John Goossens

Valinomycin-mediated transport of alkali cations through solid supported membranes

Abstract Highly flexible lipid bilayers were immobilized on gold surfaces via self-assembly of thiolipids on gold in order to obtain an appropriate matrix for the carrier valinomycin. The bilayers were created by chemisorption of thiolipids to form the first hydrophobic monolayer on the gold substrate and subsequent fusion of unilamellar 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine vesicles resulted in the second physisorbed monolayer. The synthesis of the thiolipid is based on the functionalization of phosphatidylethanolamine with a linker consisting of succinic acid hooked on a thiolated tetraethyleneglycol serving as the hydrophilic linker responsible for the flexibility of the monolayer and the anchor group. The solid supported membranes were characterized by X-ray photoelectron spectroscopy and impedance spectroscopy. The latter technique revealed that the bilayers form a considerable barrier against ions in solution. The capacitance and resistivity of the prepared bilayers amount to C m =1.0±0.2 μ F/cm 2 and R m =11 000±1000 Ω cm 2 in 10 mM Tris, 50 mM N(CH 3 ) 4 Cl, pH 7.0. Ion transport of sodium and potassium ions through the bilayers in the absence and presence of valinomycin was investigated by impedance spectroscopy in the frequency range of 10 −1 –10 6 s −1 . Valinomycin was dissolved in dimethyl sulfoxide and added to the bilayer. Data evaluation was performed using a modified model established by de Levie for carrier-mediated ion transport through free-standing lipid bilayers making use of the continuum equation. The membrane resistance showed the expected linear relation to the reciprocal of the valinomycin concentration in solution. The conductivity of the membrane in the presence of valinomycin corrected for the conductivity in the absence of the carrier vs. the concentration of alkali ions in solution showed a tenfold larger slope for potassium than for sodium ions.

Influence of praziquantel and Ca2+ on the bilayer-isotropic-hexagonal transition of model membranes

Praziquantel induces a bilayer to isotropic transition in the absence of Ca2+ in a mixed phospholipid membrane consisting of phosphatidylethanolamine (PE) and phosphatidylserine (PS) at a molar ratio of 2:1 at 25 degrees C or 35 degrees C, irrespective of the lipid/drug molar ratio (10:1; 2:1). Addition of Ca2+ at different PS/Ca2+ molar ratios (4:1, 2:1 or 1:1) leads to a transition to a hexagonal (HII) state. With a phospholipid membrane of different composition (PE/PS molar ratio 4:1) praziquantel exerts quite different effects in the presence of Ca2+ (PS/Ca2+ molar ratio 1:1). An isotropic-signal appears together with a bilayer one at 25 degrees C, while a HII-signal can be detected at 35 degrees C. Thus, two separate phases coexist at this PE/PS molar ratio, while at a PE/PS molar ratio of 2:1, praziquantel and Ca2+ induce only a HII-signal. The results with these model membranes show that praziquantel and Ca2+ exert drastic influences on bilayer-isotropic-hexagonal transitions. The possibility that this drug might act in the schistosomal tegumental membranes in the same way is discussed.