ON THE ORIGIN OF THERMAL L(BETA')-]P(BETA') PRETRANSITION IN THE LAMELLAR PHOSPHOLIPID-MEMBRANES

Small-angle X-ray diffraction as well as dynamic-light-scattering experiments with fully hydrated phosphatidylcholine suspensions both show that the lipid chain tilt, phi(tilt), in the lamellar gel (L(beta')) phase decreases with increasing temperature. This explains how and why only some lipids undergo a L(beta') --> P(beta') (Sm(C) --> Sm(A)) phase transition. Such a pretransition is the natural response to the conflict between the phi(tilt) (T)-induced tendency of the lipid molecular area in the L(beta')-gel-phase to decrease with increasing temperature and the simultaneous and increasing propensity of the highly polar lipid headgroups to increase (or at least retain) their large area in order to maximize their hydration.