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Another spectral development can take place if the solvent mixture is not able to maintain
pigments in solute state. In this case, the formation of dimers and higher aggregates of all plant
xanthophylls is very common (Takagi et al., 1983; Ruban et al., 1993a). Ethanol-water mixture
provided us with a good system, which could not only yield xanthophyll aggregates but also test
hydrophobicity of these pigments using the solvent ratio at which aggregation takes place (Ruban
et al., 1993a; Horton and Ruban, 1994). Figure 7.2b displays three types of zeaxanthin absorption
spectra: the pigment in solution, H- and J-type aggregates. Here, the variation in the observed
spectral maxima reaches more than 150 nm (
6000 cm
−1
)—a very signii cant difference, indeed.
It is therefore important to bear in mind the dependency of the carotenoid spectrum upon prop-
erties of the environment for
in vivo
analysis, which is based on the application of optical spec-
troscopies. This approach is often the only way to study the composition, structure, and biological
functions of carotenoids. Spectral sensitivity of xanthophylls to the medium could be a property to
use for gaining vital information on their binding sites and dynamics. The next sections will provide
a brief introduction to the structure of the environment with which photosynthetic xanthophylls
interact—light harvesting antenna complexes (LHC).
∼
7.3 LOCALIZATION AND FUNCTIONS OF XANTHOPHYLLS
IN LIGHT HARVESTING ANTENNA OF PLANTS
7.3.1 T
HE
N
EED
FOR
P
HOTOSYNTHETIC
A
NTENNA
The photosynthetic antenna is an assembly of pigments that is not directly involved in the charge
separation process but, by the collection of light quanta and efi cient energy transfer, enhances
the reaction center cross section by more than two orders of magnitude. The antenna is crucial
in the low-light conditions since it enhances the excitation rate of the reaction center close to its
turnover rate—a requirement for maximum energy conversion efi ciency (Clayton, 1980). The
protein is an essential part of the antenna. It binds and orients pigments in order to optimize
light energy interception and transfer. The antenna protein also tunes the excited state energies
in order to provide directionality for the energy l ow and enhances the absorption of light across
a larger wavelength range. Without the antenna, photosynthetic organisms, particularly aquatic
ones, would starve.
7.3.2 S
TRUCTURE
OF
THE
P
HOTOSYSTEM
II A
NTENNA
: X
ANTHOPHYLLS
IN
LHCII S
TRUCTURE
In higher plants, the photosynthetic machinery is almost exclusively localized in the thylakoid mem-
brane of chloroplasts (Figure 7.3). Thylakoids tend to form stacks of these membranes called grana,
which generally carry photosystem II (PSII) with the light harvesting antenna. PSII is organized as
a dimer containing two sets of reaction center proteins, D1 and D2 with their inner-antenna com-
plexes CP43 and CP47. The major part of the antenna is formed by a number of monomeric (minor
LHCII complexes) and trimeric (major LHCII complex or LHCII) pigment-protein complexes (for
review, see Dekker and Boekema (2005)). The latter can often form large oligomeric structures,
which contain several interacting trimers. The integrity of the PSII complex is ensured by various
noncovalent interactions between its multiple subunits.
The structure of the major trimeric LHCII complex has been recently obtained at 2.72 Å (Figure
7.3) (Liu et al., 2004). It was revealed that each 25 kDa protein monomer contains three transmem-
brane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chl
a
and
6 Chl
b
) and 4 xanthophyll molecules: 1 neoxanthin, 2 luteins, and 1 violaxanthin. The i rst three
xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids
by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and
hydrophobic amino acids such as tryptophan and phenylalanine.
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