Orientation Measurements on Ordered Multibilayers of Phospholipids and Sphingolipids from Synthetic and Natural Origin by ATR Fourier Transform Infrared Spectroscopy

Abstract

Fourier-transform infrared spectroscopy was applied to natural and synthetic planar membrane multibilayer systems made from phospholipids and sphingolipids to perform orientational measurements utilizing the attenuated total reflectance technique. Applying polarized infrared light, the dichroic ratios and the positions of the peak maxima of various infrared-active bands in dependence on temperature and degree of hydration as well as the phase behaviour were studied. Under certain assumptions - which are discussed - made for the order parameter S, the orientation of various functional groups with respect to the molecular axis or to the membrane normal can be determined. For the gel phase values for S between 0.80 ± 0.05 for the complete molecule and 0.40 ± 0.05 for the end methyl group and for the liquid crystalline phase a value of 0.50 ± 0.05 were approximated. Applying these data, a relatively precise determination of the angles of the oscillating dipole moments of various functional groups could be performed, which - for some vibrations - deviate considerably from those angles assumed earlier. Regarding the phase behaviour, it was found that not only the degree of hydration but the amount of bulk water of the sample has a strong influence on the phase transition temperature Tc (lyotrophism), but only a weak influence on the orientational data.