RIPPLE PHASE-FORMATION IN PHOSPHATIDYLCHOLINE - EFFECT OF ACYL-CHAIN RELATIVE LENGTH, POSITION, AND UNSATURATION

The ripple phases of a range of mixed-acid phosphatidylcholine derivat ives with one saturated C-18 chain and the other chain of variable len gth or unsaturation were studied using differential scanning calorimet ry, x-ray diffraction, and freeze-fracture electron microscopy. The te mperature dependences of their ripple wavelength (lambda), stacking re peat 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 saturat ed mixed-chain derivatives and the derivatives containing unsaturated chains were found to be larger and smaller, respectively, than those o f homoacyl derivatives of the same equivalent chain length. This was s hown to be consistent with the sliding-chain model proposed by Cevc (B iochemistry 30, 7186-7197, 1991). The tendency of phosphatidylcholine derivatives to form asymmetric ripple phases and the possible molecula r organization of such phases are discussed in terms of different mode ls. X-ray diffraction evidence was found for the existence of a second ary ripple phase with a wavelength about 1.8 times greater than the co nventional pretransition ripple phase. This phase is formed in samples freshly cooled from the liquid-crystal phase and tends to persist lon ger than the conventional ripple phase on cooling to the gel phase. Fr eeze-fracture electron microscopy suggests that it is symmetric in cro ss section possibly reflecting the interaction of two opposing pretran sition ripples.