MISCIBILITY OF PHOSPHOLIPIDS WITH IDENTICAL HEADGROUPS AND ACYL-CHAINLENGTHS DIFFERING BY 2 METHYLENE UNITS - EFFECTS OF HEADGROUP STRUCTURE AND HEADGROUP CHARGE

We have investigated the influence of the chemical structure and charg e of the hydrophillic headgroup on the miscibility of saturated phosph olipids with acyl chain lengths differing by two methylene units, name ly DMPA/DPPA, DMPC/DPPC, DMPE/DPPE and DMPG/DMPG (0.1 M NaCl). All fou r mixtures were analysed by DSC at pH 7. To study the influence of a c hange in headgroup charge, we additionally investigated DMPA/DPPA mixt ures at pH 4 and 12, and DMPG/DPPG mixtures at pH 2. The experimental DSC thermograms were fitted using methods described before [Johann et al., Biophys. J. 71 (1996), 3215-3228] to obtain the temperatures of o nset and end of melting and first approximations for the non-ideality parameters as a function of composition. The resulting phase diagrams were then fitted using a four non-ideality parameter model for non-ide al, non-symmetric mixing in both phases. The phase diagram of the syst em DMPG/DPPG has a lens-like shape, the non-ideality parameters p(g) a nd p(l) for the gel and the liquid-crystalline phase, respectively, ar e zero, indicating ideal mixing in both phases. For the other mixtures , differences in miscibility are observed depending on the structure o f the headgroup. At pH 7, p(g) > p(l), i.e., the miscibility in the li quid-crystalline phase is more ideal than in rhs gel state. All p(g) v alues are positive and the sequence for p(g) observed is PA > PE > PC > PG. Partial protonation of PA at pH 4 or complete deprotonation at p H 12 leads to negative non-ideality parameters for both phases, indica ting a preference for mixed pair formation. Protonation of PG in DMPG/ DPPG mixtures at pH 2 leads to positive non-ideality parameters for bo th phases, indicating a tendency for demixing. The results show, that the miscibility of phospholipids with identical headgroups but chain l engths differing by two methylene groups is dependent on headgroup str ucture and on headgroup charge. (C) 1998 Elsevier Science B.V.