Effect of chain length on the heat-capacity anomaly at the gel to liquid-crystalline phase transition in unilamellar vesicles of phosphatidylcholines

Dynamic heat capacity has been measured near the gel to liquid-crystalline phase transition of large unilamellar vesicle (LUV) samples of dilauroylphosphatidylcholine (DC12PC, or DLPC), ditridecanoylphosphatidylcholine (DC13PC) and dipentadecanoylphosphatidylcholine (DC15PC) with a vesicle size of 100 nm. The heat capacity showed a broad peak, which is qualitatively similar to that observed in LUV sample with the same vesicle size of dimyristoylphosphatidylcholine (DC14PC, or DMPC). It was found that the nature of the transition depends on the chain length of the lipid molecules. The observed half-widths of the anomalies are 2.4 K in DC12PC and 0.7 K in DC13PC, which are both larger than that in DC14PC. The transition is first order in DC15PC. It was shown that the results for DC12PC, DC13PC, DC14PC, and DC15PC can be described in a semiquantitative way with the theoretical calculation, which takes into account the curvature of the vesicle surface.