Interdigitation of bilayers from ether lipid analogs: (R)-PAF, (R)-lyso-PAF and the antineoplastic (R)-ET-18-OMe

The ether lipid analogs (R)-PAF, (R)-ET-18-OMe and (R)-Lyso-PAF can form bilayers. Using differential scanning calorimetry (DSC) and small angle X-ray diffraction, we have investigated bilayers formed entirely of these molecules. Small angle X-ray diffraction experiments strongly indicated that these bilayers are interdigitated at 98% relative humidity in the gel phase. The slight differences in the structures of the ether lipids affect their thermal behavior and packing properties. Of the three, (R)-Lyso-PAF shows the highest phase transition temperature, broadest phase transition peak and smallest enthalpy change. These characteristics may be attributed to hydrogen bonding of the -OH group at sn-2 position of its glycerol backbone to water or to other (R)-Lyso-PAF molecules. The interdigitated structure shows that these bilayers are similar to lipid bilayers and suggests that these molecules could easily insert themselves into membranes to reach their respective active sites or to perturb the membranes. The packing and thermal characteristics of (R)-Lyso-PAF may help explain its biological inactivity.