The phase behaviour of some mixtures of lead(II) carboxylates with phosphatidylcholines

Abstract

The phase behaviour of some binary mixtures of long-chain lead(II) carboxylates and phosphatidylcholines has been studied by DSC, optical microscopy, X-ray diffraction and multinuclear NMR. X-ray diffraction results confirm that all the soaps have the same lamellar layered structure in the solid, and that this is maintained on incorporation of phospholipids. With pure lead(II) decanoate, DSC and X-ray diffraction studies confirm that differences between recrystallized and premelted samples result from polymorphism. Didecylphosphatidylcholine (DDPC) was soluble in the liquid and L α liquid crystalline phase of lead(II) decanoate (PLC 10) up to at least 10 wt%. In either the solid or the low temperature mesophase, phase separation occurs at lower concentrations. The C 14 (up to 10 wt%) and C 18 (up to 4 wt%) phosphatidylcholines also dissolve in the liquid phase of the corresponding chain length carboxylates, and miscibility is observed up to 4 wt% in the mesophase of the C 14 system. At higher phospholipid concentrations, optical observations confirm separation into two phases. The 31P NMR spectra were run on these systems. In some cases spectra showed more than one peak, indicating different local environments for the phosphate group. The 207Pb NMR spectra were run on the C 14 system and showed similar chemical shifts in the presence and absence of phospholipid, suggesting that Pb(II) does not bind to the phosphate. However, differences were observed in the bandshape of the 207Pb NMR signal for the mesophase on incorporation of phospholipid, suggesting changes in the local dynamics. The 13C NMR spectra and the longitudinal relaxation of a DDPC/PbC 10 mixture showed that at low temperature the choline methyl group signal is split in two, possibly as a result of phase separation. This is supported by DSC data.