Probing phospholipid chain fluidity by vibrational spectroscopy including sum-frequency generation

Lipids, and in particular phospholipids, are-in terms of their importance i n biology as membrane constituents-an interesting class of chemical compoun ds. The functioning of biological membranes seems to be related to fluidity , and lipid fluidity is connected with the order/disorder stale of the hydr ocarbon chains In this paper, chain conformation (a determining the order) of a number of phospholipids (different glycerophosphatidylcholines with pa lmitoyl and oleoyl chains), is probed in dependence on the variables water content (hydration) and temperature by vibrational spectroscopy. Two of its subtechniques, conventional Fourier-transform infrared (FTIR) spectroscopy and sum-frequency spectroscopy (SFS), were used, This combination enables to characterize and, thus, to compare physical properties of the sample mol ecules in the bulk and in the superficial layer of a specimen, respectively . The results demonstrate the propensity of FTIR spectroscopy not only to c lassify the lipids with respect to their chain ordering, but also to indica te the existence of thermotropic and, as a novelty, lyotropic (hydration-dr iven) phase transitions. These findings could be, in principle, reproduced by accordingly designed SFS experiments for some of the lipids chosen thus evidencing lipid phase transitions by this method for the first time. This opens up a new way to investigate the phase behaviour of lipids in thin lay ers very similar to the bilayers existing in vivo. (C) 1999 Elsevier Scienc e B.V. All rights reserved.