Effect of the Phase Transition in Liposomes on the Fluorescence of Amphiphilic Cyanine Dyes

The dependence of fluorescence intensity of amphiphilic cyanine dyes on the thermotropic fluid-solid phase transition of lipid bilayer membranes has been investigated. N,N′-dioctadecyl oxacarbocyanine iodide or N,N′-dioctadecyl thiacarbocyanine bromide was incorporated into single lamellar liposomes, and the fluorescence intensity of each dye was observed to be modified considerably by the phase transition of the lipid bilayers. An amphiphilic cyanine dye with single long alkyl chain, N-octadecyl-N′-methyl thiacarbocyanine bromide was also incorporated in the lipid bilayers, but in this case, the phase transition showed little effect on the fluorescence intensity. These results were interpreted in terms of the effect of the bilayer membrane fluidity and rigidity on the rotational motion of the incorporated cyanine dyes. The phase transition parameters were obtained by applying those amphiphilic dyes as the fluorescence probe of the phase transition of the liposomes.