bluuebunny/crossref_metadata_2025_split huggingface dataset:
Structure and Molecular Stacking of Anthocyanins—Flower Color Variation
Authors: Goto Toshio , Kondo Tadao 
Published: December 2003

AbstractIn 1913 Willstätter 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. Willstätter 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. Willstätter'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.
Spectroscopy of Fluid Phases—The Study of Chemical Reactions and Equilibria up to High Pressures
Authors: Buback Michael 
Published: December 2003

AbstractThe properties of fluid phases can be altered considerably by the external conditions. Phase equilibria and chemical equilibria can be greatly affected, and it is possible to carry out chemical reactions by exploiting the special properties of compressed fluid phases. The use of high pressure in chemical reactions is of considerable diagnostic and preparative value. Applied research is directed towards elucidating the details of existing technical high pressure processes and to the development of novel fluid phase reactions where the application of high pressure is able to induce selectivity. In order to pursue these lines of research, and to study structure and dynamics throughout the entire range from gaseous to liquidlike states, it is important to have spectroscopic methods for characterizing systems at high pressures and temperatures. This article is concerned with quantitative absorption spectroscopy in the infrared to the ultraviolet spectral region at pressures up to about 7 kbar and temperatures up to 900 K.
The Chemistry of Rose Pigments
Authors: Eugster Conrad Hans , Märki‐Fischer Edith 
Published: December 2003

AbstractIn this article we present a survey of the pigments found in the flowers and fruits of old and modern varieties of roses. The yellow colors are produced by carotenoids, the reds by anthocyanins, and the modern oranges by a mixture of the two. The great structural diversity of the carotenoids contrasts with a surprisingly small number of anthocyanins. For the carotenoids found in roses, a clear correspondence exists between the structure and the breeding partners used; the old yellow roses, which arose from crosses with Chinese varieties, mainly contain carotenoids from early stages in the biosynthesis, while in the modern yellow roses, which are descended from Central Asian foetida types, hydroxylations, epoxidations, and epoxide transformations readily occur. A recently elucidated carotenoid degradation sequence follows the scheme C40 → C13 + C27 → C13 + C14. The C13 compounds are odoriferous substances that contribute to the scent of roses. In the physiological pH region, copigmentation with flavonol glycosides is crucial for stabilization of the anthocyanin chromophores. Many roses, including the “apothecary's rose”, which was once used medicinally, contain large amounts of strongly astringent ellagitannins, monosaccharide esters of gallic acid.
Transition‐Metal Thiolates: From Molecular Fragments of Sulfidic Solids to Models for Active Centers in Biomolecules
Authors: Krebs Bernt , Henkel Gerald 
Published: December 2003

AbstractThiolates are presently a subject of great interest in the chemistry of complexes involving transition‐metal elements and soft ligands. The manifold electronic and steric capabilities offered by the monodentate ligands RS⊖ and the bidentate chelate ligands ⊖SRS⊖ have been used to stabilize a broad spectrum of mononuclear, oligomeric, and polymeric complexes with new and remarkable structures and properties. Impetus has especially been provided by the synthesis of polynuclear cagelike homo‐ and heteroleptic metal–sulfur frameworks, which can often be regarded as “molecular fragments” of the structures of inorganic sulfides. Thiolates and mixed sulfide‐thiolates of the late open‐ and closed‐shell 3d metals (Fe, Co, Ni, Cu, Zn) and some of their homologues (Au, Cd, Hg), as well as of Mo, are of particular importance as model complexes for biologically important metal centers coordinated by sulfur. They have played an important role in increasing our understanding of the structure, bonding, and function of the reactive centers in ferredoxins, rubredoxins, nitrogenases, blue copper proteins, metallothioneins, and antiarthritic drugs.
Protease‐Catalyzed Kinetically Controlled Peptide Synthesis
Authors: Schellenberger Volker , Jakubke Hans‐Dieter 
Published: December 2003

AbstractIn spite of the enormous progress in the synthesis of peptides and proteins using commercial peptide synthesizers and the immense technological possibilities of recombinant DNA technology, a CN ligase is an indispensable tool for the racemization‐free fragment condensation of peptides. Since activation of the C‐terminal α‐carboxyl group of a peptide segment could cause partial racemization, chemical condensations of peptide fragments are prone to racemization. For the synthesis of the huge number of peptides and proteins, however, nature has only developed the ribosomal peptidyltransferase, which exhibits its full catalytic function independent of the side‐chain functions of the amino acids being coupled. However, its function requires coordination with numerous other ribosomal factors. Besides the limited possibilities of using multienzyme complexes of bacterial peptide synthesis systems, the only alternatives to peptidyltransferase are proteases, which, based on their in vivo function as hydrolases, cannot act as ideal ligases. However, by exploiting the intrinsic reversibility of hydrolytic reactions and by adjusting appropriate physicochemical reaction parameters, the protease acitivity can be used in the direction of ligation. Undoubtedly, the course of kinetically controlled, serine and cysteine protease‐catalyzed reactions can be more efficiently influenced than the equilibrium‐controlled protease‐catalyzed synthesis. This article describes the influence of the enzyme specificity on the efficiency of kinetically controlled synthesis and points the way toward a broad exploitation of serine and cysteine proteases for the catalysis of CN bond formation.
Organizing Principle of Complex Reactions and Theory of Coarctate Transition States
Authors: Herges Rainer 
Published: December 2003

AbstractThe majority of concerted reactions proceeds by a simultaneous breaking and making of only one bond at each atom involved in the reaction. If this process follows an alternating cyclic sequence, the reactions are called pericyclic. However, there is a hitherto never consistently characterized class of reactions that proceeds by breaking and making two bonds at one or more atoms at a time. The striking relationships that are purely descriptive and formal in the first instance are explained by a common physical principle. Theoretical insight not only permits predictions of mechanisms and stereochemistry; the qualitative theory is also used as an organizing principle for the known reactions and as a tool for predicting unprecedented reactions of this type.
Time‐Resolved Photoacoustic Calorimetry: From Carbenes to Proteins
Authors: Peters Kevin S. 
Published: December 2003

AbstractWhy have molecules only been seen but not heard? For over a century chemists have probed reactions with various spectroscopic methods to learn about structures, dynamics, and reactivities of their molecules. What they have not done is to listen to their molecules react. Although the photoacoustic phenomenon has been known since 1880, it is only in the last twenty years that technology has developed to the point where sound waves produced by reacting molecules can be time resolved and the information contained within the waves deciphered. The information content within the photoacoustic wave is indeed rich, for one can learn about the dynamics and the magnitude of enthalpy changes associated with the reaction as well as the changes in molecular volume. This review article chronicles the development of time‐resolved photoacoustic calorimetry and its application to a variety of reactions encountered in organic and organometallic chemistry and biochemistry.
Nitrogen Donors in Organometallic Chemistry and Homogeneous Catalysis
Authors: Togni Antonio , Venanzi Luigi M. 
Published: December 2003

AbstractHomogeneous catalysis has been responsible for many major recent developments in synthetic organic chemistry. The combined use of organometallic and coordination chemistry has produced a number of new and powerful synthetic methods for important classes of compounds in general and for optically active substances in particular. For this purpose, complexes with optically active ligands have been used, most of them coordinating through phosphorus. More recent developments have highlighted the use of “nitrogen‐donors”, particularly as they are easily obtained from the “chiral pool”. However, the remarkable achievements in this area have been based on an empirical approach. This article attempts to bridge the gap between the synthetic and the coordination chemist. The first section discusses the rates of formation and dissociation of complexes with nitrogen donors, their relationship to the rates of product formation, and presents the factors which induce homolytic cleavage of MC bonds. It also provides a summary of the main types of organometallic complexes formed by metal centers coordinated to nitrogen donors and their reactivity patterns. The second section highlights the most significant, homogeneously catalyzed reactions involving complexes with nitrogen ligands. Foremost among them are the asymmetric aspects of hydrogenation (particularly those involving boranes as reducing agents), hydrosilylation, cyclopropanations, Diels‐Alder reactions, aldol condensations, alkylation of aldehydes, conjugate addition reactions, Grignard cross‐coupling reactions, allylic alkylations, oxidation reactions, olefin epoxidations, and di‐hydroxylation of olefins.
Molecular Catalysts for Multielectron Redox Reactions of Small Molecules: The “Cofacial Metallodiporphyrin” Approach
Authors: Collman James P. , Wagenknecht Paul S. , Hutchison James E. 
Published: December 2003

AbstractThe role of metalloenzymes in important biological transformations has attracted increasing attention over the past several decades. Of the many chemical transformations mediated by enzymes, few are as challenging as multielectron redox reactions. Recent studies have revealed a partial structural and mechanistic description of these redox‐active metalloenzymes, but there is much still to be learned regarding the mechanisms of substrate transformation. Due to the complexity of the metalloenzyme systems, simplified model systems are employed to mimic structural or functional features of the enzyme. In multielectron redox enzymes, several metals are probably in‐volved in both substrate binding and the subsequent redox reactions. Thus, functional mimics of multielectron redox enzymes might also need two or more metal centers to be efficacious. The roles of multiple metal centers are to (1) increase the substrate's affinity for the catalyst, (2) increase the rate of electron transfer to the bound substrate, (3) increase the reactivity of the bound substrate, and (4) inhibit deleterious side reactions. Deter‐mining the importance of each factor may help in the development of these catalysts. Cofacial metallodiporphyrins, because of the control they provide over the geometric and electronic properties of the synthetic reaction center, are ideal bimetallic model complexes. The knowledge gained from model studies will help in understanding the mechanisms of metalloenzymes and can be used to design new homogeneous catalysts to effect multielectron transformations.
Synthesis and Reactions of Optically Active Cyanohydrins
Authors: Effenberger Franz 
Published: December 2003

AbstractCyanohydrins have always held a place of importance both as technical products and as reagents in organic chemistry. It is surprising, therefore, that optically active Cyanohydrins have been extensively investigated and employed for syntheses relatively recently. This can be explained by the fact that only in the past few years have enzymatic methods made chiral Cyanohydrins readily available in high optical purity. Chiral Cyanohydrins are widespread in nature in the form of the respective glycosides and serve roughly 3000 plants and many insects as antifeedants. For the preparative organic chemist, this class of compounds offers an enormous synthetic potential for making other chiral compounds accessible. In a few instances, the pharmacological principle of a drug also incorporates a chiral cyanohydrin as constitutive structural element. In the development of novel, physiologically active compounds all possible stereo‐isomers must be synthesized and investigated with respect to their activity range and the pathway of their metabolic transformations and/or degradation. The development of simple synthetic procedures for such compounds, which also entail a high degree of stereoselectivity, therefore has prime importance. To this end chiral Cyanohydrins may serve as stereochemically pure starting materials.In the present review, the following topics will be addressed: enantioselective addition of hydrogen cyanide (HCN), catalysed by the enzymes (R)‐and (S)‐oxynitrilase, to aldehydes and Ketones yielding (R) and (S) cyanohydrins, respectivity; enantioselective addition of HCN to aldehydes catalyzed by cyclic dipeptides; enantioselective esterification of racemic ocyanohydrins and enantioselective hydrolysis of cyanohydrin esters caytalyzed by lipases and esterases, reprectively; transformation of the nitrile groups of chiral cyanohydrins to provide optically active α‐hydroxycarboxylic acids, aldehydes, and ketones, as well as 2‐amino alcohols; sulfonylation of the OH group of chiral cyanohydrins to furnish optically active α‐sulfonyloxynitriles which undergo SN2 displacement of the activated OH group yielding α‐azido‐, α‐amino, and α‐fluoronitriles with inverted configuration.
Peptidomimetics—Tailored Enzyme Inhibitors
Authors: Gante Joachim 
Published: December 2003

AbstractPeptides and proteins (there is no clear boundary between the two classes of compounds) are absolutely essential components of organisms in many ways. While proteins have biocatalytic functions and are important components of tissues, peptides play an important role in the organism as hormones, neurotransmitters, and neuromodulators. Peptides and their analogues have long been used in medicinal chemistry as therapeutic agents for pathological conditions generally characterized by a disruption of the interplay between messenger molecules or enzyme substrates and their targets, the receptors and enzymes. For various biochemical and biophysical reasons there is an increasing tendency towards the use of chemical “Trojan horses” known as peptidomimetics. The chances that such agents are active generally increase with the magnitude of the “deceptive effect”, in other words in proportion to the degree of conversion of a peptide into a non‐peptide. Rational design has become a catchphrase which is at present applied frequently to the development of peptidomimetics. New computer programs are invaluable tools in such design processes. However, in spite of the many advances already made, we are still far from the final goal, the de novo design of peptidomimetics. Rational design is nonetheless advancing rapidly, and it is already clear that developments in the area of peptidomimetics have given a great boost to peptide chemistry as a whole. This can be expected to continue, so that in future peptide chemistry may be characterized by a type of symbiotic alliance between peptides and non‐peptides.
Recycling in the Earth's Atmosphere: The OH Radical—Its Importance for the Chemistry of the Atmosphere and the Determination of Its Concentration
Authors: Comes Franz Josef 
Published: December 2003

AbstractThe troposphere, the part of the earth's atmosphere which is closest to the surface, harbors important life processes. This important part of our environment contains primarily nitrogen and oxygen. In addition, it comprises a number of other substances, which are generally present in low concentrations. The primary reason for the low content of these trace components is the activity of a diatomic radical, namely the hydroxyl radical. OH is the main constituent of the oxidizing potential of the troposphere and causes the transformation of many trace components into water‐soluble forms, which can then rain out and be removed from the troposphere. OH has therefore acquired the title “detergent of the atmosphere”. The knowledge of local hydroxyl radical concentrations is therefore an important piece of information, especially with respect to model calculations for atmospheric chemistry. The OH concentration is very low and its determination represents a significant analytical challenge. It is controlled in a delicate manner by chemical formation and decomposition reactions, which interact to create a recycling process in the troposphere.
Vitamin B12: How the Problem of Its Biosynthesis Was Solved
Authors: Blanche Francis , Cameron Beatrice , Crouzet Joel , Debussche Laurent , Thibaut Denis , Vuilhorgne Marc , Leeper Finian J. , Battersby Alan R. 
Published: December 2003

AbstractVitamin B12 is an essential vitamin for human health, and lack of it leads to pernicious anemia. This biological activity has attracted intense interest for some time; in addition, the complex architecture of the B12 molecule has fascinated chemists and biochemists since its discovery as the first natural organocobalt complex and the establishment of its structure by X‐ray analysis. The organic ligand surrounding the cobalt displays many stereogenic centers along its periphery carrying reactive functional groups. This complexity led vitamin B12 to be rightly regarded as an extreme challenge to the synthetic chemist. Yet microorganisms achieve this synthesis in vivo with complete control of regio‐ and stereochemistry. How do they do it? This review tells the full remarkable story. Success in unraveling this biosynthetic puzzle resulted from a collaborative effort by biologists and chemists using the full range of methods available from their disciplines–from genetics at one end of the spectrum to synthesis and NMR spectroscopy at the other. This work can act as a guide for future research on the biosynthesis of yet more complex natural substances.
Chiral Recognition in π Complexes of Alkenes, Aldehydes, and Ketones with Transition Metal Lewis Acids; Development of a General Model for Enantioface Binding Selectivities
Authors: Gladysz J. A. , Boone Brian J. 
Published: December 2003

AbstractProchiral alkenes, aldehydes, and ketones constitute the most frequently used starting materials for enantioselective organic syntheses. Protocols often involve chiral binding agents or Lewis acids that can give two diastereomeric adducts, the ratios of which are measures of chiral recognition. With π adducts, the diastereomers differ in the enantioface of the CC or OC group bound to the Lewis acid. This review provides the first comprehensive analysis of such equilibria and related binding phenomena with chiral transition metal Lewis acids. An extensive body of data from the authors' laboratory for complexes of the pyramidal rhenium fragment [(η5−C5H5)Re(No)(PPh3)]+ (I) affords particular insight. Literature data for other complexes are also summarized. A general model for chiral recognition based upon the relative steric properties of four quadrants is presented. This enables binding selectivities to be individually and rationally optimized for different classes of ligands. Electronic effects are also identified and correlated with specific structural properties. Relationships between binding equilibria, reactivity, and product configurations are discussed.
Can behavioral evolution be measured on a staircase? a commentary
Authors: Moore Celia L. 
Published: December 2003

AbstractThe serious, comparative study of behavioral complexity that Greenberg et al. advocate is a progressive direction for the field, but their proposal to separate comparative psychology from its roots in evolutionary biology seems regressive. Modern evolutionary theory has been broadened within biology to include development and paleontology alongside natural selection, making closer integration with that discipline particularly timely. Such an integrated evolutionary approach in psychology would offer a useful alternative to the adaptationism popularized by evolutionary psychology. Although the differences between comparative psychologists and biologists may be blurred in the process, the behavioral sciences will be better served by a rich biological approach to evolution than by a uniquely psychological approach. © 2003 Wiley Periodicals, Inc. Dev Psychobiol 44: 16–20, 2004.
Neonatal novelty exposure, dynamics of brain asymmetry, and social recognition memory
Authors: Tang Akaysha C. , Reeb Bethany C. 
Published: December 2003

AbstractBrief and transient early‐life stimulation via neonatal handling and neonatal novelty exposure can lead to differential changes within the right and left brains. In rats, these lateralized changes have been demonstrated behaviorally, neuroanatomically, and neurophysiologically. Recently, we found that neonatal novelty exposure can prolong the duration of social recognition memory from less than 2 hr to at least 24 hr among male rats reared in social isolation and that this enhancement is associated with an initial right‐turn preference in a novel testing cage. In contrast to stable forms of asymmetry, such as handedness, we show that this turning asymmetry is dynamic—decreasing as the animal adjusts to the novel testing environment over a 2‐day period. This change in turning asymmetry was found only among animals that experienced neonatal novelty exposure during the first 3 weeks of their lives. Furthermore, individual differences in short‐term social recognition memory for a conspecific can be predicted by this change in functional asymmetry. © 2003 Wiley Periodicals, Inc. Dev Psychobiol 44: 84–93, 2004.