Bilayer characteristics of 1,3-dialkyl- and 1,3-diacyl-rac-glycero-2-phosphocholines

Series of 1,3-dialkyl- and 1,3-diacyl-rac-glycero-2-phosphocholines (alkyl tail, C₁₄, C₁₆, and C₁₈: acyl tail, C₁₂, C₁₄, C₁₆, and C₁₈) were prepared. They form stable bilayer aggregates (vesicle and lamella) in water, as confirmed by electron microscopy. Differential scanning calorimetry showed the crystal-to-liquid crystal phase transition of these bilayers to occur at higher temperatures with increasing chain lengths. The phase-transition behavior of the phosphocholine bilayers was not much affected by the mode of connection at the glycerol unit: dialkyl vs diacyl, or 1,2-disubstitution (lecithin) vs 1,3-disubstitution. Entrapment of glucosamine and ethanolamine by the synthetic 1,3-type phosphatidylcholine vesicle was probed by the fluorescamine method. Fluorescence quenching of trapped riboflavin established that the barrier function of these bilayers against alkali is similar to that of lecithin bilayer vesicles. The barrier function is lost in the liquid crystalline state. Phase separation of azobenzene-containing amphiphiles was examined in the phosphocholine and other bilayer matrices. Different phase-separation behaviors observed were explicable in terms of the head group interaction. All of these data indicate that 1,3-type phosphatidylcholines possess essentially the same bilayer characteristics as those of lecithin (1,2-type).