Study by infrared spectroscopy of the conformation of dipalmitoylphosphatidylglycerol monolayers at the air-water interface and transferred on solid substrates
A parallel investigation of the conformational changes of dipalmitoylphosphatidylglycerol (DPPG) monolayers induced by surface pressure has been studied in situ at the air water interface using polarization modulation external infrared reflection absorption spectroscopy (PM-IRRAS) and on solid substrate by attenuated total reflection (ATR) spectroscopy. For both the ATR and PM-IRRAS spectra of DPPG, the non-linear increase of the intensity of the antisymmetric methylene stretching band of DPPG with molecular surface density, indicates that the compression of the DPPG monolayer induces a change of the conformation and/or orientation of the acyl chains of the phospholipid. At surface pressures between 7 and 20 mN/m, the acyl chains of the DPPG monolayer are found to be more ordered on the germanium substrate than at the air-water interface. At higher surface pressures, the conformation of the lipid acyl chains of DPPG on both substrates are almost similar. In the liquid-condensed phase, the acyl chains are in all-trans conformation and their tilt angle with respect to the normal of the film is approximately 30 degrees. Our results also suggest the presence of DPPG solid domains in the liquid-expanded phase. Investigation of polar head group region indicates that at low surface pressures, the carbonyl groups were more oriented on the water surface than the acyl chains. Finally, the present study shows that intermolecular hydrogen bonding probably occur between the glycerol hydroxyl and the phosphate or carbonyl groups of the phospholipid. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.