroscopy as means for determining barriers to pyramidal inversion are discussed in context with a review of the highlights of experimental observations on the subject. Ambiguities that arise in the interpretation of barriers determined by DNMR are explored in detail. Factors that affect the magnitude of inversion barriers are discussed separately in four broad categories: steric effects; effects of conjugation (including (p–d)π conjugation) and hyperconjugation; effects of angular constraint; and effects of heteroatomic substitution. In the last category, critical reference is made to the question of electronegativity vs. lone pair‐lone pair repulsions, the problem of rotation vs. inversion, and the role of d orbitals.
Use of the IDC System
Authors: Lobeck Martin A.
Published: December 2003

The documentation system operated by the IDC Internationale Dokumentationsgesellschaft für Chemie mbH uses electronic data processing methods. It allows the storage and retrieval of structures, reactions, and nonstructural keyword information. Structures and reactions are coded as terms of a faceted classification, while keywords are stored in clear text. 900000 formulas from patent and journal publications, including many Markush formulas, have now been stored on magnetic tape. Searches in this store are carried out with IDC programs, which require an average computer time of less than 1 minute per inquiry (CPU time). The structure terms can be produced by computer from topological records. The field covered at present is low molecular weight organic chemistry, though extension to macromolecular chemistry is feasible, and is in fact planned.
The “Dokumentationsring der chemisch‐pharmazeutischen Industrie”; Aims and Methods
Authors: Nübling Wolfgang , Steidle Walter
Published: December 2003

The members of the Documentation Ring have chosen the “punched picture” method for coding of chemical and pharmaceutical literature. Chemical, biochemical, and medical facts gleaned from a publication are broken down into basic concepts, each of which is allocated a certain position on the punched card. This system has proved to be reliable over a period of twelve years.
Methods of Virus Formation
Authors: Sauer Gerhard
Published: December 2003

Viruses occur in a great variety of shapes and sizes, but for all their diversity in appearance they possess certain characteristics in common: all viruses contain a single nucleic acid molecule – deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) – surrounded by a protective protein coat. Among other things, the protein coat enables the genetic information stored in the nucleic acid to enter a host cell in a usable state, where it can initiate the reproduction of identical virus particles. After penetration of the cell the foreign genetic material of the virus particle first induces the synthesis of macromolecules not normally present in the cell: the viral nucleic acid undergoes replication and very many copies are produced, the protein of the virus coat is synthesized, and then these components are assembled to form a new generation of infectious virus particles. Most viruses also exhibit certain common structural features: their protein coat is built up from subunits arranged in helical or icosahedral fashion around the genetic material.
Nucleosides and Nucleotides as Potential Therapeutic Agents
Authors: Shen T. Y.
Published: December 2003

The present article summarizes recent progress in the study of nucleoside derivatives as antiviral and immunosuppressive agents. A number of 5′‐substituted 5′‐deoxy nucleosides have been found to be permeable and nonincorporable antimetabolites of 5′‐nucleotides. N6‐isopentenyladenosine and analogs show certain promising immunosuppressive activities. Encouraged by the antibody‐stimulating effect of oligonucleotides, we have developed a convenient synthesis of oligonucleotides using fully protected phosphorylated intermediates. A group of tetradeoxyribonucleotides, dApdApdApdX, was prepared for biological and physical evaluations. Nucleotide derivatives may prove valuable in the treatment of several immunological disorders.
Kinetic Evidence for Reactive Intermediates
Authors: Huisgen Rolf
Published: December 2003

Spectroscopic methods have recently been developed that allow the direct detection of reactive intermediates under favorable circumstances. The most general and reliable method, however, is still the indirect one, which makes use of the priciple of kinetic competition; this method is based on the freedom of the intermediate to choose between several reaction possibilities. The following discussion is addressed less to the specialist in reaction mechanisms than to the outsider who wishes to obtain some idea of the value and limitations of the kinetic method.
Metal‐Boron Compounds – Problems and Perspectives
Authors: Schmid Günter
Published: December 2003

“True” metal‐boron compounds have been known for about ten years. The bonding between the metal and boron atoms can vary widely in nature: Not only simple covalent bonds are encountered but also coordinate bonds and boron‐metal multicenter bonds. Furthermore, π complexes of transition metals with boron‐nitrogen systems and metal complexes containing boron(I) compounds as ligands have recently also been prepared.
Relations Between Structure and Reactivity in Free‐Radical Chemistry
Authors: Rüchardt Christoph
Published: December 2003

An analysis of the relations between structure and reactivity in free‐radical chemistry has shown that the usual interpretation of reactivities by means of the stability of the radicals involved is greatly simplified and often incorrect. The CX bond energies of the alkanes and simple alkyl derivatives can be explained qualitatively by strain effects in the ground state on the basis of the VSEPR theory and nonbonding interactions. To be able to explain reactivities in free‐radical chemistry, it is necessary to deduce information about the geometry of the transition states during free‐radical formation from experimental measurements. The relations between structure and reactivity in free‐radical chemistry are interpreted in terms of bond dissociation energies, as well as polar and steric effects.
Principles and Latest Developments in the Field of Metallic and Nonmetallic Hard Materials
Authors: Kieffer Richard , Ettmayer Peter
Published: December 2003

A survey of some fundamental aspects of the theory of hardness is followed by a discussion of recent developments in the field of sintered hard metal alloys. For example, good results have been obtained by the addition of hafnium carbide to hard metals. Niobium carbide can sometimes replace the more expensive tantalum carbide; vanadium carbide can be used to inhibit grain growth, especially in the case of ultrafine carbides. A brief outline is given of the results of investigations on ternary and quaternary systems with the components TiC, HfC, NbC, TaC, and WC. The utilization of sintered nitride and carbonitride hard metals in the field of machining appears feasible. Recently developed high‐pressure/high‐temperature autoclaves permit work on nitride systems under a high nitrogen pressure. The authors comment on the latest work in the field of non‐metallic hard materials and on diamond synthesis.
1,4‐Benzodiazepines. Chemistry and Some Aspects of the Structure‐Activity Relationship
Authors: Sternbach Leo H.
Published: December 2003

Numerous clinically used compounds having favorable tranquilizing and toxic properties possess the 1,4‐benzodiazepine skeleton. This group of active substances is readily accessible, e.g. by ring enlargement of quinazoline derivatives, by ring contraction of benzoxadiazocines, and by synthesis from aminobenzophenones, constructing the seven‐membered ring in many cases in one step. The relationships between type and position of substituents and the pharmacological properties are illustrated using 1,3‐dihydro‐5‐phenyl‐1,4‐benzodiazepin‐2‐ones as examples.
Influence of Ligands on the Activity and Specificity of Soluble Transition Metal Catalysts
Authors: Henrici‐Olivé G. , Olivé S.
Published: December 2003

The reaction of a covalently σ‐bonded ligand (alkyl group, hydrogen) with a molecule (olefin, CO) attached to the transition metal by a coordinate bond is the essence of many catalytic processes. The influence of the other ligands in the complex on this reaction is discussed in the present article. Various ways in which the ligands can act, e.g. by electronic effects via the σ and π electron systems of the complex and by steric effects, are first described separately and then illustrated by examples.
Bond Character of β‐Diketone Metal Chelates
Authors: Bock Bodo , Flatau Karsten , Junge Helmut , Kuhr Manfred , Musso Hans
Published: December 2003

The vibrational spectra of molecules labeled with 2H, 13C, and 18O show that delocalization of double and single bonds in the six‐membered ring is complete in all the metal chelates investigated of 2,4‐pentanedione (acetylacetone), but not in 2,4‐pentanedione itself. Mercury, on the other hand, is bonded to the central C atom of the ligands. The NMR spectra of the metal chelates of 3‐mesityl‐2,4‐pentanedione and 3‐anthryl‐2,4‐pentanedione show that the chelate rings have no magnetic anisotropy comparable with that of benzene. A critical appraisal is made of the questions whether it is at all justifiable to attribute any “aromatic character” to such molecules and why all comparisons with the chemical reactivity of benzene have so far led to controversial conclusions concerning the character of the bonding in the metal chelates of 2,4‐pentanedione.
The Synthesis of Antibiotic Sugars
Authors: Brimacombe J. S.
Published: December 2003

The term antibiotic sugar denotes the carbohydrate component of an antibiotic. Two alternative approaches are adopted for the synthesis of these neutral and amino sugars, many of which have unusual structures: One route starts from fairly simple non‐carbohydrate precursors, the other depending upon the modification of a carbohydrate precursor.
Synthetic Paper
Authors: Lunk Hans , Stange Karl
Published: December 2003

Synthetic paper can be made either by forming a web from synthetic fibers or by extruding a film from thermoplastic polymers. With suitable starting materials and appropriate treatment it is possible to equal the properties of conventional cellulose paper; in some respects, as in wet strength and dimensional stability, the synthetic papers are clearly superior.
Nutmeg as a Narcotic. A Contribution to the Chemistry and Pharmacology of Nutmeg (Myristica fragrans)
Authors: Kalbhen Dieter Abbo
Published: December 2003

The abuse of nutmeg for narcotic purposes has led to renewed chemical and pharmacological interest in this drug. Several allylbenzene derivatives whose biological transformation products have structures resembling mescaline and amphetamine have been identified as psychotropic constituents. It is suggested that the intensity of the hallucinogenic action of these compounds is due to the possibility of simulation of LSD‐like structural elements.
Methods and Applications of Nuclear Magnetic Double Resonance
Authors: von Philipsborn Wolfgang
Published: December 2003

In double resonance spectra, transitions between energy levels of a nuclear spin system are measured in the presence of two (or more) oscillating magnetic fields. Experiments of this nature form the basis of what is nowadays one of the most important techniques of NMR spectroscopy. Depending on the method selected, they can be used to unravel complex spectra, to measure hidden or weak resonances, or to determine the relative signs of coupling constants, as well as in stereochemical or kinetic studies. This wide and steadily growing range of applications of double resonance is described with the aid of specifilc examples.
Imaging and Analysis of Surfaces with a Scanning Electron Microscope and Electron Spectrometer
Authors: Holm Reimer
Published: December 2003

The topography and the composition of a surface are in many cases of equal importance (catalysis, electroplating, pretreatment of foils and sheet metal, corrosion, passivation, adsorption, coating of fibers, etc.), and this explains the great interest in methods of investigation that reveal both. If the demands on the resolving power, the analytical possibilities, and the thickness of the surface layer are not too exacting, combined devices like the scanning electron microscope and its analytical accessories can be used. When it is necessary to avoid the compromises involved in simultaneous imaging and analysis, the investigations must be carried out with separate equipment. As an example of a method for the analysis of surfaces we consider briefly photo‐ and Auger electron spectroscopy (ESCA).
Synthesis of Naturally Occurring Polypeptides, Problems of Current Research
Authors: Wünsch Erich
Published: December 2003

The synthesis of naturally occurring polypeptides has recently again become a center of interest. It therefore seems appropriate to make a critical appraisal of the present state of research, and at the same time to try to eliminate much of the element of chance from the solution of the problems that face us. An attempt will be made to group the preparation methods commonly used in the past into four strategies, to point out their advantages, and to uncover their disadvantages. The ultimate aim in the synthesis of naturally occurring peptides must always be a true, clean synthesis.
The Mode of Action of Hormones
Authors: Schauer Roland
Published: December 2003

Higher organisms having anatomically and functionally differentiated organ systems must be able to coordinate biochemical processes within and between the various organs or cells, even under rapidly changing environmental conditions. Hormones play an important part in the regulation processes: they promote numerous metabolic reactions and differentiation processes, and show considerable differences in the speed with which they act. Some hormones act almost instantaneously by stimulating adenyl cyclase and thus initiating the production of adenosine 3′,5′‐monophosphate (cyclic AMP), which passes on the hormone message (“second messenger”) to the intracellular metabolism by modifying enzyme activities. Delayed‐action hormones, mainly morphogenetic hormones, act primarily on the cell nucleus where they induce the synthesis of certain enzymes by activation of genes.
Structure and Function of Chromosomal Nucleohistones
Authors: Zimmermann Eberhard
Published: December 2003

The progress made in molecular biology has provided definite evidence that DNA is the genetic material of all cellular organisms. It is mainly localized within the cell nuclei and, in association with histones and acidic proteins, forms the fundamental structure of the chromosomes. The DNA‐protein complex, called nucleohistone or chromatin, represents the sum of all existing genes of one cell type and can be considered as a functional genetic unit. All genetic structures contain this fundamental element and only their steric arrangement or multiplicity determine the species‐specific differences in chromosomal morphology. The preservation of the three‐dimensional structure seems to be the function of histones which, in general, are associated with densely packed and metabolically inactive DNA sequences. The lysine‐rich histone I probably has a specific regulatory function since it inhibits the transcription of certain genes. Acidic proteins are mainly to be found in stretched chromosomal structures of high metabolic activity which accentuate the correlation between structure and function of nucleohistones. Approximately 50% of the DNA of the nucleohistone complex is not masked by proteins though only a small percentage of the free nucleotide sequences can be transcribed by RNA polymerase. The “open” structure of nucleohistones finds a plausible explanation in the fact that amino acid sequence analysis of several histones revealed an asymmetric distribution of charged groups and that therefore only these protein fractions can interact with DNA. In Crick's recently published model of the chromosomal organization the unique structure of histones is taken into account and it is assumed that the histone‐rich heterochromatin contains multiple but untranscribable control elements. Functional genes are represented by a single stranded DNA duplex within the interbands which utilize a special protein to preserve this unique conformation.
Structural Principles of Biomembranes
Authors: Kreutz Werner
Published: December 2003

The determination of the electron density distribution of the membrane cross sections of nerve myelin and the discs of the outer segments of retinal rods by a new X‐ray evaluation method indicates, as in the case of the cross‐sectional structure of the photosynthetic membrane, an “asymmetric” mass distribution. In connection with the detection of planar protein lattices in the membranes of photosynthetic bacteria, the chloroplasts of higher plants, erythrocytes, and retinal discs as well as the detection of a new category of lipid characteristics, comprising isothermal confromational and phase changes by specific ligand binding, a basis is established for the development of a general concept of biomembrane structure.
Conformational Analysis in Heterocyclic Systems: Recent Results and Applications
Authors: Eliel Ernest L.
Published: December 2003

This review deals with the following aspects of conformational analysis in saturated heterocyclic systems: 1. Conformational equilibria in 2‐aryl and 2‐ethynyl‐1,3‐dioxanes in various solvents. 2. The generalized anomeric effect in perhydro‐1,3‐diazines and its origin. 3. Unexpected conformational equilibria in 5‐heterosubstituted 1,3‐dioxanes. 4. Normal and anomalous solvent effects in conformational equilibria of 1,3‐dioxanes with heteroatoms at C‐5. Calculation of solvent effects. Correlation of ΔG0 as a function of solvent with solvatochromic transition energy ET (30); a new scale of “solvent polarity”. 5. Interplay of hydrogen bonding with dipolar, non‐bonded and eclipsing interactions in 5‐hydroxymethyl‐ 1,3‐dioxanes. 6. Highly stereoselective H‐D exchanges in anancomeric 1,3‐dithianes. 7. Potential distortion of a cyclohexane or 1,3‐dioxane ring by a tert‐butyl substituent. Evidence from 1H‐NMR and 13C‐NMR investigations and from X‐ray structure analysis.
1H‐NMR Spectra of the AA′XX′ and AA′BB′ Type – Analysis and Classification
Authors: Günther Harald
Published: December 2003

It is shown how the secular determinant for the general four‐spin case can be greatly simplified in the case of NMR spectra of the AA′XX′ and AA′BB′ type on the basis of molecular symmetry and with the aid of the so‐called X approximation. The methods for the direct analysis of these spectra are presented, and the relations between the type of spectrum and the chemical structure are discussed in detail for selected examples. In an appendix, parallels are pointed out between the formalism of the Hückel MO theory and the calculation of the eigenvalues and eigenfunctions of nuclear spin systems.
Structure Determinations of Nitrogen Compounds with the Aid of NQR, NMR, and ESCA Spectroscopy
Authors: Fitzky Hans G. , Wendisch Detlef , Holm Reimer
Published: December 2003

Since it is often necessary or desirable to determine the structures of compounds containing nitrogen directly via the nitrogen atoms, the nuclear quadrupole resonance (NQR) and the nuclear magnetic resonance (NMR) as well as the photoelectron and the Auger‐electron spectroscopy (ESCA) of nitrogen are becoming increasingly important. A comparative review of these three methods on the basis of measurement effect, information obtainable, applications, and limitations forms the subject of this article.
The Chemistry of Phenylcyclobutenediones
Authors: Ried Walter , Schmidt Arthur H.
Published: December 2003

Phenylcyclobutenediones are easily obtainable and very stable. Depending on the nature of the substituent R in position 3, they may be formally regarded as vinylogous carboxylic acid derivatives (Rhalogen, OCH3, OH, SH) or as α,β‐unsaturated diketo compounds (RH, alkyl, aryl). Their behavior in reactions justifies this approach. Ring contraction, ring expansion, and ring cleavage reactions are described.
Industrial Application of the Semiconductor Properties of Dyes
Authors: Meier Hans , Albrecht Wolfgang , Tschirwitz Ulrich
Published: December 2003

Effects of industrial interest have been detected in investigations on the dark conductivity and photoconductivity of dyes. For example, it was found that organic dyes can be used in the same way as inorganic photoconductors in vidicon television pickup tubes for the reproduction of images. Among other things, pointers were obtained to the formation of rectifying and photoelectrically sensitive pn junctions, to relatively high thermoelectromotive forces of doped and polymeric dyes, to the possibility of use in reprography, and to a high catalytic activity connected with the conductivity, which is already being utilized in fuel cells.
Thermally Reversible Photoisomerizations
Authors: Quinkert Gerhard
Published: December 2003

This article consists of a programmatic part and a pragmatic one. The programmatic aspects are the establishment of photochemistry, in the broadest definition of its scope, as a developing field directed toward the elucidation of photobiological phenomena, and an appeal for the wide development of the hitherto neglected dimension of electronically excited states. The pragmatic part is the detailed description of the photochemistry of linearly conjugated cyclohexadienones as an example of a thermally reversible photoisomerization system. The logical use of methods for obtaining information about the formal kinetics of light‐induced reactions and for the identification of short‐lived transients by low‐temperature spectroscopy is of value to both parts.
The Loose Connection between Electron Configuration and the Chemical Behavior of the Heavy Elements (Transuranics)
Authors: Jørgensen Christian Klixbüll
Published: December 2003

There is no straightforward relation between the ground electron configuration of the neutral atom and the chemistry of a given element. In particular, the choice of oxidation state depends on the comparison between consecutive ionization energies of the gaseous ions and the hydration energy or Madelung energy which by preference stabilizes increased oxidation numbers. Special ligands, such as oxide, may introduce unexpectedly high oxidation numbers. The baricenters of electron configurations are discussed with vanadium and thorium as specific examples. The chemistry of the elements with Z = 101, 102, 103, 104, 105, and 112 (of which some thousand atoms have been reported) and the (yet unknown) elements with Z=108, 109, 114, 126, 164, and 184 is discussed.
Semiempirical Use of Chirality Functions for the Description of the Optical Activity of Allene Derivatives in the Transparent Region
Authors: Ruch Ernst , Runge Wolfgang , Kresze Günter
Published: December 2003

A comparison of calculated and experimental data shows that the optical rotatory power of phenylallenecarboxylic acids is quantitatively described by the polynomial expression developed in (1). The Td component of this expression, the only relevant one for the case of methane derivatives turns out to be negligibly small in the case of allene derivatives. Therefore, and because allene derivatives belong to category a[16] the sequence of the λ‐parameter values of the ligands may be used to determine the configuration and the sign of rotation.
The Perturbation Treatment of Chemical Reactivity
Authors: Hudson Robert F.
Published: December 2003

The perturbation method of treating chemical reactions is discussed in a simple manner, by invoking the general hypothesis that the initial perturbation determines the course of a reaction. This approach is used to develop a general theory, as an alternative to the transition state theory. Such apparently diverse concepts as the symmetry rules for cyclic processes, and hard and soft acids and bases can be derived by this treatment, which is applied to radical reactions and aliphatic and aromatic substitution.
Chemistry and Biosynthesis of the Hypothalamic Releasing and Inhibiting Neurohormones
Authors: Folkers Karl , Johansson Nils‐Gunnar , Hooper Fred , Currie Bruce , Sievertsson Hans , Chang Jaw‐Kang , Bowers Cyril Y.
Published: December 2003

The hypothalamus secretes hormones which in turn affect the release of hormones from the anterior lobe of the pituitary gland. Evidence has so far been adduced for the existence of seven hypothalamic releasing hormones and three inhibiting hormones. These neurohormones are all oligopeptides and occur in only nanogram amounts in the hypothalamus. The study of synthetic analogs has provided valuable information regarding their mechanism of action.
Repair Processes on Deoxyribonucleic Acid
Authors: Berndt J.
Published: December 2003

Many cells have the ability to recognize and eliminate damage to their DNA, particularly thymine dimers formed by UV light. The elimination of this damage may be achieved by enzymatic, light‐dependent cleavage of the dimers into the monomers (photoreactivation) or more frequently by dark repair, in which the damaged part is completely removed from the, DNA. In this repair process, the DNA is incised by an endonuclease in the immediate vicinity of the thymine dimers. Oligonucleotides containing the thymine dimer are removed hydrolytically from the DNA by the 5→3′ exonuclease activity of DNA polymerase I (Kornberg enzyme). The resulting gaps are immediately closed by a de novo synthesis with the aid of the same DNA polymerase I, the complementary strand serving as a template (excision repair). The final step is the formation of the phosphodiester bond between the newly synthesized DNA fragment and the old DNA strand by a DNA ligase. Xeroderma pigmentosum patients lack the endonuclease as a result of a genetic defect; they therefore cannot eliminate thymine dimers from their DNA, and are extremely sensitive to sunlight. All information so far suggests that genetic recombination and DNA repair are performed by the same enzyme system.
Synthetic Precious Stones
Authors: Liebertz Josef
Published: December 2003

The first industrially feasible process for the synthesis of rubies was developed by Verneuil around the turn of the century. Progress in growing single crystals led in time to numerous other processes, some of which can be applied to the production of precious stones. In this way it became possible to synthesize the most important precious stones. Even crystals that do not occur in nature, but were originally of interest in other connections, have acquired importance for jewelry in recent years.
Direct Quantitative Photometry on Thin Layer Chromatograms
Authors: Hezel Ute
Published: December 2003

The principal methods for the direct photometric evaluation of thin layer chromatograms are discussed with the aid of examples. Since light is strongly scattered by TLC adsorbents, absorption measurements in reflection are preferable to transmission measurements. In the reflection measurements, an approximation method for the Kubelka‐Munk function leads to satisfactory straight calibration lines. The working range for quantitative reflection measurements is in the μg range, and the detection limit is of the order of 10−8g. The standard deviation for spots of equal concentration is less than ±1%. The signal‐to‐background ratio is improved by simultaneous measurement of reflection and transmission. The measurement of diminution in fluorescence (“fluorescence quenching”) is found to be less sensitive than the reflection at the absorption maximum. However, combined measurement of the reflection and the diminution of fluorescence gives an increase in sensitivity as compared to pure reflection measurements. As in solution photometry, fluorescence measurements are generally much more sensitive than absorption measurements. The working range is normally in the ng region, and the detection limit is around 10−11g. All the investigations described in this paper were carried out on the Zeiss chromatogram spectrophotometer.
The Cytochalasans: A New Class of Biologically Active Microbial Metabolites
Authors: Binder Michael , Tamm Christoph
Published: December 2003

A series of cytostatically active metabolites have been isolated from microorganisms in recent years. These compounds, which are chemically closely related, are of a new structural type. A highly substituted hydrogenated isoindolone unit is fused to an 11‐ to 14‐membered macrocycle. The large ring may be carbocyclic or heterocyclic (lactone or carbonic diester). The absolute configurations of two cytochalasans (phomin and a cytochalasin D derivative) have been established by X‐ray analysis. The biogenesis of phomin (cytochalasin B) is broadly known, and it is probable that all cytochalasans have a common biogenetic scheme. All the compounds of this class have a more or less pronounced specifically cytostatic action.
Iron‐Sulfur Proteins: Structural Chemistry of Their Chromophores and Related Systems
Authors: Mason Ronald , Zubieta J. A.
Published: December 2003

Intensive studies of iron‐sulfur proteins were begun only a decade or so ago but many biological and physicochemical data have since been accumulated and summarized[1–5]. As a result of the very recent X‐ray analyses of the structures of rubredoxin[6], ferredoxin (Peptococcus aerogenes)[7] and the Chromatium vinosum high potential iron protein (Hipip)[8], it has become possible to review our understanding of the nature and function of the inorganic chromophores in these proteins; to relate these findings to ‘model’ systems of varying relevance; but of more general interest, to comment on redox processes in biological systems particularly with respect to what might be termed electron transfer—allosteric effects in metalloenzymes.
Light Guide Systems for the Ultraviolet Region of the Spectrum
Authors: Dislich Helmut , Jacobsen Alfred
Published: December 2003

Whereas diverse and highly refined fiber optics have long been in technical use for the conduction of visible light, traditional but expensive silica glass lenses and prisms have had to be utilized until quite recently for the transmission of ultraviolet light. The reason for this was the lack of suitable materials. UV light cannot be transported to any technically useful extent either with pure glass‐glass (core and sheath materials) light guides or with plastic‐plastic light guides. UV light guides for wavelengths down to 200 nm can be produced by combination of a silica glass as the core material with a tetrafluoroethylene‐hexafluoropro pylene copolymer or methylpolysiloxane as the sheath material. Fiber optics of this type constitute a new aid for medical technology, medicine, and many technical and scientific fields, whose range of applications cannot yet be fully envisaged.
Extrachromosomal DNA in Bacteria
Authors: Goebel Werner
Published: December 2003

In addition to chromosomal DNA carrying the genetic information of the cell, many bacterial cells contain smaller circular DNA factors known as plasmids or episomes. These genetic elements endow the cell with additional biochemical capabilities. The fertility factors (F and F′), the antibiotic resistance factors (R), the colicinogenic factors (Col), the hemolytic factors (Hly), and other extrachromosomal DNA systems are described. These small DNA molecules can be isolated, and are therefore particularly suitable for the investigation of DNA replication and the stable establishment of genetic material in the bacterial cell.
Structural Information from the Habit of Crystals
Authors: Follner Heinz
Published: December 2003

The question whether information about the internal structure of crystals can be deduced from their external form, i.e. from their habit, has often been discussed. A rule formulated by Donnay and Harker attempts to establish the relationship on the basis of structural geometry. Deviations from this rule make it clear that purely geometric correspondence criteria are only of limited applilcation. Physical factors are also taken into account in the PBC vector method. A combination of the two methods can be of practical use in some cases for the determination of the crystal structures of organic compounds.
The Bacterial Flagella and the Flagellar Protein Flagellin
Authors: Bode Wolfram
Published: December 2003

In comparison with the relatively complicated flagella of all nucleated organisms, most of the actively motile bacteria have very much simpler locomotion organelles. These flagella are long thin filaments with a helical superstructure, anchored at one end in the cell membrane. Helical waves pass along the flagella in the distal direction during locomotion. In the flagella, identical structural units of a single protein species, flagellin, are linked to one another by non‐covalent bonds. The flagellin subunits can be separated from one another by mild methods, and their biochemistry as well as their structure and morphology can be investigated in the isolated state. Under certain conditions they can reaggregate in vitro to form polymorphous helical filaments, which are practically indistinguishable from intact flagella. In the flagellum, the elongated flagellin molecules have a layered arrangement in about ten parallel strands, which in turn form a hollow cylinder. Since this cylinder is deformed into a large helix, the individual chemically identical flagellin protomers and the protomer strands in the helical flagella are only quasi‐equivalent. The conformation and bonding pattern of the individual longitudinal flagellin lines can presumably be controlled in vitro by the basal structures, so that movement within the flagellum could conceivably occur as follows. A cyclically induced change in the length of the individual strands could result in an apparent rotation of the helical flagellum. According to hydrodynamic calculations, the resulting forward thrust is sufficient to propel the bacteria at the observed speeds of up to 50 μm/s.
Nuclear Medicine and Radiochemistry
Authors: Götte Hans , Kloss Gerhard
Published: December 2003

Nuclear‐medical diagnostic methods are widely used at present, examples being localization diagnosis e.g. of the thyroid, the kidney, and the spleen and function tests, e.g. on the thyroid and on the liver. For these tests it is essential to have organ‐specific vehicle substances that can be labeled with suitable radionuclides. For in‐vivo investigations, the exposure of the patient to radiation should be kept as small as possible, but the radiation must nevertheless be sufficient to allow the detection of the nuclide. Today, the therapeutic use of radionuclides is only small in comparison with their use in diagnosis.
Asymmetric Syntheses with the Aid of Homogeneous Transition Metal Catalysts
Authors: Bogdanović Borislav , Henc Biserka , Lösler Arnold , Meister Burkhard , Pauling Horst , Wilke Günther
Published: December 2003

The progress made in the field of homogeneous catalysis during the last five to six years has led, inter alia, to the development of highly selective catalysts for asymmetric syntheses. Homogeneous asymmetric hydrogenation, using well defined transition metal catalysts, may be achieved with optical yields of 85 to 90% or more. Catalytic reactions, in which the chiral centers are generated by CC bond formation, can result in optical yields of 70 to 80%. The hydrogenation catalysts consist primarily of rhodium(I) complexes containing “Homer phosphanes”, phosphanes with chiral C atoms, or optically active amides. Catalysts which induce optical activity through the formation of CC bonds have been developed from π‐allylnickel halides, Lewis acids, and phosphanes containing chiral C atoms. The results obtained signify a breakthrough in an area of catalysis previously restricted to syntheses involving enzymes.
Cyclooligomerization with Transition Metal Catalysts
Authors: Heimbach Paul
Published: December 2003

This report is concerned not only with the syntheses of new ring systems and the preparation of derivatives of known rings but also with new types of catalysts based upon, e. g., nickel, iron, molybdenum, manganese, and palladium. The broad applicability of the methylene insertion reaction as a method for identifying mono‐ and di‐methyl‐substituted eight‐, ten‐, and twelve‐membered rings is also demonstrated. A further section is concerned with the possible mechanism of CC bond formation in transition metal complexes; several earlier suggestions have been revised in the light of new experimental data. In addition, experimental results are discussed which indicate that the reactivity and selectivity of the complexes formed by the nickel‐ligand catalyst and olefins or alkynes depend upon the structure of the ligands.
The Variety of Thermal Pericyclic Reactions
Authors: Hendrickson James B.
Published: December 2003

The six‐electron thermal pericyclic reactions are examined systematically as to the number and kind which are possible by varying both the σ shell and the combinations of different atoms in all orientations, on both a six‐atom and a five‐atom framework. A simple unifying nomenclature is offered for these reactions, which number in the thousands. Further, in order to comprehend this very large number of possible reactions, they are also organized systematically in terms of their value for basic synthesis operations: construction, elimination, refunctionalization, etc. The methodology is aimed at providing a basis of selection for the invention of useful new reactions. A discussion of reaction energetics leads also to an analysis of molecular features which can facilitate reaction.
Mechanical Fabrication of Thermoplastic Polymers
Authors: Alfrey Turner
Published: December 2003

One of the most important features influencing the mechanical behavior of a fabricated thermoplastic item is the spatial arrangement of the polymer molecules. Inter alia processes developed for controlling the pattern of orientation of the polymer molecules are discussed.
Polymers in Medicine
Authors: Lyman Donald J.
Published: December 2003

The problems encountered in the use of artificial materials in the body are discussed, using as example the artificial heart. In the quest for suitable materials, numerous investigations have been carried out, including, inter alia, studies on the adsorption of proteins on polymer surfaces and on the passivation of such surfaces by coating with proteins or by the growth of cell cultures. The development of passive artificial substances is also discussed.
Biopolymers: Origin, Chemistry, and Biology
Authors: Calvin Melvin
Published: December 2003

A 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.
Trace Analysis of Organic Compounds
Authors: Beyermann Klaus
Published: December 2003

The trace analysis of organic compounds presents a number of additional difficulties in comparison with the trace analysis of inorganic compounds. Thus special problems arise in sampling and in the storage of samples (e. g. because of the instability of many organic compounds). It is also difficult to choose suitable mild separation methods and to find highly sensitive, molecule‐specific determination methods.
The Crystalline State of Macromolecular Substances
Authors: Zachmann H. G.
Published: December 2003

Unlike 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

Despite 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.