A liquid chromatography – tandem mass spectrometry method to measure a selected panel of uremic retention solutes derived from endogenous and colonic microbial metabolism

Chronic kidney disease (CKD) is associated with an increased risk of mortality and cardiovascular disease, which is, at least partly, mediated by the accumulation of so-called uremic retention solutes. Although there has been an increasing interest in the behavior of these solutes, derived from both the endogenous and colonic microbial metabolism, methods to simultaneously and accurately measure a broad panel of relevant uremic retention solutes remain scarce.     

Nearest-neighbor oxygen distances in liquid water and ice observed by x-ray Raman based extended x-ray absorption fine structure

We report the nearest-neighbor oxygen-oxygen radial distribution function (NN O-O RDF) of room temperature liquid water and polycrystalline ice Ih (-16.8 degrees C) obtained by x-ray Raman based extended x-ray absorption fine structure (EXAFS) spectroscopy. The spectra of the two systems were taken under identical experimental conditions using the same procedures to obtain the NN O-O RDFs. This protocol ensured a measurement of the relative distance distribution with very small systematic errors. The NN O-O RDF of water is found to be more asymmetric (tail extending to longer distances) with longer average distance (2.81 A for water and 2.76 A for ice) but a slightly shorter peak position (2.70 A for water and 2.71 A for ice). The refinement also showed a small but significant contribution from the linear O-H-O multiple scattering signal. The high sensitivity to short range distances of the EXAFS probe will set further restrictions to the range of possible models of liquid water.     

Evaluation of the interaction of propranolol with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes: the partitioning model

The sorption behavior of the amine containing beta-receptor blocking agent propranolol (Ppn) in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles was investigated. Both protonated and unprotonated Ppn were measured in the continuous phase after removal of the vesicles containing sorbed Ppn by centrifugation. In contrast, by analyzing the surface charge density, deduced from electrophoretic mobility measurements, only the sorbed protonated Ppn was determined. A partitioning model was used to describe the sorption behavior. Sensitivity analysis revealed that sufficiently reliable and independent parameters were obtained. The partition coefficient of the unprotonated Ppn was about 22 times higher than that of the protonated analogue. Statistical analysis revealed a significant increase in the intrinsic partition coefficients of both Ppn analogues with an increase in the salt concentration.     

Incomplete lipid chain freezing of sonicated vesicular dispersions of double-tailed ionic surfactants

Lipid freezing in dilute sonicated vesicular dispersions was studied using differential scanning calorimetry (DSC) and 1H NMR. For charged, anionic, or cationic lipids, approximately half of the lipids remain in a fluid state when cooled 20 degrees C below the main chain melting temperature. With a zwitterionic phospholipid, on the other hand, essentially no supercooling of the liquid state was observed. The observations are analyzed in terms of the nucleation and growth of flat solid domains in originally fluid spherical vesicles. As the solid domains grow, the remaining fluid domain is deformed, resulting in a curvature stress. Depending on the vesicle size and the bilayer bending rigidity, the solid domain growth may terminate as the gain in cohesive free energy is balanced by the curvature stress of the remaining fluid domain. It is argued that high bending rigidities are required for having a significant supercooling, which is why it is only observed for charged lipids.     

Osmotically induced morphological changes of extruded dioctadecyldimethylammonium chloride (DODAC) dispersions

Extruded vesicles, which are often used as models for living cells, can change their morphology when they are diluted into a hyperosmotic medium. Different morphological changes were observed with extruded dioctadecyldimethylammonium chloride (DODAC) vesicles after dilution with a nonionic (sucrose) and ionic (CaCl2) osmotic agent above and below the gel-to-liquid crystalline transition temperature. By means of turbidimetry, dynamic light scattering, and cryo-transmission electron microscopy, it was seen that the vesicles only deflated when they were in the gel state, whereas in the liquid crystalline state, an ionic osmotic agent could induce twinning of the vesicles, reminiscent to endocytosis. The latter could occur as a result of the combined effects of reduced repulsion, local dehydration, and reduced bending rigidity induced by the ionic agent.     

Network Density and Diene Conversion in Peroxide-Cured Gumstock EPDM Rubbers. A Solid-State NMR Study

Peroxide-cross-linked EPM and EPDM rubbers have been investigated with magic-angle spinning ¹H NMR spectroscopy and static ¹H NMR relaxometry. The results yield a consistent nanoscale picture of the chemical and physical network properties in terms of chemical cross-links formed via macro-radical combination, chemical cross-links formed via addition to the double bonds, and physical cross-links resulting from chain entanglements.