Interaction between artificial membranes and enflurane, a general volatile anesthetic: DPPC-enflurane interaction

ABSTRACT The structural modifications of the dipalmitoylphosphatidylcholine (DPPC) organization induced by increasing concentration of the volatile anesthetic enflurane have been studied by differential scanning calorimetry, small-angle, and wide-angle x-ray scattering. The interaction of enflurane with DPPC depends on at least two factors: the enflurane-to-lipid con-centration ratio and the initial organization of the lipids. At 258C (gel state), the penetration of enflurane within the lipids induces the apparition of two different mixed lipid phases. At low anesthetic-to-lipid molar ratio, the smectic distance increases whereas the direction of the chain tilt changes from a tilt toward next-neighbors to a tilt between next-neighbors creating a new gel phase called Lb9 2NNN. At high ratio, the smectic distance is much smaller than for the pure Lb9 DPPC phase, i.e., 50 Å compared to 65 Å, the aliphatic chains are perpendicular to the membrane and the fusion temperature of the phase is 338C. The electron profile of this phase that has been called Lb i, indicates that the lipids are fully interdigitated. At 458C (fluid state), a new melted phase, called La 2, was found, in which the smectic distance decreased compared to the initial pure La 1 DPPC phase. The thermotropic behavior of the mixed phases has also been characterized by simultaneous x-ray scattering and differentia