Carotenoids in Lipid Membranes - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 20

Owing to the solvatochromic effect, the position of the electronic absorption maximum on energy
scale depends on the dielectric properties of the medium (Andersson et al., 1991). The positions of the
maxima in the absorption spectra of several carotenoid pigments incorporated into the lipid mem-
brane systems, indicate that chromophores are embedded in the environment characterized by the
polarizability term of the hydrophobic core of the membrane (Gruszecki, 1999, 2004; Gruszecki and
Sielewiesiuk, 1990; Milon et al., 1986; Sujak et al., 2005). Figure 2.1 presents such a dependency plot-
ted for violaxanthin incorporated into liposomes formed with DMPC. Detailed information concern-
ing the segmental motion of acyl lipid chains, inferred on the basis of the EPR-spin label technique
(Strzalka and Gruszecki, 1994; Subczynski et al., 1992, 1993; Wisniewska and Subczynski, 1998),
NMR spectroscopy (Gabrielska and Gruszecki, 1996; Jezowska et al., 1994; Sujak et al., 2005), and
FTIR spectroscopy (Sujak et al., 2005, 2007a), indicates unequivocally that the membrane-bound
carotenoids modify profoundly the organization of the hydrophobic core of the lipid bilayers.

2.2.2 O RIENTATION
Despite the fact that both the apolar and the polar carotenoids incorporated into the hydrophobic
core of the membrane, the orientation of the long, bar-shaped molecules depends very much on the
extent of the substitution on the polar end-group, and the ability to form hydrogen bonds within
the polar headgroup zones of the membrane (Gruszecki, 1999, 2004). In general, the apolar caro-
tenoids, such as -carotene or lycopene, display a certain orientational freedom with respect to the

membrane (see the model presented in Figure 2.2). The linear dichroism study of the orientation of
β

β

-carotene led to the conclusion that the transition dipole moment of the pigment molecule, close to
the long axis of the polyene chromophore (≈15°; Shang et al., 1991), was oriented close to the plane

21,500

21,400

21,300

Violaxanthin

21,200

21,100

21,000

20,900

478 nm

20,800

20,700

20,600

n = 1.44

20,500

0.2 0.22 0.24 0.26 0.28 0.3

( n 2 - 1)/( n 2 +2)

FIGURE 2.1 Energy of the 0-0 vibrational transition in the principal electronic absorption spectrum of
violaxanthin (1 1 A g − →1 1 B u + ), recorded in different organic solvents, versus the polarizability term, dependent

on the refraction index of the solvent ( n ). The dashed line corresponds to the position of the absorption band
for violaxanthin embedded into the liposomes formed with DMPC (Gruszecki and Sielewiesiuk, 1990) and the
arrow corresponds to the polarizability term of the hydrophobic core of the membrane ( n = 1.44).