Transitions and molecular packing in highly purified 1,2-dipalmitoyl-phosphatidyl choline–water phases. II. The structures of DPPC phases with low water content

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Abstract
The structures of the polar regions are established for a series of bilayer phases with less than four water molecules per lipid molecule and shown to be closely related. For this analysis, the size and symmetry of the unit cells, determined for three phases for which single crystals were grown, were compared with the size and shape of the phosphorylcholine group and the influence of the charges on the packing. The structures of two of these crystalline phases are almost identical with that of a dimyristoyl lecithin for which some details of a structure determination were reported recently. For other phases, reference is made to the chain subcells, which are obtained from x-ray diffraction data and to the polar subcells which can be identified. The polar structures can also be related, directly or indirectly, with those in the crystalline phases from evidence of the changes occurring at the transitions. A square packing of the polar groups is observed for an anhydrous phase and for a transient phase formed at higher temperatures from samples with higher water content. The choline groups are packed in a rectangular array with an hexagonal chain packing for a phase with from one to three water molecules per lipid. In this phase, the chain axes are perpendicular to the bilayer plane and the choline groups and phosphate nonester oxygen atoms are not coplanar. This arrangement is stabilized by the proximity of another bilayer. The crystalline monohydrate phase involves a rectangular packing with coplanar phosphate nonester oxygen atoms and choline groups. This very stable polar structure is found at higher temperatures for the dihydrate and at room temperature for phases with higher water contents.