Chain fluidity and phase behaviour of phospholipids as revealed by FTIR and sum-frequency spectroscopy

Abstract

The functioning of biological membranes seems to require a certain degree of fluidity of their components. The fluidity of the lipids forming the matrix of such membranes is related to the order/disorder state of their hydrocarbon chains. In this study, vibrational spectroscopy is applied to probe the chain conformation (as determining the order) of a number of phospholipids with varying intrinsic fluidities as a function of water activity (hydration). Using conventional Fourier-transform infrared (FTIR) spectroscopy and sum-frequency spectroscopy (SFS) enables one to characterize and, thus, to compare physical properties of the molecules in the bulk and in the superficial layer of a specimen, respectively. The results demonstrate the ability of FTIR spectroscopy not only to classify the lipids with respect to chain ordering, but also to detect lyotropic (hydration-driven) phase transitions. It could be shown that the main transition of mixed-chain oleoyl/palmitoyl phosphatidylcholines (POPC, OPPC) occurs at room temperature and a defined water activity of the films investigated, as also confirmed by small-angle X-ray scattering (SAXS). Equivalent effects were found for POPC in appropriately designed SFS experiments thus evidencing lipid phase transitions by this method for the first time. This opens up a new avenue to elucidate basic aspects of lipid phase behaviour using single bilayer membranes as models of the in vivo state.