K. Semmler et al., The structure and thermotropic phase behaviour of dipalmitoylphosphatidylcholine codispersed with a branched-chain phosphatidylcholine, BBA-BIOMEMB, 1509(1-2), 2000, pp. 385-396
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.