Abstract
We studied domain formation in mixtures of the monounsaturated lipids SOPC and POPE as a function of temperature and composition by NMR. Magic angle spinning at kHz frequencies restored resolution of (1)H NMR lipid resonances in the fluid phase, whereas the linewidth of gel-phase lipids remained rather broad and spinning frequency dependent. In regions of fluid- and gel-phase coexistence, spectra are a superposition of resonances from fluid and gel domains, as indicated by the existence of isosbestic points. Quantitative determination of the amount of lipid in the coexisting phases is straightforward and permitted construction of a binary phase diagram. Lateral rates of lipid diffusion were determined by (1)H MAS NMR with pulsed field gradients. At the onset of the phase transition near 25 degrees C apparent diffusion rates became diffusion time dependent, indicating that lipid movement is obstructed by the formation of gel-phase domains. A percolation threshold at which diffusion of fluid-phase lipid becomes confined to micrometer-size domains was observed when approximately 40% of total lipid had entered the gel phase. The results indicate that common phosphatidylethanolamines may trigger domain formation in membranes within a physiologically relevant temperature range. This novel NMR approach may aid the study of lipid rafts.