High-pressure infrared spectroscopy of ether- and ester-linked phosphatidylcholine aqueous dispersions

Infrared spectra of aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and its ether-linked analogue, 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC), were measured in a diamond anvil cell at 28 degrees C as a function of pressure up to 20 kbar. Although these two lipids differ only in the linkages to the saturated hydrocarbon chains, significant differences were observed in their barotropic behavior. Most notable were the magnitudes of the pressure-induced correlation field splittings of the methylene scissoring and rocking modes, and the relative intensities of the corresponding component bands. In the case of the scissoring mode, not only can the correlation field component band be resolved at a lower pressure in DHPC (1.2 kbar, as compared with 2.2 kbar in DPPC), but the initial magnitude of the correlation field splitting in DHPC, particularly less than 9 kbar, is significantly greater than that observed in DPPC. These differences are attributed to the presence of an interdigitated lamellar gel phase in DHPC. At all pressures where the correlation field component band delta'CH2 can be resolved, the relative peak height/intensity ratio R = I delta'/I delta is greater in DPPC than in DHPC, suggesting that this parameter may be useful as a test of interdigitation.