STEROLS STABILIZE THE RIPPLE PHASE-STRUCTURE IN DIHEXADECYLPHOSPHATIDYLCHOLINE

The presence of various sterols in mixtures with dihexadecylphosphatid ylcholine (DHPC) was studied using static X-ray diffraction of tempera ture equilibrated samples, and real-time X-ray diffraction of samples undergoing temperature scans. It was found that these sterols eliminat e the interdigitation of the alkyl chains in the DHPC sub-gel and gel- state bilayers while stabilizing the ripple gel-state at the expense o f the gel-state bilayer phase. The ripple-ripple phase transition prev iously observed for dipalmitoylphosphatidylcholine in the presence of low molar concentrations of sterols (Wolfe et al. (1992) Phys. Rev. Le tt. 68, 1085-1088) was also observed for similar DHPC-sterol mixtures. In addition, we show the first evidence that the presence of 5 alpha- cholestane-3 beta,5,6 beta-triol will cause the lipid mixtures to cont inue to adopt a ripple mesophase structure even after the DHPC alkyl c hain becomes disordered.