INTERACTION OF LA3+ IONS WITH PHOSPHATIDYLCHOLINE MEMBRANES AND ITS EFFECT ON THERMAL PHASE-TRANSITION

Dilatometry and calorimetry measurements of dipalmitoylphosphatidylcholine(DPPC)/water dispersions show that the interaction of La3+ with DPPC shifts the thermal phase transition peak maximum from 316.degree. K observed with free DPPC to 321.5.degree. K for the complex. The volume extension of DPPC decreases and the change of the enthalpy increases by about 2 kcal/mol. At an ion:lipid ratio of 1:3 the maximum shift of the transition temperature is obtained. This indicates that more than 1 phospholipid molecule in the complex interacts with 1 La3+ simultaneously. On addition of La3+ to an unsonicated DPPC/D2O-dispersion the 31P chemical shift anisotropy increases by about 15 ppm. The mobility of the phosphate group is probably more restricted in the case of interaction of La3+ with the headgroup. 139La NMR is a suitable method for measuring the interaction of La3+ ions with DPPC molecules, and a large broadening of the 139La NMR peak is observed in the presence of DPPC.