STRUCTURE AND PROPERTIES OF MIXED-CHAIN PHOSPHATIDYLCHOLINE BILAYERS

The structural and thermotropic properties of the hydrated mixed-chain phosphatidylcholines (PCs), C(8):C(18)-PC and C(10):C(18)-PC, have been 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 °C (ΔH = 7.3 kcal/mol). X-ray diffraction at 0 °C (below the chain melting transition) shows a small bilayer repeat distance, d = 51.0 Å, and a sharp, symmetric wide-angle reflection at 4.1 Å, characteristic of a mixed interdigitated bilayer gel phase [see Mcintosh, T. J., Simon, S. A., Ellington, J. C, Jr., & Porter, N. A. (1984) Biochemistry 23, 4038–4044; Hui, S. W., Mason, J. T., & Huang, C. (1984) Biochemistry 23, 5570–5577]. At 30 °C (above the chain melting transition), a diffuse band is observed at 4.5 A characteristic of an Lα phase but with an increased bilayer periodicity, d = 61 Å. 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 °C. Below the chain melting transition, two lamellar phases are present, corresponding to coexisting interdigitated (d = 52.3 Å) and noninterdigitated (d = 62.5 Å) 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. © 1990, American Chemical Society. All rights reserved.