Effects of cis- and trans-unsaturated lipids on an interdigitated membrane.

The effects of adding cis- and trans-unsaturated lipid to a fully interdigitated membrane were examined using differential scanning calorimetry (DSC) and X-ray diffraction. A monofluorinated analog of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was used as the interdigitated lipid. The single fluorine atom on the end of the sn-2 chain allows 1-palmitoyl-2-(16-fluoropalmitoyl)sn-glycero-3-phosphocholine (F-DPPC) to spontaneously form the interdigitated gel phase (LβI) below the main transition temperature (Tm). The cis 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and equivalent trans lipid 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC) are strongly disfavored to form the LβI phase. Wide-angle X-ray scattering (WAXS) data demonstrate that the unsaturated lipids progressively disrupt the intermolecular packing at higher concentrations. Small-angle X-ray scattering (SAXS) data show that as the ratio of unsaturated lipid increases, the amount of interdigitated lipid decreases. The cis isomer is more disruptive and inhibits interdigitation more effectively than the trans isomer.