Phase Transition of Lipid Multilamellar Aqueous Suspension under High Pressure. IV. Chain Packing Conformation of Dipalmiloyl Phopholipid in Temperature Range between Pre- and Sub-Transition

Wide-angle X-ray scattering (WAXS) measurements for a lipid multilamellar aqueous suspension have been studied using computer profile analyses; these suggest that in the temperature range between the pre- and sub-transition, hydrocarbon chain packing conformation does not have a single structure with quasi-hexagonal or orthorhombic symmetry. Under atmospheric pressure there exists a sharp X-ray reflection at about 4.2 A, followed by a more diffuse peak at a higher angle within this intermediate zone. With increasing pressure these two reflection peak heights indicate changes that are independent of each other; one has a decreasing and the other an increasing tendency. Thus, a comparison of the WAXS profile under atmospheric pressure with that under 300 MPa obviously shows a reversal in these two peak heights. As a result, the iea that the pre- and sub-transition intermediate is composed of a two-component system with the same hexagonal symmetry is introduced.