Fluorescent Dimers of Merocyanine 540 (MC540) in the Gel Phase of Phosphatidylcholine Liposomes

Abstract

Abstract— Fluorescence emission from merocyanine 540 (MC540) dimers was observed in dipalmitoylphosphatidylcholine (DPPC) vesicles. This unusual behavior was observed only for vesicles in the gel‐phase state. No dimer fluorescence was observed either in monopalmitoylphosphati‐dylcholine (C16PC) micelles or in liquid‐crystalline DPPC vesicles, indicating that dimer fluorescence efficiency increases in highly packed interfaces. The excitonic theory of Kasha was used to interpret the spectral features. The overall fluorescence quantum yield (φr) decreases with decreasing lipid: probe ratio, not only because of the presence of a weakly fluorescent dimer that absorbs a high fraction of the total absorbed light but also due to quenching of monomer emission. This suggests the existence of probe domains. The dimer fluorescence quantum yields (φm) were estimated in DPPC large unilamellar vesicles (LUV) and DPPC multilamellar vesicles. The dependence of φr with probe concentration is compatible with values of φm lower than 0.05. The dimerization equilibrium of MC540 in C16PC micelles and DPPC‐LUV was also studied. Apparent dimerization equilibrium constants, Kdapp and dimer absorption spectrum were calculated in C16PC micelles for the first time. The dimerization equilibrium constant in DPPC‐LUV was calculated and discussed in terms of the fraction of volume occupied by the lipid phase.