LOCATION OF DIPHENYL-HEXATRIENE AND TRIMETHYLAMMONIUM-DIPHENYL-HEXATRIENE IN DIPALMITOYLPHOSPHATIDYLCHOLINE BILAYERS BY NEUTRON-DIFFRACTION

Neutron scattering experiments have been performed on oriented dipalmi toylphosphatidylcholine (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 affor ded by using proton-deuterium contrast methods the location of these f luorescent probes in the model membrane. Both probes exhibit bimodal d istributions in DPPC. The position, population and orientation in the two sites vary depending upon the physical state of the bilayer (gel o r 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 surfa ce (site I) and near the bilayer center, oriented at approximate to 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 p osition (site II) oriented at an angle of approximate to 25 degrees wi th respect to the bilayer normal, with the trimethylammonium group anc hored 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 m olecule penetrates deeper in the bilayer and adopts a approximate to 2 0 degrees tilt with respect to the surface, with an orientational dist ribution of +/- 10 degrees. In site II the molecule becomes perpendicu lar to the membrane surface. Changes in population of sites, both with DPH and TMA-DPH, are observed an going from low to high temperatures. They are however difficult to quantitate due to experimental conditio ns. The H2O-(H2O)-H-2 exchange experiments afforded an estimate of the water layer thickness as well as the maximum penetration of water int o the interior of the bilayer.