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

The functioning of biological membranes seems to require a certain degree o f fluidity of their components. The fluidity of the lipids forming the matr ix of such membranes is related to the order/disorder state of their hydroc arbon chains. In this study, vibrational spectroscopy is applied to probe t he chain conformation (as determining the order) of a number of phospholipi ds with varying intrinsic fluidities as a function of water activity (hydra tion). Using conventional Fourier-transform infrared (FTIR) spectroscopy an d sum-frequency spectroscopy (SFS) enables one to characterize and, thus, t o compare physical properties of the molecules in the bulk and in the super ficial layer of a specimen, respectively. The results demonstrate the abili ty of FTIR spectroscopy not only to classify the lipids with respect to cha in ordering, but also to detect lyotropic (hydration-driven) phase transiti ons. It could be shown that the main transition of mixed-chain oleoyl/palmi toyl phosphatidylcholines (POPC, OPPC) occurs at room temperature and a def ined water activity of the films investigated, as also confirmed by small-a ngle X-ray scattering (SAXS). Equivalent effects were found for POPC in app ropriately 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 m odels of the in vivo state. (C) 1999 Elsevier Science B.V. All rights reser ved.