TRADITIONAL STATES IN THE PRETRANSITION RANGE (L-BETA'-P-BETA') OF FULLY HYDRATED DIPALMITOYLPHOSPHATIDYLCHOLINE SYSTEMS

Transitional states formed in the pretransition (Lbeta'<->Pbeta') rang e of fully hydrated dipalmitoylphosphatidylcholine (DPPC) liposomes we re studied by using simultaneous small-and wide-angle X-ray scattering and freeze-fracture methods. A controlled incubation technique was ap plied to achieve the conditions, as close as possible, of equilibrium. After quenching and heating from the temperature domains of the phase s P-beta'(38 degrees C) and L-beta'(28 degrees C), respectively, into the transition range (31-36 degrees C), the formation of intermediate states depends not only on the temperature after quenching or heating but also on the starting temperatures and on the scan rate. The struct ural destruction in the one dimensional layer arrangement goes paralle l with the change of the hydrocarbon chain packing. The superposition of the characteristic Bragg reflections corresponding to the parent ph ases (L-beta' and P-beta', respectively), suggest phase coexistence wh ich is also observed in the electron micrographs. The effect of Ca2+ i on in small concentration on the liposomes was also investigated. In t he presence of Ca-ions some decrease of the structural coherence was f ound in both phases. Contrary to the general decrease in regularity, t he system with Ca-ions exhibits a sharper transition range.