The structure and thermotropic phase behaviour of dipalmitoylphosphatidylcholine codispersed with a branched-chain phosphatidylcholine

The structure and thermotropic phase behaviour of a fully hydrated binary m ixture of dipalmitoylphosphatidylcholine and a branched-chain phosphatidylc holine, 1,2-di(4-dodecyl-palmitoyl)-sn-glycero-3-phosphocholine, were exami ned using differential scanning calorimetry, synchrotron X-ray diffraction and freeze-fracture electron microscopy. The branched-chain lipid forms a n onlamellar phase when dispersed alone in aqueous medium. Mixed aqueous disp ersions of the two phospholipids containing less than 33 mol% of the branch ed-chain lipid form lamellar phases over the whole temperature range were s tudied (4 degreesC to 60 degreesC). When present in proportions greater tha n 33 mol% it induces a hexagonal phase in mixed aqueous dispersions with di palmitoylphosphatidylcholine at temperatures above the fluid phase transiti on. At temperatures below 35 degreesC a hexagonal phase coexists with a gel bilayer phase. The lamellar <-> nonlamellar transition can be explained sa tisfactorily on the basis of the shape of the molecule expressed in terms o f headgroup and chain cross-sectional areas. At temperatures below 35"C mac roscopic phase separation of two gel phases takes place. Freeze-fracture el ectron microscopy revealed that one gel phase consists of bilayers with a h ighly regular, periodic superstructure (macro-ripples) whereas the other ph ase forms flat, planar bilayers. The macro-ripple phase appears to represen t a relaxation structure required to adapt to the packing constraints impos ed by the incorporation of the branched-chain lipid into the dipalmitoylpho sphatidylcholine host bilayer. The data suggest that structural changes tha t take place on cooling the mixed dispersion below the lamellar <-> nonlame llar phase transition temperature cannot be adequately described using the molecular form concept. Instead it is necessary to take into account the de tailed molecular form of the guest lipid as well as its physical properties . (C) 2000 Elsevier Science B.V. All rights reserved.