Raman study of low frequency lattice modes in dipalmitoylphosphatidylcholine (DPPC) multilayers

Low frequency Raman spectra (6-30 cm^-1) of macroscopically ordered multilayers of dipalmitoylphosphatidylcholine (DPPC) were taken as a function of temperature (80-270 K) and for different water contents. A weak band is observed which is shifted from 18.5 to 14 cm^-1 between 80 and 270 K. This band is shifted to lower frequencies with increasing water content. In addition, a strong background appears at the Rayleigh line position which grows strongly with increasing temperature. The low frequency band can be explained (1) in terms of an optical mode associated with an antiphase in-plane vibration of two opposing monolayers or it can (2) be attributed to rotational vibrations of the hydrocarbon chains. The softening of this band is attributed to a decrease in the chain orientational order with increasing temperature. The increasing rotational freedom is expected to cause a transition from a monoclinic or orthorhombic to a hexagonal lattice and eventually triggers the Lβ′→L_a chain melting transition (at T=340 K for 5.3 wt. % of water). The appearance of the background can also be attributed to an increasing anharmonicity of the lattice potential upon approaching the phase transition, which leads to mode coupling. The possibility of ferroelectric order due to a collective orientation of the dipole moments of the C=O groups of the glycerol backbone is discussed.