Molecular order parameter profiles and diffusion coefficients of cationic lipid bilayers on a solid support

The molecular order and lateral diffusion in fluid cationic bilayers of chain perdeuterated DPTAP (1,2-dipalmitoyl-d(62)-trimethylammonium-propane) on a spherical solid support were studied by deuterium NMR and microcalorimetry. In comparison to the zwitterionic perdeuterated DPPC (1,2-dipalmitoyl-d(62)sn-glycero-3-phosphocholine), the molecular order parameter profile of the palmitoyl chains of DPTAP shows significantly higher molecular order toward the interface while similar values were obtained in the vicinity of the bilayer center. The lateral diffusion coefficient D of DPTAP is D = (13.2 +/- 1.5) x 10(-12) m(2)/s at 60 degreesC, slightly below that of DPPC at this temperature. Increasing the ionic strength caused the value of D to decrease further. The results suggest that despite the cationic charge the supported DPTAP bilayer features tighter molecular packing and a reduced lateral diffusivity of its constituents than for the zwitterionic DPPC on a solid support.