Infrared spectra of dipalmitoyl phosphatidylcholine bilayers in water were measured over the pressure range 0 to 35 kbar. Discontinuities in the pressure dependences of various infrared spectral parameters indicate that structural phase transitions occur at 1.7, 4.8, and 15 kbar. More than two correlation field components of the CH2scissoring mode are observed in the spectra of the highhyphen;pressure phases, suggesting that more than two acyl chains are present in the unit cell of these phases. Pressurehyphen;induced changes in the CH2bond angle and in the chain orientation are evident from the pressure effect on the intensities of the CH2stretching and CH2rocking infrared bands. Pressure also affects the dispersion of various vibrational frequencies resulting in drastic changes of the Fermi resonance interaction between ngr;sCH2and the 2dgr;CH2continuum.
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