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.