High pressure infrared spectroscopy of lipid bilayers: new tests for interdigitation.

High-pressure infrared spectroscopy is used to compare the barotropic behaviour of various interdigitated lipid bilayer systems (1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine, 1,3-dipalmitoyl-sn-glycero-2-phosphocholine and 1-palmitoyl-sn-glycero-3-phosphocholine) with non-interdigitated bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. In the pressure range between 0 and 20 kbar, we have monitored the pressure dependence of the relative peak height intensity ratio of the chain methylene scissoring band, delta CH2, and its correlation field component band, delta'CH2. We demonstrate that this parameter, in conjunction with a visual inspection of the pressure-induced correlation field splittings of the methylene scissoring and rocking mode bands, can provide reliable indications of chain interdigitation.