Steady-state fluorescence quenching for detecting acyl chain interdigitation in phosphatidylcholine vesicles

In the present study we have demonstrated the detection of the transition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar vesicles from the noninterdigitated gel to the fully interdigitated gel phase in the presence of ethanol or ethylene glycol (EG) using the method of fluorescence quenching. This method is based on the change of accessibility of 2-(12-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3- phosphatidyl-choline (NBD-PC), a membrane-buried fluorophore, to iodide, a quencher in the aqueous solution, during the phase transition. It is found that accessible fluorophore appears to increase at ethanol and EG concentrations known for inducing DPPC interdigitation. This increase in accessibility is either due to the relocation of the fluorescent moiety closer to the lipid-water interface or an increase in the ability of the quencher to penetrate into the loosely packed headgroup region of the interdigitated domain or both. Our results suggest the coexistence of interdigitated and noninterdigitated phases in the phospholipid vesicles and the method of fluorescence quenching might be useful in quantitating the percentage of phospholipids which are interdigitated.