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-crystal line phase transition of large unilamellar vesicle (LUV) samples of di lauroylphosphatidylcholine (DC12PC, or DLPC), ditridecanoylphosphatidy lcholine (DC13PC) and dipentadecanoylphosphatidylcholine (DC15PC) with a vesicle size of 100 nm. The heat capacity showed a broad peak, whic h is qualitatively similar to that observed in LUV sample with the sam e vesicle size of dimyristoylphosphatidylcholine (DC14PC, or DMPC). It was found that the nature of the transition depends on the chain leng th of the lipid molecules. The observed half-widths of the anomalies a re 2.4 K in DC12PC and 0.7 K in DC13PC, which are both larger than tha t in DC14PC. The transition is first order in DC15PC. It was shown tha t the results for DC12PC, DC13PC, DC14PC, and DC15PC can be described in a semiquantitative way with the theoretical calculation, which take s into account the curvature of the vesicle surface.