Carotenoids in Lipid Membranes - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 21

Lipid bilayer membrane

Hydrophobic
core

With apolar carotenoids With polar carotenoids

FIGURE 2.2 Model representation of organization of the lipid membrane containing apolar and polar caro-
tenoid pigments.

of the membrane formed with DOPC (Johansson et al., 1981) or was close to the magic angle in
EYPC (Gruszecki, 1999). The orientation angle of the transition dipole moment with respect to the
axis normal to the plane of the membrane, is equal to the magic angle (54.7°) and can be interpreted
as an indication of this particular mean orientation angle but one will arrive at the same result in the
case of homogeneous distribution of the transition dipoles. The angle-resolved resonance Raman
studies show that -carotene is oriented roughly parallel to the plane of the membrane formed with

DOPC but roughly perpendicular with respect to the membrane formed with SBPC (van de Ven
et al., 1984). In the case of lycopene, the mean orientation angle of the transition dipole moment with
respect to the normal to the plane of the membrane was determined as 74°, in the membranes formed
with EYPC (Gruszecki, 1999). Such a mean angle shows that the orientation of lycopene is neither
determined by the plane of the bilayer nor by the direction of the alkyl lipid chains. The x-ray analy-
sis of the electron density proi les across the lipid membranes formed with POPC (with 0.2 mol frac-
tion cholesterol) demonstrated that, in contrast to the polar carotenoids (in particularly astaxanthin),
lycopene and

β

-carotene disordered the membrane bilayer (McNulty et al., 2007). In the case of the
polar carotenoids, linear dichroism studies determined the orientations to be close to the axis normal
to the plane of the bilayer. The polar groups bound to the end-rings of the pigments examined will
tend to form hydrogen bonds with the lipid membrane headgroups and water at the membrane inter-
face. The acute orientation angles, found in the case of polar carotenoids, indicate that the molecules
adopt an orientation that allows the polar groups localized on the opposite sides to be anchored in
the opposite polar membrane zones. In the case of zeaxanthin ( (3 R ,3

β

-carotene-3,3¢-diol), lin-
ear dichroism studies determined the orientation angles of the transition dipole to be 33° in EYPC
(Sujak et al., 1999), 25° in DMPC (Gruszecki and Sielewiesiuk, 1990), and 9° in DGDG (Gruszecki
and Sielewiesiuk, 1991). As can be seen, the orientation angles negatively correlate with the thick-
ness of the hydrophobic core of the membrane: the greater the thickness of the membrane (ca. 2.3 nm
for EYPC, 2.8 nm for DMPC, and ca. 3.0 nm in the case of DGDG) the lower the orientation angle.
Such a correlation can be interpreted as a demonstration of the general rule that the orientation of
polar carotenoids is determined by a matching of the distance between the opposite polar groups of
the pigment and the thickness of the hydrophobic core of the membrane.
The studies of monomolecular layers formed by zeaxanthin-lipid mixtures at the air-water inter-
face have shown that, in contrast to the pigment molecules having an all- trans coni guration, mol-
ecules having a cis coni guration adopt an orientation within the i lm such that they are anchored
within the polar-apolar interface by both of the hydroxyl groups found at the 3 and 3

′ R )- β , β

positions
(Milanowska et al., 2003). A similar orientation of zeaxanthin molecules having cis coni gura-
tions can be expected in lipid bilayer systems. Interestingly, recent EPR experiments also led to the
conclusion that zeaxanthin in a cis coni guration is able to span the lipid bilayer, providing that the
thickness of the hydrophobic core of the membrane does not exceed the distance between the polar
groups of the pigment (Widomska and Subczynski, 2008).

′