AN INVESTIGATION OF MISCIBILITY IN MONOLAYER FILMS OF PHOSPHOCHOLINE-PHOSPHOGLYCEROL BINARY-MIXTURES

A surface thermodynamic analysis was applied to the experimentally mea sured pressure-area isotherms of mixed monolayer films of phosphocholi ne (PC) and phosphoglycerol (PG) lipids in order to quantitatively des cribe the phase miscibility of these physiologically important binary mixtures. We have calculated the excess free energies of mixing (Delta G(XS)(p)i) of binary mixtures of acyl chain saturated PC (1,2-dipalmi toyl-sn-glycero-3-phosphocholine, DPPC) with saturated PG (1,2-dipalmi toyl-sn-glycero-3-phosphoglycerol, DPPG), as well as saturated PC (DPP C) with acyl chain unsaturated PG( 1,2-dioleoyl-sn-glycero-3-phosphogl ycerol DOPG) at various mole fractions and under different subphase Ca 2+ ion conditions, Delta G(XS)(pi), for the binary mixture of DPPC + D PPG on a NaCl subphase indicates that this binary system approximates an ideal mixture at almost all mole fractions studied. The presence of 5 mM CaCl2 in the subphase results in negative values of Delta G(XS)( pi) for the DPPC/DPPG mixture, indicating a miscible, non-ideal monola yer film, where increased intermolecular interactions are stabilized i n the presence of Ca2+. The mixture of DPPC and DOPG on NaCl was found to have positive deviations from ideality at all mole fractions studi ed, indicating a phase separated binary mixture. The magnitude of the phase separation increased as the surface pressure increased. The pres ence of Ca2+ ions in the subphase was found to have little effect on t he mixing of the DPPC and DOPG binary monolayer. This system remained phase separated at all mole fractions in the presence of Ca2+, with De lta G(XS)(pi) values increasing as the isotherm surface pressure incre ased.