Orientation of solid supported lipid bilayers examined by βν-correlation analysis

The ordering properties of solid supported lipid bilayers (SSLBs) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were investigated by means of polarized attenuated total reflection (ATR) spectroscopy. The phospholipid films were prepared by two different methods at the hydrophilic surface of germanium crystal. The molecular architecture of the two films was monitored by orientation of the transition dipole moments for selected modes of lipid with respect to the molecular axis using the modified 2D correlation method called βν correlation analysis, originally proposed by Elmore and Dluhy [9] for describing relative rates of intensity changes. In present case the βν analysis has allowed to monitor in a simple intuitive manner packing properties of the phospholipid films in function of pH, time and procedure of their preparation. It was shown that the lipid molecules are more tightly packed in the film obtained from suspension of liposomes than from chloroform solution. Moreover, it was found that the molecular arrangement of hydrocarbon chains with respect to the surface normal evolves with time. The process is discussed rather in terms of changes in protonation of the phosphate group than in terms of differences in the methods of preparation of the studied films.