Methyl 4‐O‐β‐d‐galacto­pyran­osyl α‐d‐gluco­pyran­oside (methyl α‐lactoside)

Methyl α‐lactoside, C₁₃H₂₄O₁₁, (I), is described by glycosidic torsion angles ϕ (O5_gal—C1_gal—O1_gal—C4_glc) and ψ (C1_gal—O1_gal—C4_glc—C5_glc), which have values of −93.52 (13) and −144.83 (11)°, respectively, where the ring atom numbering conforms to the convention in which C1 is the anomeric C atom and C6 is the exocyclic hydroxy­methyl (–CH₂OH) C atom in both residues. The linkage geometry is similar to that observed in methyl β‐lactoside methanol solvate, (II), in which ϕ is −88.4 (4)° and ψ is −161.3 (4)°. As in (II), an inter­molecular O3_glc—H⋯O5_gal hydrogen bond is observed in (I). The hydroxy­methyl group conformation in both residues is gauche–trans, with torsion angles ω_gal (O5_gal—C5_gal—C6_gal—O6_gal) and ω_glc (O5_glc—C5_glc—C6_glc—O6_glc) of 69.15 (13) and 72.55 (14)°, respectively. The latter torsion angle differs substantially from that found for (II) [−54.6 (2)°; gauche–gauche]. Cocrystallization of methanol, which is hydrogen bonded to O6_glc in the crystal structure of (II), presumably affects the hydroxy­methyl conformation in the Glc residue in (II).     

Greatly attenuated experimental autoimmune encephalomyelitis in aquaporin-4 knockout mice

Background  

The involvement of astrocyte water channel aquaporin-4 (AQP4) in autoimmune diseases of the central nervous system has been suggested following the identification of AQP4 autoantibodies in neuromyelitis optica, an inflammatory demyelinating disease.     

4J(COCCH) and 4J(CCCCH) as probes of exocyclic hydroxymethyl group conformation in saccharides

[structure: see text] 1H NMR spectra of aldohexopyranosyl rings containing 13C-enrichment at either C1 or C3 reveal the presence of long-range 4J(C1,H6R/S) and 4J(C3,H6R/S) whose magnitudes depend mainly on the O5-C5-C6-O6 torsion angle. Using theoretical calculations (density functional theory, DFT; B3LYP/6-31G*) and conformationally constrained experimental model compounds, the magnitudes and signs of 4J(C1,H6R/S) and 4J(C3,H6R/S) have been established, and their dependencies on the geometry of the C1-O5-C5-C6-H6R/S and C3-C4-C5-C6-H6R/S coupling pathways, respectively, were determined. The latter dependencies mimic that observed previously for 4J(HH) in aliphatic compounds such as propane. DFT calculations also showed that inclusion of non-Fermi contact terms is important for accurate predictions of 4J(CH) values. Application to methyl alpha- and beta-D-glucopyranosides reveals different rotameric distributions about their hydroxymethyl groups, with the beta-anomer enriched in the gt rotamer, in agreement with recent multi-J redundant coupling analyses. 4J(C1,H6R/S) and 4J(C3,H6R/S) are expected to complement other recently developed J-couplings for the assignment of hydroxymethyl group conformation in oligosaccharides containing 1,6-glycosidic linkages.     

Differences Between Consecutive Primes

Let p_n be the nth prime. Then this paper is concerned with provingthe following result on the distribution of consecutive primes. The exponent of x in this theorem improves on the work of Heath-Brownwho proved (1) with exponent . Under the Riemann hypothesisone can prove (1) with exponent .The proof of the theorem startswith the Heath-Brown–Linnik identity which leads to aformula giving the number of primes in an interval in termsof coefficients of certain Dirichlet series. I then estimatethe coefficients by using, among other things, the informationwhich can be gained from Montgomery's mean value theorem andHuxley's version of the Hal' asz lemma. Furthermore, by usingfamiliar sieve arguments I am able to discard some of the coefficientsallowing us to gain an improvement over the previous resultof Heath-Brown. 1991 Mathematics Subject Classification: 11N05.     

Conformational analysis of beta-glycosidic linkages in 13C-labeled glucobiosides using inter-residue scalar coupling constants

Four beta-linked glucobioses selectively (13)C labeled at C1' or C2' have been prepared. The inter-residue coupling constants, J(CH), and J(CC), have been determined and related to the solution conformations of the disaccharides using Karplus-type relationships. Relying only on the experimental coupling constants, glycosidic linkage conformation in methyl alpha-sophoroside (methyl 2-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside), methyl alpha-laminarabioside (methyl 3-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside), and methyl alpha-cellobioside (methyl 4-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside) were found to be close to those observed in the solid state (39 degrees < phi(H) < 41 degrees , -24 degrees < psi(H) < -36 degrees ). The laminarabioside and cellobioside were found to have conformations that accommodate an intramolecular hydrogen bond to O5' that is observed in the solid state. In all compounds, the exocyclic hydroxymethyl groups retain a conformation close to that observed in unsubstituted glucose (gt/gg 1:1). Methyl alpha-gentiobioside (methyl 6-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside) shows greater flexibility at the psi-torsion than the other disaccharides, but the population distribution around the C5-C6 bond is essentially unaffected by substitution. None of the O2' hydroxyl groups of the beta-D-glucopyranosyl residues in any of the disaccharides appear to be involved in inter-residue hydrogen bonding since (1)JCH, (1)JCC, and (2)JCH values sensitive to C2'-O2' rotamer distribution remain close to those observed in methyl beta-D-glucopyranoside.     

Electrostatic Fluxes and Plasma Rotation in the Edge Region of EXTRAP-T2R

The EXTRAP-T2 reversed field pinch has undergone a significant reconstruction into the new T2R device. This paper reports the first measurements performed with Langmuir probes in the edge region of EXTRAP-T2R. The radial profiles of plasma parameters like electron density and temperature, plasma potential, electrical fields and electrostatic turbulence-driven particle flux are presented. These profiles are interpreted in a momentum balance model where finite Larmor radius losses occur over a distance of about two Larmor radii from the limiter position. The double shear layer of the E×B drift velocity is discussed in terms of the Biglari-Diamond-Terry theory of turbulence decorrelation.     

Experimental measurements and modelling of the structure of the radial electric field in RFX

The experimental determination of the radial electric fieldE _r and the associatedE ×B drift velocity at the edge of RFX is presented and possible mechanisms responsible for its generation are discussed.E _r measurements by means of an array of Langmuir probes and those deduced from Doppler spectroscopy of impurity lines agree fairly well. In particular the rotation velocity of the plasma edge has been determined from the Doppler shift of a C III emission line. The observation of other ions characterised by different radial positions, such as B IV and C V, has allowed an estimate of the velocity shear. Typical values of plasma rotation at the edge are around 10 km/s while the velocity shear is of the order of (10⁵−10⁶)s⁻¹ in the spontaneous layer, a few cm thick, usually observed in standard discharges. Plasma rotation has been artificially modified by both positive and negative edge biasing and the associated increase or decrease of the fluid velocity is well in agreement with the radial electric field change. The modification ofE _r during edge biasing and Pulsed Poloidal Current Drive (PPCD) are also reported and interpreted within a momentum balance model. Analytical and self-consistent Monte Carlo models at the plasma edge suggest that impurities have a relevant role in the generation of the radial electric field, due to their relatively large Larmor radius. Presented at the Workshop on Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June 2000.     

Thermal analysis and high heat flux testing of unidirectional carbon–carbon composite for infrared imaging diagnostic

Carbothermic reduction of chromite in the presence of nickel as the alloying element was investigated in a wide temperature range up to 1500 °C using thermogravimetric analysis coupled with continuous off-gas analysis (TG-DSC-MS). Both isothermal and non-isothermal linear heating tests were performed for the kinetic study with the calculation of activation energies. In order to further elucidate the reduction mechanism, the reduced products were characterized by SEM–EDS and XRD. It was concluded that the reduction sequence followed a multi-stage mechanism, reflected partly by the dependency of the activation energy on the extent of reduction. With the progress of reduction, refractory oxide layers gradually formed on/close to the surface of each chromite particle, causing the shift of the rate-limiting factor from chemical control to diffusion control. The promoting effect from the addition of Ni was evident at temperatures higher than 1300 °C due to the formation of alloys of lower melting point.     

Glycosidic linkage,N‐acetyl side‐chain,and other structural properties of methyl 2‐acetamido‐2‐deoxy‐β‐d‐glucopyranosyl‐(1→4)‐β‐d‐mannopyranoside monohydrate and related compounds

The crystal structure of methyl 2‐acetamido‐2‐deoxy‐β‐d ‐glycopyranosyl‐(1→4)‐β‐d ‐mannopyranoside monohydrate, C₁₅H₂₇NO₁₁·H₂O, was determined and its structural properties compared to those in a set of mono‐ and disaccharides bearing N‐acetyl side‐chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C—N (amide) and C—O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N—H hydrogen. Relative to N‐acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen‐bond acceptor display elongated C—O and shortened C—N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C—N and C—O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis–trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter‐residue hydrogen bonding and some bond angles in or proximal to β‐(1→4) O‐glycosidic linkages on linkage torsion angles ? and ψ. Hypersurfaces correlating ? and ψ with the linkage C—O—C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.