X-ray analysis and calorimetry on phosphatidylcholine model membranes. The influence of length and position of acyl chains upon structure and phase behaviour.

The effect of variation of the acyl chain composition of phosphatidylcholines upon thermal behaviour of multilamellar liposomes was evaluated by calorimetry and X-ray studies. A total of thirteen different phosphatidylcholines were examined. They differed from each other in the length as well as in the position of the acyl chains in the glycerol backbone. The experimental results show that the hitherto accepted phase scheme for phosphatidylcholine-water systems is incomplete and has to be extended to include the behaviour of samples that have been stored for long times at low temperatures. The X-ray results show that the structure of the new low-temperature phase is not in agreement with the hexagonal packing of the acyl chains. To explain the X-ray results, a two-dimensional orthorhombic unit cell has to be assumed in order to fit all the observed reflexes in the wide-angle region.