PHASE-BEHAVIOR OF THE ANTINEOPLASTIC ETHER LIPID 1-O-OCTADECYL-2-O-METHYL-GLYCERO-3-PHOSPHOCHOLINE

The physicochemical properties of the antineoplastic etherphospholipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine were examined in th e concentration range 1-35% (w/w) lipid, as a function of temperature (range -10 degrees C to 40 degrees C) and of different aqueous solvent s by dynamic light scattering, small- and wide-angle X-ray scattering, differential scanning calorimetry and ultrasonic speed measurements. On cooling the lipid dispersion undergoes a phase transition near 6 de grees C, transforming slowly from a micellar into a lamellar gel phase with interdigitating hydrocarbon chains. The lamellar repeat distance is nearly constant over the hydration range 65-90% buffer (d = 5.09-5 .14 nm). The size of the micelles in terms of the hydrodynamic radius is 3.8 +/- 0.1 nm, the polydispersity is low. Their average shape is s pherical. The electron density distribution across the micelle gives 2 .5 nm for the extension of the hydrocarbon chains and 1.5 nm for the p olar moiety. The existence of micelles was verified up to a concentrat ion of 35% lipid. Throughout this concentration range size and shape d o not change significantly. The kinetics of formation of the low-tempe rature phase is slow on cooling, increasing with increasing concentrat ion. Upon heating the phase behavior shows a hysteresis. The extended lamellar organizations start to break down into smaller aggregates nea r 3 degrees C. The micellar phase is reformed near 20 degrees C.