THERMOTROPIC PROPERTIES OF MODEL MEMBRANES COMPOSED OF POLYMERIZABLE LIPIDS .1. PHOSPHATIDYLCHOLINES CONTAINING TERMINAL ACRYLOYL, METHACRYLOYL, AND SORBYL GROUPS

The thermotropic phase behavior of hydrated bilayers of mono- and bis- substituted phosphatidylcholines (PC) containing either acryloyl, meth acryloyl, or sorbyl ester groups at the chain terminus was studied by differential scanning calorimetry. Each of these compounds exhibits a single endotherm which occurs at a temperature lower than that of the main phase transition T(m) of the corresponding linear saturated chain PC. Variation of the chain length of the sorbylPCs results in a prono unced odd/even alternation of the T(m). Consideration of the preferred conformation of glycerol ester lipids suggested by the crystal struct ure of dimyristoylPC dihydrate provides a basis for understanding the odd/even effect reported here. The interaction of the sn-2 chain sorby l ester carbonyl with neighboring methylene chains appears to be predo minantly intermolecular or intramolecular depending on whether the cha in length is even or odd, respectively. Intermolecular interaction is expected to decrease the T(m) to a greater extent than intramolecular interaction. The magnitude of the odd/even effect diminished with long er chain length as the free energy of stabilization contributed by van der Waals interchain interactions increased. A comparison of the T(m) of a sorbyl ether PC and a sorbyl ester PC revealed an unexpectedly l ow T(m), for the ether lipid. Analysis of this effect suggests previou sly undetected differences in the probable lipid chain conformations o f ether and ester PCs. The T(m) values of acryloyl-substituted PCs wer e somewhat higher than those of comparable chain-length sorbyl-substit uted PCs. The addition of an isomethyl to the acryloyl group, i.e., me thacryloyl, significantly depresses the T(m) values. These systematic thermotropic studies of polymerizable lipids provide new insights into the relationship of lipid phase behavior and lipid chain substitution patterns, which is crucial to the design of novel molecules and the s upramolecular assemblies formed from them.