Quenching of perylene fluorescence by Co(2+) ions in dipalmitoylphosphatidylcholine (DPPC) vesicles

We report the fluorescence quenching of perylene by CoCl2·6H2O in small unilamellar DPPC vesicles via energy transfer. At the probe-to-lipid ratio of 1∶200 and quencher to lipid ratios of ≥12.5∶1, donor-donor energy transfer between clustered perylene molecules was observed as well as energy transfer from the perylene molecules to cobalt ions bothabove andbelow the main phase transition temperature of the lipid. The fluorescence quenching of perylene by CoCl2·6H2O in the lipid gel state is shown to be well described by Förster long-range energy transfer when both donor-donor and donor-acceptor energy transfer are considered. In the liquid crystalline phase diffusion of the molecules is described as well as energy transfer. The interaction radiusR 0 for energy transfer from perylene to Co(2+) is found to be ∼13.4±1.1 Å in the gel phase at 303 K, in good agreement with the theoretical value forR 0 of 13.9 Å. In the liquid crystalline phase at 323 K the lower value obtained forR 0, ∼11.3±1.6 Å, is attributed to saturation of the Co(2+) ions at the interfacial region of the bilayer. A diffusion coefficient of (1.06±0.15)×10(-6) cm(2) s(-1) is obtained for perylene-cobalt diffusion in the liquid crystalline phase at 323K.