Structure and properties of mixed-chain phosphatidylcholine bilayers

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Abstract
The structural and thermotropic properties of the hydrated mixed-chain phosphatidylcholines (PCs), C(8):C(18)-PC and C(10):PC(18)-PC, have beem studied by X-ray diffraction and differential scanning calorimetry. For fully hydrated C(8):C(18)-PC, the reversible chain melting transition is observed at 9.9 .degree.C (.DELTA.H = 7.3 kcal/mol). X-ray diffraction at 0.degree.C (below the chain melting transition) shows a small bilayer repeat distance, d = 51.0 .ANG., and a sharp, symmetric wide-angle reflection at 4.1 .ANG., characteristic of a mixed interdigitated bilayer gel phase [see McIntosh, T. J., Simons, S.A., Ellington, J.C., Jr., and Porter, N.A. (1984) Biochemistry 23, 4038-4044; Hui, S. W., Mason, J.T., and Huang, C. (1984) Biochemistry 23, 5570-5577]. At 30.degree.C (above the chain melting transition), a diffuse band is observed at 4.5 .ANG. characteristic of an L.alpha. phase but with an increased bilayer periodicity, d = 61 .ANG.. Both the calculated lipid bilayer thickness (d1) and that determined directly from electron density profiles (dp-p) show unusual increases as a consequence of chain melting. In contrast, fully hydrated C(10):C(18)-PC shows an asymmetric endothermic transition at 11.8.degree.C. Below the chain melting transition, two lamellar phases are present, corresponding to coexisting interdigitated (d = 52.3 .ANG.) and noninterdigitated (d = 62.5 .ANG.) bilayer gel phases. The relative amounts of these phases depend upon the low-temperature incubation and/or hydration conditions, suggesting conversions, albeit kinetically complex, between metastable, and stable phases. The different behavior of C(8):C(18)-PC and C(10):C(18)-PC, as well as their positional isomers, is rationalized in terms of the molecular conformation of PC.