Homomorphisms to oriented cycles

We discuss the existence of homomorphisms to oriented cycles and give, for a special class of cyclesC, a characterization of those digraphs that admit, a homomorphism toC. Our result can be used to prove the multiplicativity of a certain class of oriented cycles, (and thus complete the characterization of multiplicative oriented cycles), as well as to prove the membership of the corresponding decision problem in the classNPcoNP. We also mention a conjecture on the existence of homomorphisms to arbitrary oriented cycles.     

Configuration and E/Z interconversion mechanism of O(S)-allyl-S(O)-methyl-N-(acridin-9-yl) iminothiocarbonate

The configuration and dynamic behavior of O-allyl-S-methyl-N-(acridin-9-yl)iminothiocarbonate (1) and its S-allyl-O-methyl regioisomer (2) were studied using quantum chemical calculations and by applying a novel graphical method to scatter maps obtained from MD simulations for evaluation of an NOE-weighted internuclear distance (r(NOE)). Energy calculations indicated that the Z configuration was predominant for each compound and, further, this was supported both by the calculated chemical shifts and the r(NOE). Both N-inversion- and rotation-type transition-state structures were also calculated for the E/Z isomerization process, the results indicating that the preferred interconversion mechanism for 1 is N-inversion, but contrastingly, interconversion via rotation is equally as probable as N-inversion for 2. This supports the notion that one or the other or both pathways can be active and each system needs to be assessed on a case-by-case basis.     

Unsymmetrical Benzotristhiazole: A New Electron‐Withdrawing Core for Octupolar Chromophores

A six‐step synthesis of the unsymmetrical trimethylbenzotristhiazole has been developed. Starting from 2‐methylbenzothiazole following nitration, reduction, acetylation, thionation, and twofold cyclization, the desired trimethylbenzotristhiazole was obtained in good yield. Its condensation with donor‐substituted benzaldehydes presents the way to new octupolar chromophores. The attempt to synthesize such benzotristhiazole from dinitroaniline failed; this procedure afforded a new benzimidazole derivative.     

Ultrasensitive impedimetric lectin based biosensor for glycoproteins containing sialic acid

We report on an ultrasensitive label-free lectin-based impedimetric biosensor for the determination of the sialylated glycoproteins fetuin and asialofetuin. A sialic acid binding agglutinin from Sambucus nigra I was covalently immobilised on a mixed self-assembled monolayer (SAM) consisting of 11-mercaptoundecanoic acid and 6-mercaptohexanol. Poly(vinyl alcohol) was used as a blocking agent. The sensor layer was characterised by atomic force microscopy, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The biosensor exhibits a linear range that spans 7 orders of magnitude for both glycoproteins, with a detection limit as low as 0.33 fM for fetuin and 0.54 fM for asialofetuin. We also show, by making control experiments with oxidised asialofetuin, that the biosensor is capable of quantitatively detecting changes in the fraction of sialic acid on glycoproteins. We conclude that this work lays a solid foundation for future applications of such a biosensor in terms of the diagnosis of diseases such as chronic inflammatory rheumatoid arthritis, genetic disorders and cancer, all of which are associated with aberrant glycosylation of protein biomarkers.

Synthesis and Characterization of the Crystalline Methyl α-Glycoside of the Repeating Unit of the O-Polysaccharide of Vibrio Cholerae O:1

Abstract

There are more than eighty serotypes of Vibrio cholerae, all causing disease with symptoms of Asian cholera. Systematic prevention of cholera by immunization has not yet been achieved because of a lack of a protective vaccine. Vibrio cholerae 0:1 Gramnegative bacteria occur as two immunologically distinct strains: Ogawa and Tnaba. The lipopolysaccharide (LPS) of both strains seem to contain the same 0-polysaccharide antigen consisting^3,4 of (1+2)-a-linked 4-amino-4,6-dideoxy-a-D-mannopyranosyl residues the amino groups of which are acylated with 3-deoxy-L-glycero-tetronic acid. Although the chemical structure of the 0-polysaccharides has been known5 since 1979, the synthesis of its monomeric repeating unit was reported only in 1988.     

The Reaction of Methyl 2,4,6-Tri-0-benzoyl-3-0-benzyl-β-D-galactopyranoside with 1,1-Dichloromethyl Methyl Ether

When methyl 2,4,6-tri-0-benzoyl-3-0-benzyl-β-D-galacto-pyranoside (JO is treated in purified chloroform at 55-60′ with excess of dichloromethyl methyl ether in the presence of a catalytic amount of freshly fused zinc chloride for 1 h the corresponding α-glycosyl chloride 2 can be isolated by column chromatography in 75-80% yield. Compound 2 is an important intermediate in the synthesis of oligosaccharides containing a glycosyl-3-0-galactosyl sequence. Under the described reaction conditions the conversion 1 - 2 is accompanied by a slow anomerisation of 1 to give methyl 2,4,6-tri-0-benzoyl-3-0-benzyl-α-D-galactopyranoside. Prolonged treatment of 2 with the used excess of the reagent results in complete debenzylation of the substrate and the conversion of the putative 2,4,6-tri-O-benzoyl-α-D-galactopyranosyl chloride into the corresponding 3-0-formyl and 3-0-dichloromethyl derivatives.     

13C Nmr Spectra of Methyl Deoxyfluoro-β-D-Galactopyranosides and Their Per-O-Acetyl Derivatives

Abstract

Methyl 6-deoxy-6-fluoro-β-d-galactopyranoside has been obtained by treatment of methyl β-d-galactopyranoside with diethyl-aminosulfur trifluoride (DAST). Improvements over the existing syntheses of methyl 2, 3-di-O-benzyl-4-deoxy-4-fluoro-β-d-galacto-pyranoside from the corresponding 6-O-substituted 4-O-arylsul-fonyl-d-gluco derivatives are described. ¹³C NMR spectra of a series of methyl deoxyfluoro-β-d-galactopyranosides and their per-O-acetyl derivatives have been measured. The data obtained can be used as an aid for the interpretation of ¹³C NMR spectra of deoxyfluoro-β-d-galactopyranose-containing oligosaccharides and related substances.     

Phase Transfer Promoted Ruthenium Oxide Oxidations of Carbohydrates II

ABSTRACT

The use of a phase transfer catalyst, benzyltriethylammonium chloride (BTEAC), is described in conjunction with the ruthenium dioxide/periodate : water/chloroform system for the oxidation of carbohydrate alcohols to the corresponding ketone, aldehyde, or carboxylic acid. The method was found to be applicable to carbohydrates appropriately protected as acetals, ethers, or containing a benzoyloxy group not positioned to readily undergo β-elimination. While the method was very suitable for the oxidation of carbohydrate secondary alcohols to ketones, it was found to be less suitable for the oxidation of a carbohydrate primary alcohol to the corresponding aldehyde or carboxylic acid. Evidence presented suggests that under the mildly basic conditions of the reaction, ruthenium tetraoxide is converted to ruthenate and perruthenate ions in the aqueous solution and then the perruthenate ion is carried by the phase transfer catalyst into the organic layer where oxidation of the substrate occurs. A number of examples illustrating the scope of the method are presented.     

Left and right centers in quasi-Poisson geometry of moduli spaces

We introduce left central and right central functions and left and right leaves in quasi-Poisson geometry, generalizing central (or Casimir) functions and symplectic leaves from Poisson geometry. They lead to a new type of (quasi-)Poisson reduction, which is both simpler and more general than known quasi-Hamiltonian reductions. We study these notions in detail for moduli spaces of flat connections on surfaces, where the quasi-Poisson structure is given by an intersection pairing on homology.     

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes - Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late-stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron-donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near-infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG-functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red-shifted absorptions (Δλ_abs up to 25 nm) and emissions (Δλ_em up to 73 nm, Φ_PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in ¹H NMR spectroscopy (218 K), N−CH₂ protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol⁻¹. This phenomenon is due to planar chiral conformations (S_p and R_p configurations), induced by the geometry of the alkyl (n-propyl) side-chains next to the acetylenic substituents. Ion pairing studies with Δ-TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.