Chemistry Reference
In-Depth Information
1.2
9
λ
ex
= 488 nm
1.0
k
D
0.8
0.6
0
0.4
0.2
LHCII
0.0
940
950
960
970
980
Wavenumber (cm
-1
)
FIGURE 7.9
n
4
resonance Raman spectra for neoxanthin in LHCII in different quenching states. The varia-
tion in the extent of quenching is illustrated by the arrow indicating variation of the nonradiative constant from
0 in trimers to 9 in highly aggregated complexes. Structure of neoxanthin is displayed on the right with arrows
pointing toward the most distorted areas in the backbone of the molecule.
A close analysis of the trimers order in the crystal revealed that the exposed part of neoxanthin
molecule is completely free from interactions with any protein or pigment components (Pascal et
al., 2005). In addition, an examination of the neoxanthin coni guration, taken from the structure of
LHCII, points toward strong distortion of the
cis
-end of the molecule (Figure 7.9). This fact sug-
gests that the twist most likely occurs within the protein interior, implying that some movement in
the LHCII monomer must take place during the transition into dissipative state. Apparently, this
movement affects not only lutein 1, as previously discussed, but also neoxanthin.
It has been important to determine if the neoxanthin distortion signature could be detected dur-
ing the nonphotochemical quenching
in vivo
. Resonance Raman measurements on leaves and chlo-
roplasts of various
Arabidopsis
mutants have revealed a small increase in the 950 cm
−1
region. The
relationship between the amplitude of this transition and the amount of NPQ suggests that the
LHCII aggregation may be the sole cause of the protective chlorophyll l uorescence quenching
in
vivo
(Ruban et al., 2007).
7.7 IDENTIFICATION OF PERIPHERAL XANTHOPHYLLS:
THE XANTHOPHYLL CYCLE
The fourth binding site in LHCII structure is occupied by violaxanthin—the most polar xanthophyll
of the xanthophyll cycle (Figures 7.3 and 7.5). The question of whether zeaxanthin formed upon the
deepoxidation of violaxanthin is bound differently or remains in the structure at all is a controver-
sial subject (Ruban et al., 1999; Verhoeven et al., 1999; Morosinotto et al., 2002). It is likely that the
low afi nity of violaxanthin/zeaxanthin binding to LHCII in the presence of detergent is responsible
for these discrepancies. The fact that LHCII trimers can be prepared with one zeaxanthin per mono-
mer using gentle solubilization procedures suggests that this xanthophyll must be a normal struc-
tural component of the antenna complex (Ruban et al., 1999; Johnson et al., 2007). The manner by
which the lumen-associated deepoxidase accesses the stroma-facing epoxy group of violaxanthin
also remains controversial.
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