Effect of liposomal size on the calorimetric behavior of mixed-chain phosphatidylcholine bilayer dispersions.

The effect of liposomal size on the endothermic transition profiles of the saturated mixed-chain phosphatidylcholines 1-stearoyl-2-myristoyl-sn-glycero-3-phosphocholine and 1-stearoyl-2-caproyl-sn-glycero-3-phosphocholine has been investigated. Liposomal bilayer dispersions of progressively smaller average diameter were prepared by extrusion of coarse multilamellar preparations of these lipids through polycarbonate membrane filters of decreasing pore size from 3-to 0.2-micrometers diameter. These samples were then investigated by high-sensitivity differential scanning calorimetry and negative-strain electron microscopy. It was found that coarse dispersions of the above lipids are composed of liposomes whose average diameter is considerably smaller than that typically associated with multilamellar liposomes of synthetic phosphatidylcholines. This fact, coupled with an analysis of the size dependence of the transition cooperativity, leads to the conclusion that the small size of the liposomes limits the transition cooperativity in coarse dispersions of these mixed-chain phosphatidylcholines. Thus, the broad and highly asymmetric transition profiles that have been observed in previous studies of these phosphatidylcholines are postulated to arise largely from this size dependence, rather than from characteristic packing properties of the phospholipid acyl chains as has been previously suggested.