Formation of the antiplanar-antiplanar phosphate conformation of dilauroylphosphatidylcholine bilayers.

Infrared spectroscopy was used to investigate lipid conformational changes that occur in dilauroylphosphatidylcholine (diC12PC) bilayers with and without fatty-acid-amino-acids as guest molecules in the membrane. Incorporating 2.5 mole% N-decanoylglycine (decgly) into diC12PC liposomes caused formation of the antiplanar-antiplanar (ap-ap) phosphodiester conformation which was stable in room temperature IR spectra. Several other fatty-acid-amino-acids incorporated into diC12PC bilayers were found to also elicit the ap-ap phosphodiester conformation. Unlike these diC12PC/fatty-acid-amino-acid mixed bilayers, pure diC12PC bilayers would form the ap-ap phosphodiester conformation only under low temperature incubation conditions. Dry diC12PC films incubated at 5 degrees C for 0.5 h (brief incubation) or 16 h (prolonged incubation), and then rapidly hydrated (i.e., vortexed at 25 degrees C in D2O), caused the ap-ap phosphodiester conformation to persist in the diC12PC liposomes equilibrated to room temperature. Slow hydration for 16 h at 5 degrees C in both buffered and non-buffered D2O of diC12PC lipid films also produced the ap-ap phosphodiester conformation. In contrast, slow hydration for 16 h at 5 degrees C in PBS/D2O of diC12PC/decgly mixed films caused the greatest number of ap-ap phosphodiester conformers. Using pure diC12PC bilayers, infrared data indicate that incubation of diC12PC films causes the headgroup phosphodiester conformation to change from gauche-gauche (g-g) conformation to the ap-ap conformation. Under all liposome formation conditions examined, no changes in hydration of either the phosphate group or the carbonyl ester group were detected and in addition, no trans/gauche conformational changes in the acyl chain were observed.