Abstract
The effects of high NaCl concentrations on DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine) multilamellar and vesicle mesophases have been investigated by electron spin resonance spectroscopy with the spin labels 5-nitroxide stearic acid and di- tert-butyl-nitroxide to get information on the molecular order and fluidity of the bilayers. In the presence of up to 3 M NaCl the multilamellar phase shows an increase in molecular order, mainly in the gel phase, and a decrease of the bilayer permeability. An upward shift from about 34.3 to 38.5°C and from 40.6 to 42.5°C of both the pre- and main-phase transition temperatures is also observed. Negligible effects are observed on the liquid crystal phase. An increase in the permeability and a decrease in molecular order with a downward shift of the main transition temperature from 37.5 to 36.0°C are observed for small sonicated unilamellar vesicles. Furthermore, the effect on the liquid-crystal phase is very strong. The data are interpreted in terms of ion-induced perturbations either of the hydrogen bond network or of the electrostatic interaction in the polar zone of the DPPC bilayers. The different radii of curvature of the multilamellar and vesicle mesophases triggers the phenomena observed.
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