Effect of Phase Transition on Photosensitized Radical Polymerization of Diene-Containing Lipids as Liposomes¹

l,2-Bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) was polymerized in liposomes by organic free radicals which were generated by photosensitized cleavage of the added azo-type radical initiators such as azobis(isobutyronitrile) (AIBN) or azobis(2-amidinopropane) dihydrochloride (AAPD). Selective polymerization of diene groups on the 1- or 2-acyl chain of DODPC was carried out with water-insoluble AIBN or water-soluble AAPD, respectively, at any temperature, even at temperatures lower than the phase transition temperature (T_c) of the DODPC liposomes. The diene group in the 1-acyl chain locates in the hydrophobic region of the bilayer, whereas that in the 2-acyl chain faces an aqueous phase at any temperature. When AAPD was added to the outer aqueous phase of DODPC liposomes, polymerization conversion initiated by the photosensitized AAPD radicals reached around 50% above the T_cbut it remained 27% below the T_c. This confirms that AAPD added to the outer aqueous phase of the liposome can penetrate the lipid membrane above the T_c, but not below it. The liposomes which were polymerized in either the outer or the inner half of the bilayer membrane were therefore prepared by applying this penetration profile. The liposomes polymerized only in the outer layer were as stable as those fully polymerized. The initial polymerization rate of DODPC initiated by photosensitized AIBN radicals in the lipid membrane also reflected the membrane viscosity. Lower polymerization conversions, reflecting considerable recombination of free radicals, were found below T_cdue to higher membrane microviscosity.