Effects of phospholipid acyl chain structure on physical properties: I. Isobranched phosphatidylcholines

All of the isobranched fatty acids of 12 to 18 carbons have been synthesized in gram quantities by a convenient acetylene coupling reaction followed by catalytic hydrogenation. The corresponding phosphatidylcholines (PCs) have been synthesized and their thermotropic phase behavior investigated by differential thermal analysis. The isobranched acyl phosphatidylcholines show gel-to-liquid-crystalline phase transition temperature (T_cs) some 20°C below those of the corresponding straight-chain PCs and appear to exhibit two slowly interconverting low-temperature phases below T_c. The observed strong alternation of T_cs between isobranched PCs with odd- and even-carbon number acyl chains contrasts with the behavior of the straight-chain PCs and suggests that the acyl chains of the branched-chain PCs are strongly tilted with respect to the bilayer normal below and/or above T_c while those of the straight-chain PCs are not. These results clearly indicate significant differences in the overall packing of branched-and straight-chain PCs in the gel and possibly the liquid-crystalline state.