New evidence for gel-liquid crystalline phase coexistence in the ripple phase of phosphatidylcholines

Experimental evidence supporting the hypothesis of gel-liquid crystalline p hase coexistence in the stable ripple phase of diacylphosphatidylcholines h as been obtained from time-resolved X-ray small- (SAXS) and wide-angle diff raction (WAXS) in the millisecond to second time domain. The pretransition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) exhibits a thin lamel lar liquid crystalline intermediate phase (designated L-alpha*) if driven f ar away from equilibrium by an infrared temperature jump (T-jump) technique . The findings can be described by a two-step model. (1) Instantaneously wi th the T-jump, an anomalously thin lamellar liquid crystalline intermediate phase (d = 5.6-5.8 nm) forms, coexisting with the original gel-phase L-bet a'. Within the first seconds, the lamellar repeat distance of the intermedi ate increases to a value of about 6.7 nm. A closer examination of these kin etics reveals two relaxation components: a fast process, proceeding within tenths of a second, and a slow process, on the time scale of a few seconds. (2) Finally, both the liquid crystalline and the gel-phase relax into the stable ripple phase P-beta'. The total process time of the transition is ne arly independent of the addition of NaCl, but varies strongly with the chai n length of the lecithin species.