Bullatantriol,a sesquiterpene from annona bullata

Bullatantriol has been isolated from Annona bullata. Its constitution and relative configuration have been established by X-ray analysis. Its absolute configuration has been assigned by chiroptical investigation of the corresponding 7-ketone.     

Chemical modification of neamine

The aminocyclitol antibiotic neamine has been modified by changing the configuration of one or two hydroxyl groups of the aminocyclitol moiety to elucidate the relationship between configuration and antimicrobial activity. 5-Epi-, 6-epi-, and 5,6-diepineamine have been prepared and their antimicrobial activity has been determined against several micro-organisms.     

Identification and determination of absolute and anomeric configurations of the 6-deoxyaltrose residue found in polysialoglycoprotein of Salvelinus leucomaenis pluvius eggs. The first demonstration of the presence of a 6-deoxyhexose other than fucose in glycoprotein

An oligosaccharide alditol, dHex-GalNAc-Gal-Gal-GalNAcol, has been isolated from polysialoglycoprotein, which was derived from the unfertilized eggs of Savelinus leucomaenis pluvius (a salmonid fish, Iwana in Japanese), by alkaline borohydride treatment followed by exhaustive digestion with sialidase. First, the structure of the terminal dHex residue in the above pentasaccharide has been assigned as 6-deoxyaltrose (= dAlt in pyranoid form) by a combination of structural methods (GLC, TLC, mass spectrometry, and 400-MHz 1H-NMR spectroscopy). The occurrence of a 6-deoxyhexose other than L-fucose in glycoprotein has not been previously reported. Next, the absolute configuration of this unusual sugar residue has been assigned as D on the basis of the exciton-splitting study of tris-p-bromobenzoate derivatives of methyl 6-deoxyaltrosides. The usefullness of this circular dichroic exciton-splitting method in the determination of the absolute configuration of carbohydrate components, only available in minute amounts, is emphasized. The anomeric configuration of the glycosidic linkage of the D-altropyranosyl residue was deduced from 400-MHz 1H NMR spectroscopy. The 6-deoxy-beta-D-altropyranosyl residue thus established has the same configuration as alpha-L-fucose but with the C-5 methyl group inverted, suggesting that the biosynthetic incorporation of D-dAlt parallels that of L-fucose, and a possible pathway is also considered.     

Synthesis and absolute configuration of optically pure enantiomers of a kappa-opioid receptor selective agonist

The enantiomers of U50,488, ligands highly selective for kappa-opioid receptors, have been prepared by a refined procedure and their optical purity demonstrated. The absolute configuration of (+)-trans-2-pyrrolidinyl-N-methylcyclohexylamine, a chemically versatile intermediate for synthesis of analogs of kappa-opioid receptor ligands with defined chirality, has been determined to be 1S,2S by X-ray crystallographic analysis. This intermediate has been used to synthesize the optically pure U50,488 enantiomers with known absolute configuration.     

Absolute configuration determination of isoflavan-4-ol stereoisomers

Elucidation of the correct stereochemistry of the metabolite is essential for the mechanistic study of bioactive compounds. Isoflavan-4-ol has the same chiropical chromophore as THD, the biosynthetic precursor of the potent phytoestrogen S-equol. Interested in the correct absolute configuration of isoflavan-4-ol stereoisomers and to compare the available practical approaches for the absolute configuration determination, complete absolute configuration analysis of isoflavan-4-ol stereoisomers has been carried out with by means of ECD and VCD spectroscopy as well as modified Mosher method. Theoretical TD-DFT computations resulted in a poor simulation of the observed experimental ECD spectra, and thus inconclusive absolute configuration assignments of isoflavan-4-ol stereoisomers were obtained. However, DFT-assisted VCD spectroscopic analyses successfully determined correct absolute configurations, and further confirmed by modified Mosher method.     

HPLC study on the 'history' dependence of gramicidin A conformation in phospholipid model membranes

A novel HPLC methodology for the study of gramicidin A reconstituted in model membranes has been tested in comparison with circular dichroism data. It is shown that this chromatographic technique not only corroborates most of the recent spectroscopic results but allows one to explain them in terms of mass fractions of different actual conformational species of GA in the phospholipid assemblies. In particular, the dependence of the inserted peptide configuration on the organic solvent and other parameters involved in the 'history' of the sample preparation and handling has been analyzed by HPLC in two phospholipid model systems: small unilamellar vesicles and micelles. Moreover, a slow conformational transition of GA towards a beta 6.3-helical configuration, accelerated by heat incubation, has been also chromatographically visualized and quantitatively interpreted.     

Acetoxonium ions from acetoxyoxiranes and orthoesters: their conversion into ethylidene acetals,rearrangement, and solvolysis

The acid-catalysed ethylidenation of some methyl pentopyranosides has been studied and the configuration at the acetal carbon atom assigned by p.m.r. spectroscopy. There is a strong preference for the isomer in which the methyl group has the endo configuration. Several cyclic alkyl orthoacetates derived from methyl pentopyranosides have been prepared by orthoester exchange and the endo C-methyl isomer shown to preponderate. Treatment of vicinal acetoxyoxiranes and orthoacetates with boron trifluoride followed by lithium borohydride, or with diborane, yields ethylidene acetals in which the C-methyl group is endo. Rearrangements of the hexachloroantimonate salts of acetoxonium ions derived from methyl lyxo- and arabino-pyranosides, possessing trans-vicinal acetoxyl groups, have been studied. The ions having the arabino configuration are preferred in both the α and β series. The reaction of cyclic orthoacetates of methyl β-L-arabinopyranoside and some derivatives with dry acetic acid proceeds via an acyclic acetoxonium ion to yield only products having the L-arabino configuration.     

Ordering of p–n-alkoxybenzoic acids at phase transition temperatures: a comparative computational analysis

A comparative analysis of molecular ordering of nematogenic p-n-alkoxybenzoic acids has been carried out with respect to translatory and orientational motions for the acids with seven (7OBAC), eight (8OBAC), nine (9OBAC) and 10 (10OBAC) carbon atoms in the alkyl chain. The CNDO/2 method has been used to compute the net atomic charge and dipole moment components at each atomic center. Modified Rayleigh-Schrodinger perturbation theory with multicentered-multipole expansion method has been used to evaluate long-range intermolecular interactions while a '6-exp' potential function has been assumed for short-range interactions. The total interaction-energy values obtained by these computations were used to calculate the probability of each configuration at the phase-transition temperature using the Maxwell-Boltzmann formula. The flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. A comparative picture of molecular parameters like total energy, binding energy and total dipole moment has been given. An attempt has been made to explain the nematogenicity of these acids in terms of their relative order with the molecular parameter introduced in this paper.     

Stereochemical course of phosphokinases. The use of adenosine [gamma-(S)-16O,17O,18O]triphosphate and the mechanistic consequences for the reactions catalyzed by glycerol kinase, hexokinase, pyruvate kinase, and acetate kinase.

We report the synthesis of adenosine [gamma-(S)-16O,17O,18O]triphosphate, an isotopically labeled species of ATP that is chiral at the gamma-phosphoryl group, the configuration of which has been confirmed by independent stereochemical analysis. This molecule has been used as a substrate in the reactions catalyzed by glycerol kinase and by acetate kinase. The resulting samples of isotopically labeled sn-glycerol 3-phosphate and of acetyl phosphate have been used as substrates in the alkaline phosphatase mediated transfer of the chiral phosphoryl groups to (S)-propane-1,2-diol, whence the configuration at phosphorus has been determined [Abbott, S. J., Jones, S. R., Weinman, S. A., & Knowles, J. R. (1978) J. Am. Chem. Soc. 100, 2558]. It is shown that glycerol kinase and acetate kinase (and, by virtue of an earlier correlation, pyruvate kinase and hexokinase) proceed by pathways that result in inversion of the configuration at phosphorus. The sterochemical approach provides an access to the otherwise cryptic events that are involved in phosphoryl-group transfer within the ternary complexes of these kinases and their substrates.     

Stereochemistry of phosphoryl transfer

A general method has been developed for the synthesis of chiral [16O,17O,18O]phosphate monoesters of known absolute configuration. An analytic method for determining the absolute configuration of chiral phosphate esters has also been developed, which is based on the isotope effects of 17O and 18O at phosphorus in the 31P nuclear magnetic resonance spectrum. These methods have shown that phosphoryl transfer catalysed by hexokinase, phosphofructokinase and pyruvate kinase occurs with inversion of configuration. This is most simply interpreted as an "in-line' transfer of the phosphoryl group between substrates in the enzyme-substrate ternary complex.     

Lilioside C,a glycerol glucoside from Lilium lancifolium

A new glycerol glucoside, lilioside C, has been isolated from the leaves and stems of Lilium lancifolium. Its structure including the configuration of the aglycone moiety has been elucidated by chemical and spectroscopic means and by its synthesis.     

Determination of the anomeric configuration of D-xylose with D-xylose isomerases.

The anomeric configuration of D-xylose, resulting from hydrolysis of β-D xylopyranosides by β-D-xylosidase from Bacillus pumilus, has been determined by an enzymic procedure, based on the stereospecificity of D-xylose isomerases. The initial hydrolysis product is α-D-xylose. β-D-Xylosidase from Bacillus pumilus thus acts by inversion of configuration in contrast to most other glycosidases.     

The Habitat Amount Hypothesis implies negative effects of habitat fragmentation on species richness

The habitat amount hypothesis (HAH) predicts that species richness in a habitat site increases with the amount of habitat in the ‘local landscape’ defined by an appropriate distance around the site, with no distinct effects of the size of the habitat patch in which the site is located. It has been stated that a consequence of the HAH, if supported, would be that it is unnecessary to consider habitat configuration to predict or manage biodiversity patterns, and that conservation strategies should focus on habitat amount regardless of fragmentation. Here, I assume that the HAH holds and apply the HAH predictions to all habitat sites over entire landscapes that have the same amount of habitat but differ in habitat configuration. By doing so, I show that the HAH actually implies clearly negative effects of habitat fragmentation, and of other spatial configuration changes, on species richness in all or many of the habitat sites in the landscape, and that these habitat configuration effects are distinct from those of habitat amount in the landscape. I further show that, contrary to current interpretations, the HAH is compatible with a steeper slope of the species–area relationship for fragmented than for continuous habitat, and with higher species richness for a single large patch than for several small patches with the same total area (SLOSS). This suggests the need to revise the ways in which the HAH has been interpreted and can be actually tested. The misinterpretation of the HAH has arisen from confounding and overlooking the differences in the spatial scales involved: the individual habitat site at which the HAH gives predictions, the local landscape around an individual site and the landscapes or regions (with multiple habitat sites and different local landscapes) that need to be analysed and managed. The HAH has been erroneously viewed as negating or diminishing the relevance of fragmentation effects, while it actually supports the importance of habitat configuration for biodiversity. I conclude that, even in the cases where the HAH holds, habitat fragmentation and configuration are important for understanding and managing species distributions in the landscape.     

Test of the Inverse Monte Carlo Method for the Calculation of Interatomic Potential Energies in Atomic Liquids

Abstract

The principle purpose of this paper is to demonstrate the use of the Inverse Monte Carlo technique for calculating pair interaction energies in monoatomic liquids from a given equilibrium property. This method is based on the mathematical relation between transition probability and pair potential given by the fundamental equation of the “importance sampling” Monte Carlo method. In order to have well defined conditions for the test of the Inverse Monte Carlo method a Metropolis Monte Carlo simulation of a Lennard Jones liquid is carried out to give the equilibrium pair correlation function determined by the assumed potential. Because an equilibrium configuration is prerequisite for an Inverse Monte Carlo simulation a model system is generated reproducing the pair correlation function, which has been calculated by the Metropolis Monte Carlo simulation and therefore representing the system in thermal equilibrium. This configuration is used to simulate virtual atom displacements. The resulting changes in atom distribution for each single simulation step are inserted in a set of non-linear equations defining the transition probability for the virtual change of configuration. The solution of the set of equations for pair interaction energies yields the Lennard Jones potential by which the equilibrium configuration has been determined.     

SC-CNNs for chaotic signal applications in secure communication systems

In this paper a CNNs based circuit for the generation of hyperchaotic signals is proposed. The circuit has been developed for applications in secure communication systems. An Saito oscillator has been designed by using a suitable configuration of a four-cells State-Controlled CNNs. A cryptography system based on the Saito oscillator has been implemented by using inverse system synchronization. The proposed circuit implementation and experimental results are given.     

电子圆二色谱技术在微生物次级代谢产物结构研究中的应用研究进展

微生物次级代谢产物的化学结构十分复杂,对其绝对构型的确定十分困难。近年来,电子圆二色谱(electronic circular dichroism,ECD)由于其用量少、精度高等优点,在测定绝对构型方面的应用越来越多,已经成为研究微生物次级代谢产物结构的重要方法。本文就电子圆二色谱在微生物次级代谢产物结构研究中的应用进行综述,以期为今后的研究奠定基础。     

A cross-flow reactor with immobilized pectolytic enzymes for juice clarification

Summary A new system for continuous juices clarification is presented. The bioreactor combines microporous plates commercially available and industrial pectinases immobilized on nylon membranes in a cross-flow configuration. The kinetic behaviour of the reactor for different recirculation flow rates has been determined. Fresh apricot juice has been continuously clarified in the bioreactor with excellent results.     

Synthesis and structures of new salen complexes with bulky groups

A new series of Schiff base complexes [Fe(III), VO(II), Pd(II), Cu(II), and Ni(II)] has been developed. The ligand possesses bulky t-pentyl groups at the 3- and 5-positions. The iron (III) complex is obtained in monomeric form with a square-pyramidal configuration while the copper complex is with square-planar configuration.     

Circular dichroism, a powerful tool for the assessment of absolute configuration of flavonoids

Circular dichroism is a powerful tool for establishing the absolute configuration of flavonoids and proanthocyanidin analogues. It has been utilized to study the configuration of flavanones, dihydroflavonols (3-hydroxyflavanones), flavan-3-ols, flavan-4-ols, flavan-3,4-diols, flavans, isoflavans, isoflavanones, pterocarpans, 6a-hydroxypterocarpans, rotenoids, 12a-hydroxyrotenoids, neoflavonoids, 3,4-dihydro-4-arylcoumarins, 4-arylflavan-3-ols, auronols, homoisoflavanones, proanthocyanidins, and various classes of biflavonoids. Results relevant to the correlation of circular dichroic data and the absolute configuration of the diastereoisomers of some of the above classes of compounds will be discussed.