ABSTRACT
A rod-like molecule of carotenes lacks polar groups and therefore may be
expected to be localized in the hydrophobic core of the lipid membrane, owing to
the requirement of energy minimization in a system. On the other hand, the
terminal groups of polar carotenoids are expected to interact with the polar head
group regions of a lipid bilayer via hydrogen bonds. Such a localization of
carotenoids, deduced on the basis of their chemical structure, has a strong
experimental support from the analysis of a position of light absorption maxima
of carotenoid pigments incorporated to lipid membranes (1-4). Specifically, the
positions of absorption maxima of carotenoid pigments incorporated to lipid
membranes correlate with the polarizability term of the hydrophobic core of the
lipid bilayer, representing dielectric properties of the chromophore environment
and calculated on the basis of a refractive index. Figure 1 presents such a
dependency plotted for lutein, dissolved in a series of organic solvents and
incorporated into liposomes formed with dipalmitoylphosphatidylcholine
(DPPC). The value of the polarizability term for the hydrophobic core of
DPPC correlates very well with the position of the 0-0 transition in the absorption
band of lutein incorporated into this membrane system. The rule that polar end
groups of xanthophyll pigments have to remain in direct contact with polar
groups of lipid molecules, realized in most cases by hydrogen bonding,
determines the orientation of carotenoid molecules with respect to the lipid
bilayer, as will be discussed below. Both localization and orientation of
carotenoid molecules in the membrane are directly responsible for molecular
mechanisms of carotenoid-lipid interaction that influence basic physical pro-
perties of lipid membranes such as membrane thickness, fluidity, permeability,
energy, and cooperativity of phase transitions, etc. Selected aspects of
physiologically relevant carotenoid-lipid interactions, directly dependent on
carotenoid orientation with respect to the lipid bilayer will also be addressed.
