CIRCULAR DICHROIC PROPERTIES OF PHOSPHATIDYLCHOLINE LIPOSOMES

The temperature dependence of the circular dichroic properties of lipo somes from different phosphatidylcholines has been investigated with t he aim of clarifying whether circular dichroism (CD) measurements can give information on the conformational flexibility in the region of th e head group of phospholipids within the bilayer of liposomes. These C D measurements reflect electronic transitions of the intrinsic ester c hromophors localized near the chiral center of the phospholipids, and they support earlier NMR and molecular dynamics studies which have sho wn that a considerable conformational rigidification in the glycerol p art of the phosphatidylcholine molecules in Liposomes exists. These co nformational restrictions are more significant at lower temperature, s howing a significant increase in CD intensity around 220 nm on cooling the liposomes below the main phase transition temperature T-c. To pro be the hydrophobic domain of the liposomes. in which no intrinsic chro mophor is localized, the achiral, linear, water insoluble 1,6-diphenyl -1,3,5-hexatriene (DPH) molecule has been solubilized in liposomes. Be low T-c DPH shows an (induced) CD spectrum with multiple maximums in t he region of the absorption maximums of DPH (300-400 nm), the sign of the CD signal being dependent on the chirality of the phospholipids us ed. Above T-c, DPH in the liposomes is not CD active. On the basis of these findings, the use of the induced CD of DPH is proposed as an alt ernative method for the estimation of T-c of phospholipid bilayers.