Structure and mechanical properties of giant lipid (DMPC) vesicle bilayers from 20.degree.C below to 10.degree.C above the liquid crystal-crystalline phase transition at 24.degree.C
We have used micromechanical tests to measure the thermoelastic properties of the liquid and gel phases of dimyristoylphosphatidylcholine (DMPC). We have found that the rippled P.beta.'' phase is only formed when a vesicle is cooled to temperatures below the main acyl chain crystallization transition, Tc, under zero or very low membrane tension. We also found that the P.beta.'' surface ripple or superlattice can be pulled flat under high membrane tension into a planar structure. For a ripple structure formed by acyl chains perpendicular to the projected plane, the projected area change that results from a flattening process is a direct measure of the molecular crystal angle. As such, the crystal angle was found to increase from about 24.degree. just below Tc to about 33.degree. below the pretransition. It was observed that the P.beta.'' superlattice did not form when annealed L.beta.'' phase vesicles were heated from 5.degree. C to Tc; likewise, ripples did not form when the membrane was held under large tension during freezing from the L.alpha. phase. Each of these three procedures could be used to create a metastable planar structure which we have termed L*.beta.'' since it is lamellar and plane-crystalline with acyl chains tilted to the bilayer plane. However, we show that this structure is not as condensed as the L.beta.'' phase below 10.degree. C. On the basis of observed changes in vesicle projected area at the main transition and comparison of the elastic area compressibility moduli measured for each of the structures (L.alpha., LL*.beta., and L.beta.''), the P.beta.'' phase is shown to be a soft crystalline solid that possesses some degree of chain disorder and slight liquid-like character. Below the pretransition temperature, bilayers exhibited a much lower compressibility indicative of further condensation to a more solid crystalline phase. At intermediate temperature for the P.beta.'' phase, we observed that the rippled surface behaved in a weakly elastic manner at low membrane tensions. On the basis of this observation, we have developed a mechanical model for the rippled phase that represents the material as a pleated surface where extensional deformations of the projected plane are derived from bending deformations of the surface facets (analogous to a "corrugated spring"). This model predicts that the ripple surface elastic modulus is proportional to the bilayer bending or curvature elastic modulus and inversely proportional to the square of the ripple amplitude. The model correlates very well with the observed mechanical behavior of the P.beta.'' phase surface and yields a value for the bilayer bending modulus of 3 .times. 10-12 erg.