OSMOTIC RESPONSE OF LARGE UNILAMELLAR VESICLES OF PHOSPHATIDYLCHOLINE - FACTORS DETERMINING THE RATE OF THE PROCESS AND THE PROPERTIES OF THE SHRUNKEN VESICLES

Large unilamellar vesicles of dipalmitoylphosphatidylcholine shrink due to water outflux under conditions of hypertonic osmotic shock imposed by the addition of salt to the external medium, The rate of the process depends upon the osmotic unbalance, the temperature, and the presence of additives (n-octanol or gramicidin D) incorporated to the bilayer, On the other hand, the type of electrolyte employed to impose the osmotic unbalance has little relevance upon the rate and extent of the process, The osmotic shock and/or the presence of ions capable of binding to the phospholipid heads (Ca2+) do not modify the properties of the hydrocarbon matrix of the bilayer. On the other hand, the adsorption of both charged (ANS, TNS, or merocyanine) and uncharged (n-octanol) solutes increases after the shock, reflecting significant alterations at the interface level, For the charged species, the increase is more significant in the presence of Ca2+ ions in the inner and/or outer interface. (C) 1995 Academic Press, Inc.