Orientation studies of hydrated dipalmitoylphosphatidylcholine multibilayers by polarized FTIR-ATR spectroscopy

Polarized Fourier-transform infrared-attenuated total reflection spectroscopy has been applied to explore the temperature-dependence of molecular orientations in multibilayers of 1,2-dipalmitoyl-sn-glycero-3- phosphocholine (DPPC) of various degrees of hydration. The order parameter of the hydrocarbon chain, evaluated from the dichroic ratios of the antisymmetric and symmetric CH2 stretching bands, was drastically decreased at the main (or gel to liquid-crystalline phase) transition temperature (Tm) irrespective of the water content, suggesting that the hydrocarbon chain is in a disordered state as a result of chain-melting associated with an increase in the number of the gauche conformers. On the other hand, the dichroic ratios of the polar bands of hydrated DPPC assignable to the symmetric PO2- stretching and asymmetric N+ (CH3)3 stretching modes were increased mainly at the pretransition temperature (Tp), except for less hydrated case. The dichroic ratios of both the OH stretching and OH2 bending bands of water showed the same temperature-dependence as those of the polar bands. These results indicate that the pretransition is ascribable mainly to the reorientation of the polar groups of the DPPC and bound water, while the main transition is due to the orientational disorder of the hydrocarbon chains. For less hydrated DPPC, the reorientation of the polar groups and water did not occur around Tp, but in the higher temperature region around Tm. This is in accord with the previously reported observation that the pretransition disappears for less hydrated DPPC. In this case, the polar groups and water may reorient following the reorientation of the hydrocarbon chains near Tm.