Phase Coexistence in Ternary Lipid Mixtures Containing POPC and Phytosterols, Ergosterol or 7-Dehydrocholesterol

Abstract

2H-NMR spectroscopy was used to investigate the occurrence of phase coexistence in multilamellar vesicles of DPPC and POPC (33mol%: 33mol%) with either stigmasterol, brassicasterol, ergosterol or 7-DHC (each 33mol%). In all cases, the sn-1 chains of DPPC and POPC were deuterated in turn, and 2H-NMR spectra were measured for both lipid components as a function of temperature between 5 oC and 48 oC. The chain order of DPPC was found to be greater than that of POPC in all the above mixtures. An equimolar DPPC/POPC membrane exhibits a clear solid-ordered (so)-liquid disordered (ld) transition between 5 oC and about 33 oC as observed for each lipid by a fairly steep reduction in the average spectral width. Adding sterol generally results in a more gradual change in average spectral width over this temperature range. Depending on the particular sterol, NMR spectra taken in this temperature range either appeared poorly resolved (“blurry”) or displayed coexisting liquid ordered (lo) and ld spectra. Liquid-liquid phase coexistence is observed in DPPC/POPC/ergosterol mixtures. The spectra for DPPC/POPC/ brassicasterol only exhibited liquid-liquid coexistence when a small concentration (0.02 mol%) of the fluorescent lipid dye TR-DHPE was added to the system. Only one liquid phase was observed for either DPPC/POPC/7DHC or DPPC/POPC/stigmasterol. In summary, our interest was to study how liquid phase coexistence in DPPC/POPC/sterol is modified by small changes in the sterol structure. These studies show that ergosterol is the most effective sterol in fostering phase coexistence. Note that in comparison to cholesterol, which does not cause micron scale phase coexistence in DPPC/POPC, ergosterol has an extra double in the fused ring, as well as a double bond at C22. Possible connections between the detailed sterol structures and these observations will be discussed.