Characterization of the pretransition in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine by Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy has been used to study the infrared-active acyl chain vibrational modes of fully hydrated multibilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (L-DPPC) over the temperature range 0-55 degrees C. Frequencies, bandwidths, and other spectral parameters were measured as a function of temperature for the methylene scissoring, rocking, and wagging modes, as well as for the C-H stretching modes, and they were used to monitor the packing of the acyl chains. Particular emphasis was placed on determining the nature of the pretransition event. It is shown that between 36 and 38 degrees C the spectral changes are indicative of a phase change in the acyl chain packing from an orthorhombid to a hexagonal subcell. It is also concluded that in the gel phase, at all temperatures below the main transition, the acyl chains are predominantly in all-trans conformations and that the temperature-dependent variations of spectral parameters result from changes in interchain interactions.