7Li NMR and DSC investigations of lithium phospholipid interactions

Abstract

The effect of lithium on several phospholipids was investigated by 7 Li nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The interaction is very small in the case of dipalmitoylphosphatidylcholine (DPPC) and mixed dipalmitoylphosphatidylglycerol (DPPG)/DPPC vesicles. The main phase transition properties of DPPC were not changed by the addition of lithium ions; the lithium NMR spectrum however reflected the phase structure of the lipid. 7 Li NMR was found to be a sensitive new method for probing structural changes of the interfacial water layer in phospholipid mesophases. In bilayers containing both DPPC and DPPG (4:1), lithium redued the transition temperature and enthalpy change by only a small amount (0.8°C and 0.7 kcal/mol). Both concentrated DPPC and DPPC/DPPG dispersions showed equal quadrupolar splittings of 4.7 kHz in the 7 Li NMR spectrum at room temperature, indicating that direct binding is not enhanced by the addition of anionic DPPG. The phase behavior of dimyristoylphosphatidylserine (DMPS) in the presence of lithium depended critically on hydration conditions and lithium concentration. A reproducible endotherm at 40.6°C (>Δ H = 7.1kcal/mol) could be obtained by hydrating the lipid in 0.05 M LiCl at 90°C. Hydration at 55–60°C produced a presumably less-hydrated lipid species which melts at 81°C (Δ H = 17.1kcal/mol). X-ray diffraction of DMPS at 20°C in either 0.05 or 0.5 M LiCl showed a sharp line at 4.2A˚, characteristic of the untilted L β phase.