CONDITION FOR THE APPEARANCE OF THE METASTABLE P-BETA' PHASE IN FULLYHYDRATED PHOSPHATIDYLCHOLINES AS STUDIES BY SMALL-ANGLE X-RAY-DIFFRACTION

In the ripple phase of fully hydrated multilamellar vesicles of dipalm itoylphosphatidylcholine (DPPC), two kinds of small-angle x-ray diffra ction profiles are observed on cooling through the main transition. On e is a seemingly normal profile similar to that observed on heating an d the other is the superposition of the diffraction profiles for the p rimary (normal) and the secondary ripple structures. We found that the profile obtained depended on the cooling rate. Increasing the cooling rate from 0.1-degrees-C/min to 1-degrees-C/min caused the peaks origi nating from the secondary ripple structure to diminish. After a coolin g scan at 43-degrees-C/min, the profile became similar to that of the normal ripple structure, although a trace of the secondary ripple stru cture remains. The results are interpreted in terms of the rise and fa ll of three-dimensional correlated domains composed of both primary an d secondary ripple structures. At slow cooling rates, correlated domai ns of both kinds of ripple structures develop. As the cooling rate is increased, the domain of the primary ripple structure remains correlat ed, while that of the secondary ripple structure becomes less correlat ed. In addition, the multipeak profile appears even at rapid cooling r ates, if the final low temperature lies just below the T(m) for the ma in transition. This results suggests that formation of the correlated domains of the secondary ripple structure requires a certain time inte rval during which the DPPC vesicles experience the temperature just be low the main transition. The secondary ripple structure takes place in phosphatidycholines having more than 15 carbons in each hydrocarbon c hain upon cooling through the main transition.