SURFACE CHANGES INDUCED BY OSMOTIC SHRINKAGE ON LARGE UNILAMELLAR VESICLES

Osmotic shrinkage changes the surface properties of dipalmitoylphospha tidylcholine large unilamellar vesicles depending on the phase state o f the bilayer. In the gel state, shrinkage produces an increase in the adsorption of hydrophobic dyes, such as Merocyanine 540 (MC540) monom ers, toluidine and anilinonaphthalene sulfonic acid (TNS, ANS). In the fluid state, shrinkage does not affect the bilayer surface when gradi ents between the inner and the outer compartments below 0.2-0.25 M NaC l (higher concentration outside) are applied. Larger differences in co ncentrations produce an increase in packing as inferred from the desor ption of the MC monomers. Kinetic experiments show that the surface ch anges correlate with the volume decrease produced by the water extrusi on from the vesicle interior. It is interpreted that the decrease of w ater content compels the vesicles to a state in which defects at the m embrane surface are likely to occur when the bilayer is in the gel sta te.