Traditional states in the pretransition range (L-beta'-P-beta') of fully hydrated dipalmitoylphosphatidylcholine systems

Transitional states formed in the pretransition (Lbeta'<->Pbeta') range of fully hydrated dipalmitoylphosphatidylcholine (DPPC) liposomes were studied by using simultaneous small-and wide-angle X-ray scattering and freeze-fracture methods. A controlled incubation technique was applied to achieve the conditions, as close as possible, of equilibrium. After quenching and heating from the temperature domains of the phases 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 structural destruction in the one dimensional layer arrangement goes parallel with the change of the hydrocarbon chain packing. The superposition of the characteristic Bragg reflections corresponding to the parent phases (L-beta' and P-beta', respectively), suggest phase coexistence which is also observed in the electron micrographs. The effect of Ca2+ ion in small concentration on the liposomes was also investigated. In the presence of Ca-ions some decrease of the structural coherence was found in both phases. Contrary to the general decrease in regularity, the system with Ca-ions exhibits a sharper transition range.