DETERMINATION OF THE DIMERIZATION CONSTANT OF MEROCYANINE-540 AT THE MEMBRANE INTERFACE OF LIPID VESICLES IN THE GEL STATE

In the present work, the dimerization of merocyanine 540 (MC540) at th e interface of DPPC bilayers in gel state is investigated using a meth od by which the amount of probe remaining in the aqueous phase is negl igible. A procedure is developed in order to demonstrate that monomer- dimer equilibrium takes place only in the membrane phase. In these con ditions, absorption and emission studies show that the apparent dimeri zation constant is strongly dependent on the volume of lipid phase pre sent in the dispersion The correction by volume lipid fraction brings the dimerization constants within the order of that estimated in penta nol. The corrected K-d values increase with increasing vesicle diamete r, thus indicating that the lipid interface of multilamellar liposomes , large unilamellar vesicles and sonicated vesicles have different sol vent properties. This is congruent with the finding that the MC 540 di mer is fluorescent in gel state membranes suggesting that the environm ent for the dimer is different to that found in water or in fluid memb ranes in which it is non-fluorescent.