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 expe riments with fully hydrated phosphatidylcholine suspensions both show that the lipid chain tilt, phi(tilt), in the lamellar gel (L(beta')) p hase decreases with increasing temperature. This explains how and why only some lipids undergo a L(beta') --> P(beta') (Sm(C) --> Sm(A)) pha se transition. Such a pretransition is the natural response to the con flict between the phi(tilt) (T)-induced tendency of the lipid molecula r area in the L(beta')-gel-phase to decrease with increasing temperatu re 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.