EFFECT OF CHOLESTEROL ON THE ETHANOL-INDUCED INTERDIGITATED GEL PHASE IN PHOSPHATIDYLCHOLINE - USE OF FLUOROPHORE PYRENE-LABELED PHOSPHATIDYLCHOLINE

It is now recognized that many amphiphilic molecules such as ethanol can induce the formation of the fully interdigitated gel phase (L-beta-I) in phosphatidylcholines (PC's). In the present study, we have developed a simple detection method for the L-beta-I phase using pyrene-labeled PC (PyrPC), which is a PC analogue with a covalently coupled pyrene moiety at the end of one of its acyl chains. The intensity ratio of its fluorescence vibrational bands is a reflection of the polarity of the environment of the fluorophore. We have tested this fluorophore in several established interdigitated lipid systems, including 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (1,2-DPPC) in the presence of high concentrations of ethanol and 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) and 1,3-dipalmitoyl-sn-glycero-2-phosphocholine (1,3-DPPC) in the absence of any additives. We have found in each of these systems that the ratio of the intensities of band III (387.5 nm) to band I (376.5 nm) is sensitive to the lipid phase change from the noninterdigitated L-beta' phase to the interdigitated L-beta-I phase. By comparison of the III/I ratios for PyrPC in the lipid systems with the III/I ratios for methylpyrene in organic solvents, it was shown that the polarity of the PyrPC environment in the L-beta-I phase is similar to that of pentanol or ethanol. Using this method, we investigated the effect of cholesterol on the ethanol induction of the interdigitated gel phase in 1,2-DPPC. We found that the ethanol induction of the interdigitated gel phase is prevented by the presence of 20 mol % cholesterol.