Location of diphenyl-hexatriene and trimethylammonium-diphenyl-hexatriene in dipalmitoylphosphatidylcholine bilayers by neutron diffraction.

Neutron scattering experiments have been performed on oriented dipalmitoylphosphatidylcholine (DPPC) bilayers containing diphenylhexatriene (DPH) or its trimethylammonium analog (TMA-DPH). DPH and TMA-DPH were either protonated or deuterated in one of the phenyl rings which afforded by using proton-deuterium contrast methods the location of these fluorescent probes in the model membrane. Both probes exhibit bimodal distributions in DPPC. The position, population and orientation in the two sites vary depending upon the physical state of the bilayer (gel or fluid) and the presence or absence of the TMA group. In gel (L beta') phase lipids DPH is located close and parallel to the bilayer surface (site I) and near the bilayer center, oriented at approximately 30 degrees with respect to the normal to the surface (site II). On going to the fluid (L alpha) phase, a distribution of orientations around the parallel to the surface is only observed for site II. Orientation of DPH in site I is unchanged. In the gel phase TMA-DPH is found in a position close and parallel to the bilayer surface (site I) and in a position (site II) oriented at an angle of approximately 25 degrees with respect to the bilayer normal, with the trimethylammonium group anchored in the head group domain. On going to the fluid phase there is a change in molecular orientation of each of the sites. In site I the molecule penetrates deeper in the bilayer and adopts a approximately 20 degrees tilt with respect to the surface, with an orientational distribution of +/- 10 degrees. In site II the molecule becomes perpendicular to the membrane surface. Changes in population of sites, both with DPH and TMA-DPH, are observed on going from low to high temperatures. They are however difficult to quantitate due to experimental conditions. The H2O-2H2O exchange experiments afforded an estimate of the water layer thickness as well as the maximum penetration of water into the interior of the bilayer.