EFFECT OF UNILAMELLAR VESICLE SIZE ON ETHANOL-INDUCED INTERDIGITATIONIN DIPALMITOYLPHOSPHATIDYLCHOLINE

Unilamellar liposomes are widely used as model membranes to represent and study the properties of biological membranes and as potential drug delivery systems. It is well established that ethanol and other amphi philes induce the interdigitated L(beta)I phase in multilamellar vesic les of dipalmitoylphosphatidylcholine (DPPC). However, all of the work on this phase has been performed using the hand shaken multilamellar preparations. In the present report, we have studied the induction of interdigitation in a series of unilamellar vesicles prepared by sonica tion and by extrusion. The methods used to characterize the vesicles w ere freeze fracture electron microscopy and quasielastic light scatter ing (QELS). Two fluorescence methods were used to detect interdigitati on, the DPH fluorescence quenching method (Nambi, P., Rowe. E.S. and M clntosh, T.J. (1988) Biochemistry 27, 9175-9182) and the pyrene-PC flu orescence method (Komatsu, H. and Rowe, E.S. (1991) Biochemistry 30: 2 463-2470). It was found that sonicated vesicles are not stable in the presence of interdigitating concentrations of ethanol; they form highe r aggregates at all temperatures examined. The behavior of the extrude d vesicles was different from that of the SUV: each size studied was s table in the presence of ethanol, although they exhibited an increase in size. It was shown that extruded vesicles having a 200-nm or greate r diameter become interdigitated in the presence of ethanol. The thres hold concentration for interdigitation in vesicles is greater than tha t for MLVs and it decreases with increasing vesicle size, approaching the MLV value for the largest vesicles.