J. Lobau et al., Chain fluidity and phase behaviour of phospholipids as revealed by FTIR and sum-frequency spectroscopy, J MOL STRUC, 481, 1999, pp. 407-411
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.