Vesicle adsorption and lipid bilayer formation on glass studied by atomic force microscopy

The adsorption of phosphatidylcholine (PC) vesicles (30, 50, and 100 nm nominal diameters) and of dye-labeled PC vesicles (labeled with 6% Texas Red fluorophore (TR) and encapsulated carboxy fluorescein (CF)) to glass surfaces was studied by contact mode atomic force microscopy in aqueous buffer. These studies were performed in part to unravel details of the previously observed isolated rupture of dye-labeled PC vesicles on glass (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), specifically to differentiate partial rupture, that is, pore formation and leakage of entrapped dye, from full rupture to form bilayer disks. In addition, the adhesion potential of PC vesicles on glass was calculated based upon the adhesion-driven flattening of adsorbed vesicles and a newly developed theoretical model. The vesicles were found to flatten considerably upon adsorption to glass (width-to-height ratio of approximately 5), which leads to an estimate for the adhesion potential and for the critical rupture radius of 1.5 x 10(-4) J/m2 and 250 nm, respectively. Independent of vesicle size and loading with dye molecules, the adsorption of intact vesicles was observed at all concentrations below a threshold concentration, above which the formation of smooth lipid bilayers occurred. In conjunction with previous work (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), these data show that 6% TR 20 mM CF vesicles adsorb to the surface intact but undergo partial rupture in which they exchange content with the external buffer.     

The role of the π-bonding network for trigonal level splittings of tris-bidentate Cr(acac)3 and Cr(ox) 3 3−

The varying -bonding contributions in the title compounds caused by the different electronic and molecular structure of the chelate rings are used for explaining the large band splittings in the absorption spectra by trigonal symmetry. It is shown that usual ligand field theory and the angular overlap model are not able to account for the trigonal level splitting of Cr(acac)₃ for which the coordination sphere of oxygen atoms is nearly octahedrally arranged. The experimental finding can, however, be rationalized by an extended angular overlap model which considers the phase coupling of -orbitals in the ligands leading to non-additive contributions to the metal-ligand bond energy.On leave of absence from the Bulgarian Academy of Sciences, Sofia, Bulgaria     

Spin-orbit electronic states of octahedral Pt-group d 6 complexes as derived from reflectance spectra and ligand field calculations

The 90 K diffuse reflectance spectra of some octahedral Rh(III), Ir(III) and Pt(IV) complex compounds with chloride, thiocyanate and amine ligands are reported. The spectra show in the visible and near ultraviolet distinct bands due to spin-orbit components of singlet-triplet d-d transitions which are explained by ligand field calculations including spin-orbit coupling considering all possible d-electron configurations. Model parameter sets are obtained by fitting the measured band peaks to possible transitions between calculated energy levels. For some of the bands the vibrational structure could be resolved which is assigned to metal-ligand stretching vibrations of the electronically excited complex octahedra with vibrational fundamentals lower than those of the ground state.     

Capillary electrophoresis separation of peptide diastereomers that contain methionine sulfoxide by dual cyclodextrin‐crown ether systems†

A dual‐selector system employing achiral crown ethers in combination with cyclodextrins has been developed for the separation of peptide diastereomers that contain methionine sulfoxide. The combinations of the crown ethers 15‐crown‐5, 18‐crown‐6, Kryptofix® 21 and Kryptofix® 22 and β‐cyclodextrin, carboxymethyl‐β‐cyclodextrin, and sulfated β‐cyclodextrin were screened at pH 2.5 and pH 8.0 using a 40/50.2 cm, 50 μm id fused‐silica capillary and a separation voltage of 25 kV. No diastereomer separation was observed in the sole presence of crown ethers, while only sulfated β‐cyclodextrin was able to resolve some peptide diastereomers at pH 8.0. Depending on the amino acid sequence of the peptide and the applied cyclodextrin, the addition of crown ethers, especially the Krpytofix® diaza‐crown ethers, resulted in significantly enhanced chiral recognition. Keeping one selector of the dual system constant, increasing concentrations of the second selector resulted in increased peak resolution and analyte migration time for peptide‐crown ether‐cyclodextrin combinations. The simultaneous diastereomer separation of three structurally related peptides was achieved using the dual selector system.