The interfacial region of dipalmitoylphosphatidylcholine bilayers is perturbed by fusogenic amphipaths

Several structural methods were used to probe the influence of three fusogenic and four nonfusogenic amphipaths on large, unilamellar dipalmitoylphosphatidylcholine (DPPC) vesicles. For four of these structural measurements there was a correlation observed between the ability of an amphipath to favor poly(ethylene glycol) (PEG)-induced fusion and the structural perturbation reported by each method. First, the fluorescence anisotropy of 1-[4-(trimethylamino)phenyl]-6-phenyhexa-1,3,5-triene (TMA-DPH), which probes the upper region of the bilayer, decreased in the range of PEG concentrations previously found to cause fusion of membranes containing fusogenic amphipaths. For nonfusogenic amphipaths, the anisotropy increased monotonically with PEG concentration. The properties of similar probes that locate in the hydrophobic core of the bilayer showed no correlation with fusogenicity, nor did the properties of probes purported to sense the aqueous surface of the membrane. Second, the frequency of the C=O stretch increased and then decreased dramatically as fusogenic but not nonfusogenic membranes were heated through their phase transition. Third, there was a dramatic increase in the frequency of the C-O-C ester stretch at the membrane order/disorder phase transition for membranes containing fusogenic amphipaths, twice the increase observed for nonfusogenic amphipaths. The spectral characteristics of phosphate, choline, and acyl chain motions showed no such correlation with fusogenicity. Finally, calorimetric measurements showed that low levels of fusogenic amphipaths eliminated the "pretransition" (L beta-->P beta) in DPPC membranes, whereas other amphipaths shifted but did not eliminate this transition. Taken together, these results indicate that fusogenic amphipaths perturb the interface or "backbone" region of the bilayer rather than the hydrophobic core, the headgroup, or the water interface regions of DPPC bilayers.