DYNAMICS OF MEROCYANINE-540 IN MODEL BIOMEMBRANES - PHOTOISOMERIZATION STUDIES IN SMALL UNILAMELLAR VESICLES

The fluorescence lifetime, tau(f), of merocyanine 540 (MC540) in small unilamellar vesicles was measured as a function of temperature and ch olesterol content by using phase modulation fluorometry. These vesicle s were formed by probe sonication of aqueous suspensions of egg phosph atidylcholine (PC) and cholesterol. The fluorescence lifetime of MC540 in these vesicles decreased with increasing temperature, but was inde pendent of cholesterol. The decrease in tau(f) with temperature is att ributed to trans-cis photoisomerization. At low temperatures, the inve rse of tau(f), or the fluorescence rate constant, k(f), approaches a c onstant value of 0.45 +/- 0.02 ns(-1), which corresponds to the value of the radiative rate constant, k(r), of the dye. The photoisomerizati on rate constant, k(iso), was obtained by subtracting k(r) from k(f). The temperature dependence of k(iso) is well described by an Arrhenius equation, with an activation energy of 31.5 +/- 0.9 kJ mol(-1). This Arrhenius behavior is rationalized in terms of the Smoluchowski limit for the Kramers theory for activated barrier crossing. The electronic spectra and k(iso) for MC540 in these vesicles are consistent with the dye being located in the polar headgroup region of the lipid bilayer.