Carotenoids in Lipid Membranes - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 22

-diol), determined in
different lipid membrane systems, was always larger (less acute orientation angle) relative to the
normal to the membrane than in the case of zeaxanthin, despite the very similar chemical structure
of these pigments. For example, the angle was greater by 30° in EYPC, by 20° in DPPC, and by 10°
in DHPC (Sujak et al., 1999). Moreover, the molecular organization of two-component carotenoid-
DPPC monomolecular layers was substantially different in the case of lutein and zeaxanthin (Sujak
and Gruszecki, 2000). The higher molecular area values, observed in the lutein-containing lipid
i lms, as compared to the zeaxanthin-containing monolayers, correlates with greater orientation
angles when observed in the lipid bilayers. From the structural point of view, the main difference
between zeaxanthin and lutein, is the position of the double bond in one terminal ring: between C5

Interestingly, the orientation angle of lutein ( (3 R ,3 ′ R ,6 ′ R )- β , ε -carotene-3,3 ′

′

and C6 in the case of lutein. Owing to such a

difference, this particular double bond is conjugated with the double bonds system in the case of
zeaxanthin and is not in the case of lutein. It seems that the relative rotational freedom of the entire
ε

′ in the case of zeaxanthin and between C4 ′ and C5 ′

single bond, may be a determinant of the different orientation
of the xanthophylls with respect to the lipid membranes. It has to be kept in mind that the orien-
tation angle, determined by means of the linear dichroism technique, represents a mean value.
Consequently, one can determine the orientation angle in the sample in which chromophores are all
oriented in the same direction or in a very different sample, in which the chromophores are distrib-
uted in two orthogonal pools: for example, parallel and perpendicular to the plane of the membrane.
Interestingly, the x-ray analysis of the electron density proi les of lipid bilayers show that the effect
of lutein on the membrane is very different from the effect of zeaxanthin and resembles the effect
of apolar carotenoids oriented homogenously or parallel to the plane of the membrane (McNulty
et al., 2007). No differences have been observed in the orientation of lutein and zeaxanthin in the
membranes formed with DMPC in the samples in which the pigments were largely in the aggre-
gated form (Sujak et al., 2002).
As a rule, the carotenoid pigments substituted with the keto groups in the C4 and C4

-ring of lutein about the C6 ′ -C7 ′

positions,
demonstrate almost vertical orientation with respect to the axis normal to the plane of the membrane:
26° astaxanthin in EYPC, 29° canthaxanthin in EYPC (Gruszecki, 1999), or 20° canthaxanthin in
DPPC (Sujak et al., 2005). In the latter case, the orientation of the pigment molecules was found
to depend on the actual concentration in the membrane and larger orientation angles were found at
canthaxanthin concentrations approaching the aggregation threshold in the lipid phase (Sujak et al.,
2005). Formally, the terminal keto groups of carotenoids can only form hydrogen bonds in which
they act as proton acceptors. Owing to this fact, a direct interaction of the ketocarotenoids with
the ester carbonyl groups, at the border of the hydrophobic and the polar zones of the membrane,
via hydrogen bonding is not possible and the pigment molecule may penetrate more deeply within
the hydrophilic membrane zone. In the case of vertical orientation of the carotenoid pigment with
respect to the membrane, one can expect the strongest ordering effect of the rigid, apolar backbone
of the pigment with respect to the alkyl lipid chains.

′

2.2.3 I NCORPORATION R ATES
The binding of carotenoids within the lipid membranes has two important aspects: the incorpora-
tion rate into the lipid phase and the carotenoid-lipid miscibility or rather pigment solubility in the
lipid matrix. The actual incorporation rates of carotenoids into model lipid membranes depend on
several factors, such as, the kind of lipid used to form the membranes, the identity of the carote-
noid to be incorporated, initial carotenoid concentration, temperature of the experiment, and to a
lesser extent, the technique applied to form model lipid membranes (planar lipid bilayers, liposomes
obtained by vortexing, sonication, or extrusion, etc.). For example, the presence of 5 mol% of caro-
tenoid with respect to DPPC, during the formation of multilamellar liposomes, resulted in incor-
poration of only 72% of the pigment, in the case of zeaxanthin, and 52% in the case of -carotene

(Socaciu et al., 2000). A decrease in the l uidity of the liposome membranes, by addition of other

β