Solid‐state NMR spectroscopy of Pb‐rich apatite

Pb‐containing hydroxylapatite phases synthesized under aqueous conditions were investigated by X‐ray diffraction and solid‐state nuclear magnetic resonance (NMR) techniques to determine the Pb, Ca distribution. ³¹P and ¹H magic‐angle spinning (MAS) NMR results indicate slight shifts of the isotropic chemical shift with increased Ca content and complex lineshapes at compositions with near equal amounts of Ca and Pb. ³¹P{²⁰⁷Pb} and ¹H{²⁰⁷Pb} rotational‐echo double resonance (REDOR) results for intermediate compositions show that resolved spectral features cannot be assigned simply in terms of local Ca, Pb configurations or coexisting phases. ²⁰⁷Pb MAS NMR spectra are easily obtained for these materials and contain well‐resolved resonances for crystallographically unique A1 and A2 Pb sites. Splitting of the A1 and A2 ²⁰⁷Pb resonances for pure hydroxyl‐pyromorphite (Pb₁₀(PO₄)₆(OH)₂) compared to natural pyromorphite (Pb₅(PO₄)₃Cl) suggests symmetry reduced from hexagonal. We find that ²⁰⁷Pb{¹H} CP/MAS NMR is impractical in Pb‐rich hydroxylapatites due to fast ²⁰⁷Pb relaxation. Copyright © 2009 John Wiley & Sons, Ltd.     

Discrete network models of interacting nephrons

The kidney is one of the major organs involved in whole-body homeostasis, and exhibits many of the properties of a complex system. The functional unit of the kidney is the nephron, a complex, segmented tube into which blood plasma is filtered and its composition adjusted. Although the behaviour of individual nephrons can fluctuate widely and even chaotically, the behaviour of the kidney remains stable.In this paper, we investigate how the filtration rate of a multi-nephron system is affected by interactions between nephrons. We introduce a discrete-time multi-nephron network model. The tubular mechanisms that have the greatest effect on filtration rate are the transport of sodium and water, consequently our model attempts to capture these mechanisms. Multi-nephron systems also incorporate two competing coupling mechanisms–vascular and hemodynamic–that enforce in-phase and anti-phase synchronisations respectively. Using a two-nephron model, we demonstrate how changing the strength of the hemodynamic coupling mechanism and changing the arterial blood pressure have equivalent effects on the system. The same two-nephron system is then used to demonstrate the interactions that arise between the two coupling mechanisms. We conclude by arguing that our approach is scalable to large numbers of nephrons, based on the performance characteristics of the model.     

A novel class of azatricyclononanes: pentasubstituted cyclopropanes from an uncatalysed reaction

The direct and versatile stereoselective synthesis of a novel class of cyclopropyl derivatives of pyroglutamic acid is reported, using substituted diaryldiazo compounds. The course of the reaction has been investigated, and yields of substituted fluorenylcyclopropanes are greater than those of diphenylmethylcyclopropane analogues.     

Pursuit of optimal carbohydrate-based anticancer vaccines: preparation of a multiantigenic unimolecular glycopeptide containing the Tn, MBr1, and Lewis(y) antigens.

A novel preparation of nonnatural glycoamino acids starting from n-pentenyl glycosides is described. The approach involves a Horner-Emmons olefination with a suitably protected glycine-derived phosphonate, followed by catalytic asymmetric hydrogenation, which proceeds with excellent diastereomeric selectivity. The synthetic methodology was useful for the preparation of glycoamino acids containing the Tn antigen, the MBr1 antigen (Globo-H), the Le(y) antigen, and lactose. These glycoamino acids can also serve as units for peptide synthesis. The synthesis of polyvalent glycopeptides containing three different antitumor antigens is described (28 and 29), and these have been prepared for conjugation to carrier protein in order to access the immunogenicity for tumor immunotherapy applications.     

19-electron intermediates and cage-effects in the photochemical disproportionation of [CpW(CO)3]2 with Lewis bases

The role of 19-electron intermediates in the photochemical disproportionation of [CpW(CO)3]2 (Cp = C5H5) with Lewis bases (PR3; R = OMe, Bu, Ph) is investigated on the ultrafast time scale using femtosecond VIS-pump, IR-probe spectroscopy. Formation of a 19-electron (19e) species CpW(CO)3PR3*by coordination of PR3 with photogenerated 17-electron (17e) radicals CpW(CO)3* is directly observed, and equilibrium is established between the 17e radicals and the 19e intermediates favoring 19e intermediates in the order: Bu > OMe > Ph. Steric effects dominate the 17e/19e equilibrium when the cone-angle of the Lewis base exceeds a certain limiting value (between 132 degrees and 145 degrees ), but below this value electronic properties of the Lewis base control the 17e/19e dynamics. Disproportionation occurs in less than 200 picoseconds by electron transfer between a solvent caged 17e radical and 19e, highly reducing species. The rate and extent of ultrafast disproportionation depends on both the identity and concentration of the Lewis base. In low concentrations of PR3 (typically 1-2 M or less) or with Lewis bases whose equilibrium heavily favors 17e radicals (e.g., PPh3), disproportionation is rate-limited by breakdown of the solvent cage. Density functional theory calculations on vibrational frequencies and charge distributions of the various complexes support the experimental results.     

Synthesis and characterization of poly(ethylene glycol) derivatives

Five general routes for the preparation of polyoxyethylene [generally referred to as poly(ethylene glycol) or PEG] derivatives are described. These routes are (1) nucleophilic displacements with the alkoxide of PEG, (2) nucleophilic displacement on PEG–tosylate, –mesylate, or –bromide, (3) reductive amination of PEG–aldehyde, (4) reductive amination of PEG–amine, and (5) nucleophilic displacements on the s-triazine derivatives prepared from s-triazine trichloride (cyanuric chloride) and PEG. Eighteen derivatives are prepared and potential applications to catalysis, cell purifications, and other areas are discussed briefly.     

Diels-Alder polymers. III. Polymers containing phenylated phenylene units

The Diels-Alder reaction of biscyclopentadieneones with diacetylenes produces colorless, soluble, phenylated polyphenylenes of high molecular weight (M?_n ? 40,000) in nearly quantitative conversions. The polymers are noncrystalline, form clear films, and are stable in air to 550°C. Under nitrogen, the polymers lost approximately half the phenyl groups attached to the phenylene main chain to give brown-black insoluble polyphenylenes of very low crystallinity.     

Structure–property relationships of poly(vinyl alcohol). I. Influence of polymerization solvents and temperature on the structure and properties of poly(vinyl alcohol) derived from poly(vinyl acetate)

The solubility properties of poly(vinyl alcohol) (PVA) vary with the method of preparation of the poly(vinyl acetate) (PVAc) from which it is derived. PVAc was prepared with free-radical catalysts over a range of temperatures from ?78 to 90°C. with solvents of varying chain-transfer ability. The corresponding PVA samples varied in their resistance to dissolution in water. Their high-resolution proton nuclear magnetic resonance spectra showed on differences in tacticity. Data on 1,2-diol content showed only minor differences. Hence, the increase in resistance of PVA to dissolution in water arising from changes in chain-transfer activity of the solvent used in vinyl acetate polymerization is largely attributable to a decrease in molecular weight, and the increase in resistance of PVA to dissolution in water arising from a decrease in the temperature of the vinyl acetate polymerization is largely attributable to a decrease in both long and short branches. Evidently, with polar polymers having small side groups, tacticity is not the only factor influencing property variation; that is, variations in stereoregularity influence more the crystallinity of the sample as measured by density or x-ray methods than the ultimate crystallizability under conditions of mechanical and thermal treatment. In this regard polar polymers having small side groups differ from nonpolar polymers.