Atomic force microscopic investigations of the gel phase of phosphatidylcholines containing omega-cyclohexyl fatty acids

The gel phase of 1,2-di-13-cyclohexyltridecanoyl-sn-glycero-3-phosphocholine (13cyPC) and 1,2-di-14-cyclohexyltetradecanoyl-sn-glycero-3-phosphocholine (14cyPC) was investigated by atomic force microscopy. 13cyPC forms a gel phase with a very high degree of chain order resembling the subgel phase of PCs with saturated linear chains. For 13cyPC, flat surfaces, ripple structures, and singular ripple lines at domain borders were observed. Ripples developed only in multilamellar stacks (5 or more lamellae) on the top surface of the stack. The ripple periodicity was 33 nm and the ripple amplitude only 0.3 nm. 14cyPC shows a gel phase with much higher degree of disorder. For 14cyPC, ripples were observed on all layers, the ripples having much larger amplitudes (3 nm) but a similar periodicity of 37 nm. In addition, irregular bilayer deformations appearing almost like honeycomb structures were observed. The apparent periodicity of the holes in this structure was 30 nm and thus similar to the ripple period. The hole depth was 2 nm or larger. These deformations were only very pronounced for every second layer. For 14cyPC, the higher disorder in the gel phase seen by spectroscopic techniques enables larger amplitudes for the deformation of the lamellae. These deformations may be related to the convex-concave bilayer deformations seen by freeze fracture electron microscopy in other systems (H. W. Meyer, K. Semmler, W. Rettig, W. Pohle, A. S. Ulrich, S. Grage, C. Selle and P. J. Quinn, Chem. Phys. Lipids, 2000, 105, 149).