Self-Distribution of Dye and Isoflurane in the DPPC Bilayer

Abstract

Volatile anesthetics are postulated to partition to the lipid-water interface. Lipid-soluble dyes, such as diphenylhexatriene (DPH) and its tetramethylammonium analog (TMA-DPH), have been measured, using quenching by tail-bound nitroxide spin labels, to span a bilayer leaflet, with the polar end of TMA-DPH residing ∼4Å closer to the interface than DPH. Using empirical, non-polarizable force fields for the dyes and for isoflurane, and various starting positions, we assessed dye and anesthetic positions after 10-ns dynamics simulations at 330 K and isotropic pressure. Structural equilibration was evaluated by tracking tetragonal simulation cell dimensions. Dye molecules rapidly (<1 ns) migrated spontaneously into one of the 64-molecule dipalmitoylphosphatidylcholine leaflets. TMA-DPH resided closer to the surface of the bilayer with its axis parallel to the bilayer normal, whereas DPH often assumed tilted positions. Isoflurane molecules (4, 8, 32 or 128) were positioned in the bulk water (3363 molecules) near a bilayer with either a pair of DPH or TMA-DPH molecules, corresponding to anesthetic molalities of 0.06-1.94 m. In the high density simulations, they aggregated into a separate phase surrounded by water, and few molecules entered the lipid bilayer. This is consistent with the solubility of isoflurane in water, 0.0133 M at room temperature and declining with increasing temperature, ∼2-fold/20 K. At the lower two densities, several or most anesthetic molecules were absorbed into the bilayer. First, they adsorbed to the headgroup region, where they paused briefly, but then they diffused to the tail region, where they typically stayed the remainder of the simulation. These studies lay the groundwork for assessment of the effect of anesthetics on the wobble motions of the dye molecules in the bilayer, for comparison to fluorescence anisotropy measurements.