bluuebunny/crossref_metadata_2025_split huggingface dataset:
Protein Folding and Protein Association
Authors: Jaenicke Rainer 
Published: December 2003

AbstractThe biosynthesis of proteins leads to linear chain molecules with a well‐defined amino acid sequence that is unambiguously determined by a corresponding polynucleotide sequence at the level of the gene. The underlying mechanism of transcription and translation is based upon the complementarity of pyrimidine and purine bases, on the one hand, and the genetic code, on the other. As indicated by denaturation/renaturation experiments, the one‐dimensional structural information encoded in the polypeptide chain generates a unique three‐dimensional structure. In consequence, one would expect a “folding code” to exist which governs the spontaneous acquisition of the thermodynamically stable “native” structure of a given protein. The rate of folding (in vivo as well as in vitro) is fast, despite the astronomically high number of potential conformations. The mechanism of folding must therefore be kinetically controlled. Whether the native structure represents the “global” or a “local” minimum on the energy hyperspace is an open question. Above a critical size, protein molecules tend to be organized as assemblies made up of identical or non‐identical subunits. The observed uniformity of a given “quaternary structure” requires a close correlation of folding and association as well as a high specificity of subunit recognition. Both are corroborated by in vitro reconstitution experiments.
Carbene Complexes in Organic Synthesis [New Synthetic Methods (47)]
Authors: Dötz Karl Heinz 
Published: December 2003

AbstractTransition metals are finding increasing use in organic synthesis on the borderline between “organic” and “inorganic” chemistry. Advantage is taken thereby of the fact that metal‐induced CC bond formation often takes place with remarkable selectivity. The rapid development that has taken place in this area of chemistry is clearly demonstrated by the carbene complexes, examples of which are now known for almost all transition elements, and which have transformed from organometallic curiosities into synthetically useful reagents in less than two decades since the first studies of E. O. Fischer. They are not only suitable as carbene‐transfer agents but also undergo interesting cycloadditions with other ligands in the co‐ligand sphere. Their manipulation requires techniques no more complicated than those for Grignard reactions. Thus, carbene complexes can also be used in the synthesis of natural products such as vitamins or antibiotics.
Asymmetric Diels‐Alder and Ene Reactions in Organic Synthesis. New Synthetic Methods (48)
Authors: Oppolzer Wolfgang 
Published: December 2003

AbstractRapidly accumulating evidence shows the value of asymmetric Diels‐Alder and ene reactions for the syntheses of enantiomerically pure molecules. This article presents a systematic and critical treatment of various stereoface‐differentiating principles, including very recent spectacular advances. The chiral information is mainly provided by covalently bound auxiliaries, some of which are crystalline, inexpensive, and readily available from naturally occurring monoterpenes, hydroxy acids, amino acids, steroids, and sugars. Non‐destructive transfer of chirality leads to the efficient creation of up to four chiral centers with predictable relative and absolute configurations. Regenerative cleavage of the auxilary group from the diastereomerically pure adducts furnishes a range of polyfunctional, optically pure building blocks. Their synthetic potential is illustrated by strategic applications to the syntheses of physiologically interesting, chiral natural products such as prostaglandins, antibiotics, terpenoids, shikimic acid, alkaloids, and kainoids.
Consequences of Strain for the Structure of Aliphatic Molecules
Authors: Rüchardt Christoph , Beckhaus Hans‐Dieter 
Published: December 2003

AbstractThe chemist is accustomed to deriving structures and preferred conformations of organic compounds from rigid molecular models and standard values for bond lengths, bond angles, and torsional profiles. In the case of strained compounds, this rigid structural model has to be abandoned and replaced by a flexible one which takes individual conditions of strain into consideration. It is shown, on the basis of new experimental structure data, that the force field method is suitable and highly reliable for the calculation of structural parameters and preferred conformations of strained compounds. It is, therefore, capable of replacing the rigid molecular model. Furthermore, the systematic analysis of strain induced angle and bond deformation gives a new pivot for the development of a qualitative discussion of deformation in strained molecules and hence for improved conformational analysis. — In the course of this work we were able to isolate two rotamers of D,L‐3,4‐di(1‐adamantyl)‐2,2,5,5‐tetramethylhexane; this is the first isolation of a rotamer pair of an aliphatic hydrocarbon.
Selective Reactions Using Organoaluminum Reagents [New Synthetic Methods (54)]
Authors: Maruoka Keiji , Yamamoto Hisashi 
Published: December 2003

AbstractIn addition to their high oxyphilicity, organoaluminum compounds are endowed with ambiphilic character. These properties can be successfully utilized in developing new synthetic reactions with unique selectivities. Especially noteworthy are the organoaluminum‐promoted Beckmann rearrangement of oxime sulfonates, new syntheses of polyamino macrocycles via reductive cleavage of aminals and amidines by diisobutylaluminum hydride, the diastereoselective cleavage of chiral acetals by organoaluminum compounds leading to optically active secondary alcohols, allylic alcohols, and β‐substituted carbonyl compounds, and biomimetic terpene syntheses. These and other examples, which illustrate the characteristics of organoaluminum chemistry, are used to demonstrate the distinct advantages of organoaluminum reagents in selective organic synthesis.
Error Rates of the Replication and Expression of Genetic Information
Authors: Englisch Uwe , Gauss Dieter , Freist Wolfgang , Englisch Sabine , Sternbach Hans , von der Haar Friedrich 
Published: December 2003

AbstractBiological evolution is based on continual changes in the genomes of cells. If stable organisms are to develop nonetheless in the course of this process, low error rates (high accuracy) in both the replication of genes and their transcription and translation into proteins have to be assured. The overall error rates of these processes are measurable and can be estimated: they result from the error rates of the individual enzyme‐catalyzed steps. In many cases, for example in the discrimination of the amino acids valine and isoleucine during protein biosynthesis, it is not possible to achieve a sufficient accuracy only on the basis of the difference of the free energies of binding of the correct and incorrect substrates (the two amino acids differ only in a CH2 group). The necessary low error rate is maintained by an additional proofreading step, which is carried out by the enzymes involved after formation of the enzyme‐substrate complex, either during catalysis or before product release. In this step, an incorrect intermediate or product is hydrolyzed. The energy input necessary for the synthesis of the incorrect intermediate or product, which is provided by hydrolysis of adenosine or guanosine triphosphate, is the price paid for the low error rates. In this article, the proofreading mechanisms of several enzymes—mainly those involved in DNA replication and the aminoacylation of transfer RNA during protein biosynthesis—are discussed.
The Role of Hydrothermal Synthesis in Preparative Chemistry
Authors: Rabenau Albrecht 
Published: December 2003

Abstract“Hydrothermal synthesis” usually refers to heterogeneous reactions in aqueous media above 100°C and 1 bar. The previously common distinction between hydrothermal conditions below and pneumatolytic conditions above the critical point is no longer made, since no discontinuities are observed upon exceeding the critical conditions. Under hydrothermal conditions, reactants otherwise difficult to dissolve go into solution as complexes, in whose formation water itself or very soluble “mineralizers” can participate. Thus, one can obtain the conditions of chemical transport reactions,[1] of which hydrothermal syntheses can be considered a special case. During recent decades in the geological sciences—in which the method is also historically rooted—it has received a strong impulse, whose effect on preparative solid state chemistry is discussed here.
[3+2] Cycloaddition Approaches to Five‐Membered Rings via Trimethylenemethane and Its Equivalents [New Synthetic Methods (55)]
Authors: Trost Barry M. 
Published: December 2003

AbstractThe power of cycloadditions for ring formation derives from the potential for chemo‐, regio‐, diastereo‐, and enantioselectivity. Trimethylenemethane (TMM) and its equivalents offer the possibility of extending such benefits to the synthesis of five‐membered rings. Three types of conjunctive reagents appear to be promising candidates: (1) selected 4‐alky‐lidene‐4,5‐dihydro‐3H‐pyrazoles, (2) 2‐[(trimethylsilyl)methyl]allyl esters and halides, and (3) alkylidenecyclopropanes. Thermal reactions, especially intramolecular ones, effect the cycloaddition of the pyrazoles to olefins–especially electron‐poor olefins. The latter two precursors require a catalyst, notably a palladium complex, to unlock the synthetic potential. With the 2‐[(trimethylsilyl)methyl]allyl esters, a general chemo‐, regio‐, and diastereoselective cycloaddition with typical Diels‐Alder dienophiles forms methylenecyclopentanes in both inter‐ and intramolecular fashion. A tin analogue allows extension of this cycloaddition to aldehydes, and, in a related sequence, to ketones and imines to form 3‐methylenete‐trahydrofurans and 3‐methylenepyrrolidines with excellent diastereoselectivity. The 2‐[(tri‐methylsilyl)methyl]allyl esters also serve to effect net methylenecyclopentane annulation to cyclic enols. Such adducts can further undergo three‐carbon intercalation or combined ring contraction‐spiroannulation simply by the use of either a nucleophilic or electrophilic trigger. Alkylidenecyclopropanes undergo cycloadditions to both electron‐rich and electron‐poor olefins when stimulated by metal catalysts, especially palladium. A complementary regiochemistry to the approach based upon silicon conjunctive reagents is observed. The availability of these cycloadditions provides new strategies for the synthesis of natural products.
Cluster Beam Chemistry—from Atoms to Solids
Authors: Martin T. Patrick 
Published: December 2003

AbstractHow do the properties of a solid gradually evolve as atoms are brought together to form increasingly larger units? In order to answer this question one must study aggregates of atoms too large to be called molecules but still too small to have even the structure of a crystal. Recent advances in both theory and experiment have made possible some first explorations in this long neglected but fascinating field of study. This review will emphasize the generation, growth, and properties of homo‐ and heteronuclear clusters in the vapor phase, particular attention being paid to mass spectrometric investigations.
Orthophosphates in the Ternary System Al2O3P2O5H2O
Authors: Kniep Rüdiger 
Published: December 2003

AbstractOrthophosphates of aluminum have been the subject of numerous studies since the middle of the nineteenth century. They have acquired importance in such diverse fields as the earth sciences, chemical engineering, and medicine. The chemical problems associated with this Al2O3P2O5H2O system mainly arise from the marked tendency to form polymeric aluminum orthophosphate complexes in solution; equilibrium is reached only with difficulty, and crystallization of single phases is the exception rather than the rule. However, the crystal structures of several aluminum orthophosphates (including minerals) have been elucidated, and it is now possible to give a systematic description of this class of compounds and the conditions necessary for their formation.
Photochemistry of Transition‐Metal Atoms: Reactions with Molecular Hydrogen and Methane in Low‐Temperature Matrices
Authors: Ozin Geoffrey A. , McCaffrey John G. , Parnis J. Mark 
Published: December 2003

AbstractReactions of metal atoms with molecular hydrogen and alkanes are prototypical of more complex systems involving metal‐mediated reactions in solution, on supports, and on surfaces. The simplicity of metal‐atom reactions makes them particularly attractive for fundamental experimental and theoretical studies. Following the first reports of H2 and CH4 activation by photoexcited metal atoms in cryogenic matrices, the behavior of a number of transition‐metal atoms in their ground and selected electronically excited states has been examined with molecular hydrogen, methane, and ethane. A wealth of structural, electronic, reactivity, and selectivity information is emerging from these studies and it is now becoming possible to recognize the controlling factors at work in these fundamental chemical reactions. In this article, some experimental results for a number of metal‐atom systems, in particular, those involving silver, copper, manganese, and iron atoms, are presented along with a discussion of the factors that are believed to be important in the understanding of the observed physical and chemical behavior.
New Reductive Transformations of Unsaturated Cyclic Hydrocarbons [New Synthetic Methods (66)]
Authors: Müllen Klaus 
Published: December 2003

AbstractThe successive reduction of fully conjugated cyclic hydrocarbons leads to singly and multiply charged ions with unusual bonding. The charge distribution in these ions can be determined spectroscopically, and the information so obtained is then used in kinetically controlled trapping reactions for the regioselective introduction of electrophilic groups. When non‐benzenoid substrates are used, syntheses become possible which can either not be carried out or can only be carried out with great difficulty in other ways. Examples of new preparative applications are cycloannelation and bridging reactions as well as polymerization reactions. The ion pair structure of the intermediate and the type of electrophile used are of paramount importance in controlling the mechanism of these reductive transformations.
Allylamine Derivatives—a New Class of Active Substances in Antifungal Chemotherapy
Authors: Stütz Anton 
Published: December 2003

AbstractFungal infections (mycoses) are found throughout the world. Only a few structural classes of compounds currently satisfy the demands of modern chemotherapy in their treatment; hence, the quest for new types of active substances is of major scientific and therapeutic importance. The first representative of a new class of substances–the allylamine derivatives–namely naftifine ((E)‐N‐methyl‐N‐(1‐naphthylmethyl)‐3‐phenylallylamine) was discovered by accident and has recently become commercially available as a topical antimycotic. The exploration of structure‐activity relationships on the basis of naftifine and new synthetic strategies led to the discovery of the currently most active compound of this type, terbinafine, the first pharmaceutical agent to contain a (E)‐1, 3‐enyne structural element. Terbinafine exhibits considerably higher activity than the original “lead” structure naftifine both in vitro and in vivo; it is also up to one order of magnitude more effective than standard preparations in various chemotherapeutic animal tests after topical or oral administration. According to clinical experience gained so far, terbinafine is well tolerated and shows promising activity against various types of mycoses. Allylamines act as potent, selective inhibitors of fungal squalene epoxidase via a novel mechanism, which, unlike in the case of other inhibitors of steroid biosynthesis, does not depend on cytochrome P450.
The Pyrolysis of Azides in the Gas Phase
Authors: Bock Hans , Dammel Ralph 
Published: December 2003

AbstractThe chemistry of the non‐metallic elements has in recent years passed through a period of rapid development, often referred to as its “renaissance”. To emphasize just one of the key facets: numerous short‐lived molecules containing multiple bonds to elements of the third and higher periods have been discovered, often accompanied by the planned synthesis of derivatives which are sterically shielded by bulky groups and thus kinetically stabilized. Thus today molecules such as silabenzenes H6C6−nSin and silaethenes H2SiCH2 or R2SiCR2, disilenes R2SiSiR2 and diphosphenes RPPR, silaphenylisonitrile H5C6NSi, or methylidyne‐phosphanes RCP, are all well‐known species. Sandwich compounds with P6 rings or silicon centers demonstrate that there are now hardly any barriers to impede the imagination of the non‐metal chemist. In sharp contrast is our lack of knowledge regarding the “microscopic” pathways of chemical reactions: thus apart from information provided for example by molecular beam experiments, or from exact numerical calculations involving species consisting of only a few atoms, it remains largely unknown from which directions medium‐sized molecules must approach each other to successfully collide and form a “reaction complex”, in which way their structures are changed in such a process or which role is played by molecular dynamics in the energy transfer.–The pyrolysis of azides XN3, i.e. compounds which tend to explode violently when ignited in the condensed phase but can be heated in low‐pressure gas flow systems without much risk, illustrates that studies of reactive intermediates are of interest not only because novel molecules may be discovered and isolated, and thereby possibilities for synthesis expanded. Moreover, some aspects of the “microscopic” pathways of these azide pyrolyses can be described satisfactorily on the basis of calculated energy hypersurfaces, and the influence of molecular dynamics becomes experimentally visible in the “chemical activation” of intermediates which leads to their “thermal explosion”.
The Use of Light in the Study of Interfacial Electrochemical Reactions
Authors: Gerischer Heinz 
Published: December 2003

AbstractSpectroscopic analysis techniques have become particularly important in the study of interfacial electrochemical processes since it has become possible to increase their sensitivity to the extent where changes occurring in the absorption of light by fractions of monolayers can be detected. At the same time the development of new techniques in surface physics, though most of these can only be used in ultra‐high vacuum, has also had a considerable influence on electrochemistry since it has enabled studies to be carried out on electrodes after transfer to UHV. The availability of well‐defined semiconductors in which photocurrents can be generated by the absorption of light has led to the development of photoelectrochemical methods that can be used for the study of electrode reactions at semiconductors or at metal surfaces which are covered with a semiconducting film. Finally, it has been observed during the study of semiconductor electrodes that redox reactions at an electrode can result in the generation of luminescence. This, in turn, can lead to important information on the mechanism of the charge transfer process. These recent developments are reviewed in the present article.
Enzymic Baeyer–Villiger Oxidations by Flavin‐Dependent Monooxygenases
Authors: Walsh Christopher T. , Chen Y.‐C. Jack 
Published: December 2003

AbstractThere has recently been a revival of interest in microbial biochemistry. Part of this resurgence is due to the increased presence of man‐made organic compounds in the biosphere and the resultant interest in microorganisms which can degrade these xenobiotic molecules. Also, advances in genetic engineering have raised the possibility of utilizing the chemical machinery of bacteria for commercial profit and social benefit. The Baeyer‐Villiger reaction is a useful transformation in organic synthesis which provides chemists with a gentle means of converting ketones into esters or lactones. The reaction, however, suffers from the problems of low yield and the need, in some cases, to utilize harsh conditions. There exist bacteria, capable of growth on aliphatic molecules, that contain enzymes which can catalyze Baeyer‐Villiger reactions. These enzymes, known as monooxygenases, are involved in the breakdown of acyclic and alicyclic ketones to provide simpler carbon units for further catabolism. The gamut of reactions catalyzed by some of these enzymes is remarkable. This diversity, plus their availability in pure form in quantity by genetic engineering raises the possibility that these biocatalysts can be useful as reagents in organic synthesis.
Pheromones in Nanogram Quantities: Structure Determination by Combined Microchemical and Gas Chromatographic Methods [New Analytical Methods (35)]
Authors: Attygalle Athula B. , Morgan E. David 
Published: December 2003

AbstractWhat can a chemist do with a few nanograms of valuable organic substance, pure or impure? The answer may be, a great deal. Its components may be quantified, separated, isolated, degraded, derivatized, extracted by solvent partition, as well as the more obvious recording of their mass spectra. Provided a substance is not contaminated by large quantities of solvent, manipulations are possible without much loss or dilution. Necessity is the mother of invention. The examples given in this review show what can be accomplished today in nanochemistry and should act as a stimulus to direct the scientist to discover other applications appropriate to his needs and his circumstances. The examples are largely taken from the field of insect chemistry, but their general applicability is emphasized. This review illustrates how gas chromatography and reaction gas chromatography, alone or combined with mass spectrometry are potent tools in the hands of the chemist to deduce the unique structure of volatile organic substances available only in nanogram quantities.
The Cyclopropyl Group in Studies of Enzyme Mechanism and Inhibition
Authors: Suckling Colin J. 
Published: December 2003

AbstractThe cyclopropyl group—particularly in compounds such as cyclopropylmethanol, phenylcyclopropylamine, and cyclopropanone hydrate—has proved to be very useful in the study of enzyme reactions. Whether the cyclopropane ring is opened to form a radical or remains intact can provide much information about reactions occurring at the active site.
Tellurium‐Rich Tellurides
Authors: Böttcher Peter 
Published: December 2003

AbstractThe tellurides represent a class of compounds which exhibit a wide range of physical and chemical properties. In addition to the salt‐like compounds, some tellurides show metallic properties while others have strong covalent bonds. Like tellurium itself, many of these are semiconductors, e.g., the II‐VI compounds CdTe and MnTe, and in this respect are of special interest. Other tellurides, e.g., NbTe4 and ZrTe5, have low‐dimensional electronic transport properties. Some ternary transition metal‐main group metal tellurides appear in an amorphous state with spin glass properties. The tellurium‐rich tellurides are characterized without exception by directed Te‐Te bonds. It turns out that general structural chemical relationships exist between them which allow these different compounds to be treated as a single comprehensive class of substances. They represent a link between the Zintl phases on the one hand, and molecular compounds on the other.