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 the interface of DPPC bilayers in gel state is investigated using a method by which the amount of probe remaining in the aqueous phase is negligible. A procedure is developed in order to demonstrate that monomer-dimer equilibrium takes place only in the membrane phase. In these conditions, absorption and emission studies show that the apparent dimerization constant is strongly dependent on the volume of lipid phase present in the dispersion The correction by volume lipid fraction brings the dimerization constants within the order of that estimated in pentanol. The corrected K-d values increase with increasing vesicle diameter, thus indicating that the lipid interface of multilamellar liposomes, large unilamellar vesicles and sonicated vesicles have different solvent properties. This is congruent with the finding that the MC 540 dimer is fluorescent in gel state membranes suggesting that the environment for the dimer is different to that found in water or in fluid membranes in which it is non-fluorescent.