Ripple phase formation in phosphatidylcholine: Effect of acyl chain relative length, position, and unsaturation

The ripple phases of a range of mixed-acid phosphatidylcholine derivatives with one saturated ${\mathrm{C}}_{18}$ chain and the other chain of variable length or unsaturation were studied using differential scanning calorimetry, x-ray diffraction, and freeze-fracture electron microscopy. The temperature dependences of their ripple wavelength (λ), stacking repeat distance ${(d}_s),$ and the monoclinic angle ${\theta }_m$ defining their unit cell were measured and found to be consistent with an inherently asymmetric ripple phase with an amplitude that slowly increases with temperature. The temperature spans of the ripple phases of the saturated mixed-chain derivatives and the derivatives containing unsaturated chains were found to be larger and smaller, respectively, than those of homoacyl derivatives of the same equivalent chain length. This was shown to be consistent with the sliding-chain model proposed by Cevc (Biochemistry 30, 7186–7197, 1991). The tendency of phosphatidylcholine derivatives to form asymmetric ripple phases and the possible molecular organization of such phases are discussed in terms of different models. X-ray diffraction evidence was found for the existence of a secondary ripple phase with a wavelength about 1.8 times greater than the conventional pretransition ripple phase. This phase is formed in samples freshly cooled from the liquid-crystal phase and tends to persist longer than the conventional ripple phase on cooling to the gel phase. Freeze-fracture electron microscopy suggests that it is symmetric in cross section possibly reflecting the interaction of two opposing pretransition ripples.