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Showing papers in "ChemInform in 1991"


Journal ArticleDOI
TL;DR: The authors provides a concise summary of the concepts, limitations, and paradigms of biological self-assembly processes, and surveys covalent selfassembly processes in the synthesis of 3-dimensional molecules and ordered assemblies.
Abstract: This article first provides a concise summary of the concepts, limitations, and paradigms of biological self-assembly processes, then surveys covalent self-assembly processes in the synthesis of 3-dimensional molecules and ordered assemblies

608 citations


Book ChapterDOI
TL;DR: In this article, the Bragg and Williams approximation of the Ising model is used to describe spin-state transitions in metal complexes which are driven by a change of temperature T or pressure p are always associated with a considerable reorganization of molecular geometry, the change involves metal-ligand bond lengths R, bond angles and a variation of ligand orientation.
Abstract: Spin-state transitions in metal complexes which are driven by a change of temperature T or pressure p are always associated with a considerable reorganization of molecular geometry. The change involves metal-ligand bond lengths R, bond angles, and a variation of ligand orientation. In particular, the elongation 4R by up to ∼ 10% occurring in the course of the LS → HS conversion produces an expansion of molecular volume ΔV ≌ 25 A3 per metal atom. The average crystal structure for given values of T and p is reproduced by the fractional occupancy of the individual structures of the high-spin (HS) and low-spin (LS) isomer. The transitions are reasonably well described by a number of theoretical models which are equivalent to the Bragg and Williams approximation of the Ising model. The dynamics of the spin-state transitions in solution, based on measurements by ultrasonic and photo-perturbation techniques, is in general rapid with rate constants between 4 × 105 and 3 × 108 s−1. Similar results are obtained for the spin conversion in solid complexes where the line shape analysis of Mossbauer spectra based on the theory of Blume and Tjon is applied. The dynamics may be rationalized employing one-dimensional cross sections through Gibbs free-energy surfaces G = G(R), an alternative being the comparison of the results with quantum-mechanical calculations for a radiationless non-adiabatic multiphonon process.

361 citations


Journal ArticleDOI
TL;DR: The Hammett equation has been widely used for the study and interpretation of organic reactions and their mechanisms as discussed by the authors, and it is astonishing that u constants, obtained simply from the ionization of organic acids in solution, can frequently predict successfully equilibrium and rate constants for a variety of families of reactions in solution.
Abstract: The Hammett equation (and its extended forms) has been one of the most widely used means for the study and interpretation of organic reactions and their mechanisms. Although the Hammett methodology has been criticized by theoreticians because of its empirical foundation, it is astonishing that u constants, obtained simply from the ionization of organic acids in solution, can frequently predict successfully equilibrium and rate constants for a variety of families of reactions in solution. Almost every kind of organic reaction has been treated via the Hammett equation, or its extended form. The literature is so voluminous and extensive that there is no complete review of all that has been accomplished. Hammett's success in treating the electronic effect of substituents on the rates and equilibria of organic reactions1P2 led Taft to apply the same principles to steric and inductive and resonance effects? Then, more recently, octanol/ water partition coefficients (P) have been used for rationalizing the hydrophobic effects of organic compounds interacting with biological systems? The use of log P (for whole molecules) or n (for substituents), when combined with electronic and steric parameters, has opened up whole new regions of biochemical and pharmacological reactions to study by the techniques of physical organic chemistry.sf3 The combination of electronic, steric, hydrophobic, hydrophilic, and hydrogen-bonding7 parameters has been used to derive quantitative structure-activity relationships (QSAR) for a host of interactions of organic compounds with living systems or parts thereof. The binding of organic compounds to proteins,8 their interaction with enzymess and with cellsloJ1 and tiasues,12 their inhibition of organelles,l' and as antimalarial^'^

243 citations


Book ChapterDOI
TL;DR: In this article, the general rules governing the fragmentation reactions, the types of functional groups present, as well as the manner in which they are joined to form a unique molecular structure may be deduced from the mass spectrum.
Abstract: For over three decades, mass spectrometry has played a central role for the structure elucidation of natural products because it can be used to determine the molecular weights and elemental compositions of substances with a wide range of chemical and physical properties In addition, if the molecular ion is formed with sufficient excitation energy, it will undergo a series of unimolecular reactions to give fragment ions, the relative abundances of which are normally unique to the structure of the molecular ion By understanding the general rules governing the fragmentation reactions, the types of functional groups present, as well as the manner in which they are joined to form a unique molecular structure may be deduced from the mass spectrum

211 citations




Journal ArticleDOI

159 citations


Journal ArticleDOI
TL;DR: The removal of such harmful substances from effluents and waste waters by microbe-based technologies may provide an alternative or additional means of metal/radionuclide recovery for economic reasons and/or environmental protection.
Abstract: Bacteria, and other microorganisms, exhibit a number of metabolism-dependent and-independent processes or the uptake and accumulation of heavy metals and radionuclides. The removal of such harmful substances from effluents and waste waters by microbe-based technologies may provide an alternative or additional means of metal/radionuclide recovery for economic reasons and/or environmental protection. Both living and dead cells as well as products derived from or produced by microorganisms can be effective metal accumulators and there is evidence that some biomass-based clean-up processes are economically viable. However, many aspects of metal-microbe interactions remain unexploited in biotechnology and further development and application is necessary, particularly to the problem of radionuclide release into the environment.

157 citations


Journal ArticleDOI
TL;DR: Ethanol produced from cellulosic biomass is examined as a large-scale transportation fuel in this article, where desirable features include ethanol's fuel properties as well as benefits with respect to urban air quality, global climate change, balance of trade, and energy security.
Abstract: Ethanol produced from cellulosic biomass is examined as a large-scale transportation fuel. Desirable features include ethanol's fuel properties as well as benefits with respect to urban air quality, global climate change, balance of trade, and energy security. Energy balance, feedstock supply, and environmental impact considerations are not seen as significant barriers to the widespread use of fuel ethanol derived from cellulosic biomass. Conversion economics is the key obstacle to be overcome. In light of past progress and future prospects for research-driven improvements, a cost-competitive process appears possible in a decade.

156 citations





Journal ArticleDOI
TL;DR: A number of important photophysical properties of C{sub 60} have been determined, including its lowest triplet state energy (near 33 kcal/mol), lifetime, and triplet-triplet absorption spectrum.
Abstract: A number of important photophysical properties of C{sub 60} have been determined, including its lowest triplet-state energy (near 33 kcal/mol), lifetime, and triplet-triplet absorption spectrum. The triplet state is formed in near quantitative yield and produces a very high yield of singlet oxygen by energy transfer. C{sub 60} does not react with singlet molecular oxygen and quenches it only slowly by an unknown mechanism. These results are discussed in terms of the unusual geometry of this molecule.

Journal ArticleDOI
TL;DR: 9-[(dimethylamino)methyl]-10-hydroxycamptothecin (4, SK&F 104864) for development as an antitumor agent demonstrated broad-spectrum activity in preclinical tumor models and is currently undergoing Phase I clinical trials in cancer patients.
Abstract: Water-soluble analogues of the antitumor alkaloid camptothecin (1) were prepared in which aminoalkyl groups were introduced into ring A or B. Most of the analogues were prepared by oxidation of camptothecin to 10-hydroxycamptothecin (2) followed by a Mannich reaction to give N-substituted 9-(aminomethyl)-10-hydroxycamptothecins (4-12) or by subsequent modification of Mannich product 4 (13, 15, 17, 19, 21). Others were obtained by modification of the hydroxyl group of 2 (25,26) or by total synthesis (35,42,43). These analogues, as well as some of their synthetic precursors, were evaluated for inhibition of topoisomerase I, cytotoxicity, and antitumor activity. Although there was not a quantitative correlation between these assays, compounds that inhibited topoisomerase I were also cytotoxic and demonstrated antitumor activity in vivo. Further evaluation of the most active water-soluble analogue led to the selection of 9-[(dimethylamino)methyl]-10-hydroxycamptothecin (4, SK&F 104864) for development as an antitumor agent. In addition to its water solubility, ease of synthesis from natural camptothecin, and high potency, 4 demonstrated broad-spectrum activity in preclinical tumor models and is currently undergoing Phase I clinical trials in cancer patients.






Book ChapterDOI
TL;DR: 7-hydroxycoumarin [umbelliferone, (2)], one of the most widely distributed coumarins, is often regarded as the parent, in a structural sense and also biogenetically, of a large number of the structurally more complex coumarin.
Abstract: Over the past 150 years, since Vogel in 1820 isolated the parent oxygen heterocycle, coumarin (1) from Coumarouna odorata = Dipteryx odorata (1), coumarins have been recognized as widely distributed plant products. The vast majority carry an oxygen substituent at C-7; consequently 7-hydroxycoumarin [umbelliferone, (2)], one of the most widely distributed coumarins, is often regarded as the parent, in a structural sense and also biogenetically, of a large number of the structurally more complex coumarins. A number of review articles on natural plant coumarins have appeared (154, 302, 344, 420, 525, 541, 548) since the last in this series (153), the most comprehensive, albeit dealing solely with coumarins of the Umbelliferae, being that of Nielsen in 1970 (419).

Journal ArticleDOI
David E. Cane1

Journal ArticleDOI
TL;DR: The reverse transcriptase assay developed for the detection of the enzyme in virions involving polyadenylic acid (poly rA.oligodeoxythymidylic acid) and radiolabeled thymidine 5'-triphosphate (TTP) can be applied as a simple method for screening the HIV-1 RT inhibitory potential of natural products as discussed by the authors.
Abstract: Inhibition of human immunodeficiency virus reverse transcriptase is currently considered a useful approach in the prophylaxis and intervention of acquired immunodeficiency syndrome (AIDS), and natural products have not been extensively explored as inhibitors of this enzyme. We currently report that the reverse transcriptase assay developed for the detection of the enzyme in virions involving polyadenylic acid.oligodeoxythymidylic acid (poly rA.oligo dT) and radiolabeled thymidine 5'-triphosphate (TTP), can be applied as a simple method for screening the human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT) inhibitory potential of natural products. As reported herein, 156 pure natural products have been examined in this system. Benzophenanthridine alkaloids such as faragaronine chloride [1] and nitidine chloride, which are known inhibitors of avian myeloblastosis virus reverse transcriptase, demonstrated potent activity in the HIV-1 RT system, and 1 (IC50 10 micrograms/ml) was adopted as a positive-control substance. Additional inhibitors found were columbamine iodide [2] and other protoberberine alkaloids, the isoquinoline alkaloid O-methylpsychotrine sulfate [3], and the iridoid fulvoplumierin [4]. A number of indolizidine, pyrrolizidine, quinolizidine, indole, and other alkaloids, as well as compounds of many other structural classes, were tested and found to be inactive. A total of 100 plant extracts have also been evaluated, and 15 of these extracts showed significant inhibitory activity. Because tannins and other polyphenolic compounds are potent reverse transcriptase inhibitors, methods were evaluated for the removal of these from plant extracts prior to testing. Polyphenolic compounds were found to be responsible for the activity demonstrated by the majority of plant extracts. After appropriate tannin removal procedures were established, the bioassay system was shown to be generally applicable to both pure natural products and plant extracts. The method also proved useful in directing an isolation procedure with Plumeria rubra to yield fulvoplumierin [4] as an active compound (IC50 45 micrograms/ml).

Journal ArticleDOI
TL;DR: Organic solar cells (OSC) as discussed by the authors use polymers instead of semiconductors, such as silicon or gallium arsenide, which are used in traditional or inorganic solar cells.
Abstract: Solar cells are devices that utilize the light energy of the sun and convert it into electrical energy, which is needed for powering any electronic device. While organic solar cells(OSC) have the same fundamental structure as traditional or inorganic solar cells(ISC), OSCs use polymers instead of semiconductors, such as silicon or gallium arsenide, which are used in ISCs. While the efficiencies of OSCs are still somewhat low in relation to other solar cells, their advantages, such as being light in weight, flexible, low in material and processing costs, failure resistant, and relatively environmentally friendly make them a worthy competitor of ISCs. Considering fossil fuels are limited and alternate forms of energy need to be considered, OSCs will play a major role in supplying power to people around the world.

Journal ArticleDOI
TL;DR: In this paper, a simple, inexpensive way of preparing a ceramic material that contains such weak interfaces is described, where Silicon carbide powder is made into thin sheets which are coated with graphite to give weak interfaces and then pressed together and sintered without pressure.
Abstract: THE major problem with the use of ceramics as structural materials is their brittleness. One way of overcoming this problem is to introduce weak interfaces which deflect a growing crack1. Polymer composites of this sort can be easily prepared by surrounding fibres with liquid plastic. To make similar structures with ceramic matrices and fibres is difficult and expensive, however, because traditional ceramic processing techniques of powder compaction and sintering prevent densification and cause cracking2–4. Here we describe a simple, inexpensive way of preparing a ceramic material that contains such weak interfaces. Silicon carbide powder is made into thin sheets which are coated with graphite to give weak interfaces and then pressed together and sintered without pressure. Relative to the monolithic material, the apparent fracture toughness for cracks propagating normal to the weak interfaces is increased more than fourfold, and the work required to break the samples increases by substantially more than a hundredfold. The technique should be readily applicable to other ceramics.

Journal ArticleDOI
TL;DR: In this paper, the color variation and stabilization of anthocyanins in aqueous solution could have other causes, namely self-association, copigmentation and intramolecular sandwich-type stacking.
Abstract: In 1913 Willstatter made the striking observation that the same pigment can give rise to different colors. Thus, the same pigment, cyanin, is found in the blue cornflower and in the red rose. Willstatter attributed the variety of flower colors to different pH values in solution. Indeed, anthocyanin changes its color with pH; it appears red in acidic, violet in neutral, and blue in basic aqueous solution. Willstatter's pH-theory for explaining flower color variation is still to be found in major text books of organic chemistry. Very recently, however, reinvestigation has disclosed that the color variation and stabilization of anthocyanins in aqueous solution could have other causes, namely self-association, copigmentation and intramolecular sandwich-type stacking. The stacking would be mainly brought about by intermolecular or intramolecular hydrophobic interaction between aromatic nuclei such as anthocyanidins, flavones and aromatic acids. In addition, hydrogen bonds and charge transfer interactions may also be involved. The most interesting molecular complexes of anthocyanins are the metalloanthocyanins such as commelinin and protocyanin (blue cornflower pigment). These seemingly pure blue complexes each consist of six anthocyanin and six flavone molecules and two metal ions; their molecular weight is nearly 10000. A structure is proposed for commelinin.


Journal ArticleDOI
TL;DR: The role of hydrogen bonding in catalysis has been discussed in this article, although mainly in terms of the salicylate ion as a leaving group, and it seems likely that this will herald other systems where the role of a strong hydrogen bond may serve as the key step in a catalytic process.
Abstract: Publisher Summary This chapter discusses the hydrogen bonding, which has been recognized as the single most important intermolecular interaction, The chapter presents the hypothesis that there are three kinds of hydrogen bond: weak, strong, and very strong. These are determined by the shape of the potential energy well and the respective positions of hydrogen and deuterium within the well, which can be used to provide information about the well that applies in a particular example. The role of hydrogen bonding in catalysis has been discussed, although mainly in terms of the salicylate ion as a leaving group. With its strong intramolecular hydrogen bond playing an essential part in the reaction mechanism, it seems likely that this will herald other systems where the role of a strong hydrogen bond may serve as the key step in a catalytic process.