Weak coupling of a Reynolds model and a Stokes model for hydrodynamic lubrication

The Reynolds model is a reduced Stokes model, valid for narrow lubrication regions. In order to be able to handle locally non‐narrow regions such as pits or grooves, often displaying rapid geometrical variations, there is a need to be able to transit to the more accurate Stokes model. A fundamental problem is how to couple the two models in a numerical simulation, preferably allowing for different meshes in the different domains. In this paper, we present a weak coupling method for Reynolds and Stokes models for lubrication computations, including the possibility of cavitation in the different regions. The paper concludes with a numerical example. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparison of direct mass spectrometry methods for the on‐line analysis of volatile compounds in foods

For the on‐line monitoring of flavour compound release, atmospheric pressure chemical ionization (APCI) and proton transfer reaction (PTR) combined to mass spectrometry (MS) are the most often used ionization technologies. APCI‐MS was questioned for the quantification of volatiles in complex mixtures, but direct comparisons of APCI and PTR techniques applied on the same samples remain scarce. The aim of this work was to compare the potentialities of both techniques for the study of in vitro and in vivo flavour release. Aroma release from flavoured aqueous solutions (in vitro measurements in Teflon bags and glass vials) or flavoured candies (in vivo measurements on six panellists) was studied using APCI‐ and PTR‐MS. Very similar results were obtained with both techniques. Their sensitivities, expressed as limit of detection of 2,5‐dimethylpyrazine, were found equivalent at 12 ng/l air. Analyses of Teflon bag headspace revealed a poor repeatability and important ionization competitions with both APCI‐ and PTR‐MS, particularly between an ester and a secondary alcohol. These phenomena were attributed to dependency on moisture content, gas/liquid volume ratio, proton affinities and product ion distribution, together with inherent drawbacks of Teflon bags (adsorption, condensation of water and polar molecules). Concerning the analyses of vial headspace and in vivo analyses, similar results were obtained with both techniques, revealing no competition phenomena. This study highlighted the equivalent performances of APCI‐MS and PTR‐MS for in vitro and in vivo flavour release investigations and provided useful data on the problematic use of sample bags for headspace analyses. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis of Non-glucosamino Glucan Oligosaccharides Related to Heparin and Heparan Sulphate

Abstract

A series of three oligosaccharides, α-d-Glc-(1→4)-β-d-GlcA-1ωe, β-d-GlcA-(1→4)-α-d-Glc-(1→4)-β-d-GlcA-lωe and α-d-Glc-(1→4)-β-d-GlcA-(1→4)-α-d-Glc-(1→4)-β-d-GlcA-1ωe was prepared by a short synthetic route, using maltose and glucuronic acid derivatives as starting materials. The oligosaccharides contain glucose residues instead of glucosamines, and have a less complicated structure than the corresponding unsulphated structures found in native heparin and heparan sulphate. This simplification in structure has diminished the number of synthetic steps and raised the total yield compared to the preparation of the corresponding heparin/heparan sulphate structures which have been found to bind acidic and basic FGF.     

Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis

The conditions required for the accurate measurement of the sulfur content of cellulose nanocrystals (CNCs) by conductometric titration are discussed. CNCs from sulfuric acid hydrolysis are electrostatically stabilized in aqueous suspension due to the introduction of charged sulfate ester groups onto the surface of the crystallites during reaction. The sulfur content thus largely reflects the surface charge of the crystals, and is crucial to the characterization and understanding of material properties. Conductometric titration is commonly used to quantify the sulfur content of CNCs, however, the exhaustive removal of free acid by dialysis and the necessity, type, quantity and duration of ion-exchange resin treatments are not always consistent. Here we explore the standard conditions of dialysis, ion-exchange, and the reproducibility of titration results. Extensive dialysis is found to be effective in the removal of free acid, but similar results are also achieved in shorter times and with less water using membrane ultrafiltration. It is also shown that the conditions of ion-exchange most commonly employed in the literature can lead to inaccurate sulfur contents. Finally, good agreement is obtained between the sulfur contents of different CNC batches prepared using the same hydrolysis conditions, and from titration and elemental analysis when thoroughly purified, well-dispersed samples, and appropriate resin conditions are used.     

Hybrid phospholipid bilayer coatings for separations of cationic proteins in capillary zone electrophoresis

Protein separations in CZE suffer from nonspecific adsorption of analytes to the capillary surface. Semipermanent phospholipid bilayers have been used to minimize adsorption, but must be regenerated regularly to ensure reproducibility. We investigated the formation, characterization, and use of hybrid phospholipid bilayers (HPBs) as more stable biosurfactant capillary coatings for CZE protein separations. HPBs are formed by covalently modifying a support with a hydrophobic monolayer onto which a self‐assembled lipid monolayer is deposited. Monolayers prepared in capillaries using 3‐cyanopropyldimethylchlorosilane (CPDCS) or n‐octyldimethylchlorosilane (ODCS) yielded hydrophobic surfaces with lowered surface free energies of 6.0 ± 0.3 or 0.2 ± 0.1 mJ m^?2, respectively, compared to 17 ± 1 mJ m^?2 for bare silica capillaries. HPBs were formed by subsequently fusing vesicles comprised of 1,2‐dilauroyl‐sn‐glycero‐3‐phosphocholine or 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine to CPDCS‐ or ODCS‐modified capillaries. The resultant HPB coatings shielded the capillary surface and yielded reduced electroosmotic mobility (1.3–1.9 × 10^?4 cm² V^?1s^?1) compared to CPDCS‐ and ODCS‐modified or bare capillaries (3.6 ± 0.2 × 10^?4 cm² V^?1s^?1, 4.8 ± 0.4 × 10^?4 cm² V^?1s^?1, and 6.0 ± 0.2 × 10^?4 cm² V^?1s^?1, respectively), with increased stability compared to phospholipid bilayer coatings. HPB‐coated capillaries yielded reproducible protein migration times (RSD ≤ 3.6%, n ≥ 6) with separation efficiencies as high as 200 000 plates/m.     

A quantitative LC/MS method targeting urinary 1-methyl-4-imidazoleacetic acid for safety monitoring of the global histamine turnover in clinical studies

Anaphylaxis is a potentially life-threatening condition triggered mainly by the release of inflammatory mediators, notably histamine. In pharmaceutical research, drug discovery, and clinical evaluation, it may be necessary to accurately assess the potential of a compound, event, or disorder to promote the release of histamine. In contrast to the measurement of plasma histamine, determination of the stable metabolite 1-methyl-4-imidazoleacetic acid (tele-MIAA) in urine provides a noninvasive and more reliable methodology to monitor histamine release. This study presents a repeatable high-performance liquid chromatography coupled to electrospray mass spectrometry (LC–ESI–MS) method where tele-MIAA is baseline separated from its structural isomer 1-methyl-5-imidazoleacetic acid (pi-MIAA) and an unknown in human urine. The ion-pairing chromatography method, in reversed-phase mode, based on 0.5 mM tridecafluoroheptanoic acid demonstrated high repeatability and was applied in a clinical development program that comprised a large number of clinical samples from different cohorts. The inter- and intra-run precision of the method for tele-MIAA were 8.4 and 4.3 %, respectively, at the mean urinary concentration level, while method accuracy was between ?16.2 and 8.0 % across the linear concentration range of 22–1,111 ng mL^?1. Overall, method precision was greater than that reported in previously published methods and enabled the identification of gender differences that were independent of age or demography. The median concentration measured in female subjects was 3.0 μmol mmol^?1 of creatinine, and for male subjects, it was 2.1 μmol mmol^?1 of creatinine. The results demonstrate that the method provides unprecedented accuracy, precision, and practicality for the measurement of tele-MIAA in large clinical settings.

Bistriazoles Connected Through a B−B Bridge,Synthesized by Highly Selective Dipolar Cycloaddition Reactions of a Diazido-diborane(4)

In this work we report the first cycloaddition reactions between a diazido diborane(4) and terminal alkynes, providing unique access to bis-1,2,3-triazoles connected by a B−B bridge. The catalyst-free reactions are highly selective, yielding exclusively the thermodynamically disfavored bis-1,4-triazoles. The reactions are enabled by the high thermal stability of the diazido-diborane [B(hpp)(N₃)]₂ (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate). Due to the tetra-coordinate boron atoms in this reagent, the reactions are tolerant with respect to the introduction of Lewis-basic groups at the alkyne. The scope and limitations of the new reactions are discussed.     

Interplay and Competition Between Two Different Types of Redox-Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono- as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.