Immobilization of liposomes in nanostructured layer-by-layer films containing dendrimers

Artificial vesicles or liposomes composed of lipid bilayers have been widely exploited as building blocks for artificial membranes, in attempts to mimic membrane interaction with drugs and proteins and to investigate drug delivery processes. In this study we report on the immobilization of liposomes of 1,2-dipalmitoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (Sodium Salt) (DPPG) in layer-by-layer (LbL) films, alternated with poly(amidoamine) G4 (PAMAM) dendrimer layers. The average size of the liposomes in solution was 120 nm as determined by dynamic light scattering, with their spherical shape being inferred from scanning electron microscopy (SEM) in cast films. LbL films containing up to 20 PAMAM/DPPG bilayers were assembled onto glass and/or silicon wafer substrates. The growth of the multilayers was achieved by alternately immersing the substrates into the PAMAM and DPPG solutions for 5 and 10 min, respectively. The formation of PAMAM/DPPG liposome multilayers and its ability to interact with BSA were confirmed by Fourier transform infrared spectroscopy (FTIR). The structural features and film thickness were obtained using X-ray diffraction and surface plasmon resonance (SPR).     

Membrane structure characterization using variable-period x-ray standing waves

The variable-period x-ray standing wave (XSW) technique is emerging as a powerful tool for studying membrane structure. However, two significant problems arise when the method is used to characterize membranes of thickness dL < 100 A. First, the surface roughness, sigma(r), of the supporting reflecting mirror convolutes with the intrinsic half-width of the marker atom distribution in the membrane, sigma(in), and contributes to an apparent half-width, sigma, which is measured in the XSW experiment. Here we show how the latter terms are related quantitatively [sigma(in) = (sigma2 - sigma(r)2)(1/2)], such that rough mirrors give rise to larger marker atom distribution widths, sigma, and how the required quantity sigma(in) can be determined in the XSW measurement. Second, when the mean position of the marker atom layer, (z), is close to one or both boundaries of the membrane, its distribution function is truncated at the boundary. In such cases, we show why marker atom distribution should be expressed in terms of its first and second moments. We also demonstrate by numerical simulations of realistic samples how the physical parameters, sigma(r), sigma, (z), and dL, affect x-ray reflectivity and fluorescence yield profiles as an aid in their interpretation.     

Hydrogen-assisted 1,2,3-trichloropropane dechlorination on supported Pt–Sn catalysts

The chemical kinetics behavior of Pt–Sn/C catalysts for hydrogen-assisted 1,2,3-trichloropropane dechlorination is understood in terms of a modification of Pt by both Sn and Cl. Catalysts with Pt:Sn atomic ratios ranging from 9:1 to 1:6 exhibit a 10–20 h time-on-stream (TOS) transient period characterized by a 2–3 fold decrease in activity. This transient behavior is followed by a ‘steady state’ period during which the activity remains essentially constant (5–7% change in conversion during 30–60 h TOS); however, the relative rates of hydrogenation and hydrodechlorination decrease. For catalysts with a Pt:Sn atomic ratio ≥3, the decrease in relative rates manifests itself as an increase in propene selectivity at the expense of propane. In contrast, allyl chloride selectivity increases at the expense of propene for the catalysts with Pt:Sn ratio ≤1. Steady state turnover frequencies (TOFs) decrease with decreasing Pt:Sn atomic ratio to a minimum for the catalyst with the Pt:Sn=1. Further decreasing the Pt:Sn ratio results in a TOF increase, with the Pt1Sn6 catalyst TOF exceeding that of monometallic Pt/C.     

Dopaminergic modulation of early signs of excitotoxicity in visualized rat neostriatal neurons

Cell swelling induced by activation of excitatory amino acid receptors is presumably the first step in a toxic cascade that may ultimately lead to cell death. Previously we showed that bath application of N-methyl-D-aspartate (NMDA) or kainate (KA) produces swelling of neostriatal cells. The present experiments examined modulation of NMDA and KA-induced cell swelling by dopamine (DA) and its receptor agonists. Nomarski optics and infra-red videomicroscopy were utilized to visualize neostriatal medium-sized neurons in thick slices from rat pups (12-18 postnatal days). Increase in somatic cross-sectional area served as the indicator of swelling induced by bath application of glutamate receptor agonists. NMDA induced cell swelling in a dose-dependent manner. Activation of DA receptors in the absence of NMDA did not produce swelling. DA and the D1 receptor agonist SKF 38393, increased the magnitude of swelling produced by NMDA. This effect was reduced in the presence of the D1 receptor antagonist, SCH 23390. In contrast, activation of D2 receptors by quinpirole decreased the magnitude of NMDA-induced cell swelling. DA slightly attenuated cell swelling induced by activation of KA receptors. Quinpirole produced a significant concentration-dependent reduction in KA-induced swelling while SKF38393 increased KA-induced swelling, but only at a low concentration of KA. Together, these results provide additional support for the hypothesis that the direction of DA modulation depends on the glutamate receptor subtype, as well as the DA receptor subtype activated. One possible consequence of these observations is that endogenous DA may be an important contributing factor in the mechanisms of cell death in Huntington's disease.     

Dehalogenative oligomerization of dichlorodifluoromethane catalyzed by activated carbon-supported Pt–Cu catalysts: effect of Cu to Pt atomic ratio

Activated carbon-supported Pt–Cu catalysts with a Cu to Pt atomic ratio in the range of 2–18 catalyze the formation of oligomerization hydrocarbon products from an equimolar mixture of CF₂Cl₂ and H₂ at 523 K. The steady-state selectivity toward C₂₊ products is 42% for the Pt1Cu2/C and increases to more than 70% when the Cu/Pt atomic ratio reaches 18:1. All catalysts deactivate with time on stream. The results of the TEM investigation are consistent with the suggestion that deactivation is attributed to carbon deposition and not to particle sintering. All of the catalysts have approximately the same average size of Pt-containing particles, independent of Cu/Pt atomic ratio, and the average size is essentially the same for the freshly reduced and used Pt–Cu catalysts. As the Cu to Pt atomic ratio is increased, a larger fraction of Cu is unalloyed with Pt. The performance of the catalysts in the CF₂Cl₂+H₂ reaction is discussed in terms of the different active sites, which catalyze different elementary reaction steps.     

The dynamic surface chemistry during the interaction of CO with ceria captured by Raman spectroscopy

The interaction of CO with ceria under conditions typically used to measure the oxygen storage capacity (OSC) of automotive three way catalysts (TWC) has been investigated by in situ Raman spectroscopy. During exposure of the ceria to CO at 623 K vibrational bands at 1582–1600 and 1331–1340 cm⁻¹ appeared; these bands increased with increasing time of exposure to CO. The band positions are consistent with phonon modes of carbon; however, assignment to carboxylate species or carbonate species cannot be excluded. Subsequent exposure to O₂ at room temperature resulted in a decrease in the intensities of the 1582–1600 and 1331–1340 cm⁻¹ bands by more than 90%. As well, exposure to O₂ at room temperature also resulted in the appearance of Raman modes characteristic of formate and peroxide surface species. The mechanism by which formate forms upon room temperature O₂ exposure is discussed in the context of the assignment of the 1582–1600 and 1331–1340 cm⁻¹ bands to carbon phonon modes which result from the disproportionation of CO on reduced ceria.     

Oligomerization pathways of dichlorodifluoromethane hydrodechlorination catalyzed by activated carbon supported Pt-Cu, Pt-Ag, Pt-Fe, and Pt-Co

Activated carbon-supported Pt-Cu, Pt-Ag, Pt-Co, Pt-Fe, and Pt catalyze the formation of oligomerization products from CF₂Cl₂ and H₂ mixture (1:1 ratio) at 523 K. All catalysts deactivate with time on stream. The Pt-Co/C catalyst exhibits the highest selectivity toward C₂–C₃ hydrocarbons (50%), whereas Pt-Cu/C is the most selective toward tetrafluoroethylene (20%). The other catalysts (Pt, Pt-Ag, Pt-Fe) exhibit negligible oligomerization activity, CH₄ and partially halogenated C₁ molecules are the main products. The performance of each catalyst is understood in terms of the difference in the stability of bimetallic particles toward segregation under dechlorination conditions.