Chemistry Reference
In-Depth Information
Lutein 1
Lutein 2
a
603
Lut2
mon
1
mon
2
(a)
(b)
FIGURE 7.7
(a) Structure of the LHCII trimer showing lutein 2 from the monomer 1 (
mon1
) interacting with
the chlorophyll
a
603 from the neighboring monomer (
mon2
). Inset displays the lutein 2-exposed side of the
chlorophyll
a
603. (b) Comparison of the structures of two LHCII luteins. Arrows and black balls indicate the
atoms with bonds in lutein 2, which are the most affected by distortion in addition to those of lutein 1.
7.5.3 I
DENTIFICATION
OF
THE
C
HLOROPHYLL
E
XCITATION
Q
UENCHER
IN
A
GGREGATED
LHCII
The identii cation studies described in Sections 7.5.1 and 7.5.2 recently played a crucial role in
the search for the excitation energy quencher in LHCII. In plants, the photosynthetic apparatus
responds to a harmful excess of sunlight by changing the conformation of the PSII light harvest-
ing antenna leading to a decrease in the amount of the excitation energy funneled into the reaction
center (Horton et al., 1996). This down regulation is achieved by creating a new, energy-dissipative
channel in the antenna, which becomes competitive with the transfer of energy toward the PSII
reaction center. This channel can be easily monitored by measurements of the chlorophyll l uo-
rescence from antenna. A parameter known as the nonphotochemical chlorophyll
a
l uorescence
quenching (NPQ), as opposed to the l uorescence quenching caused by reaction centers and called
photochemical quenching (qP), can be simply derived from the measurements.
T he nat u re of N PQ -associate d a lterat ions i n LHCI I, as wel l as t he physica l me cha n ism of quench-
ing, has been a key focus of photosynthesis research for a number of decades. In the early 1990s, the
group of Horton and coworkers put forward the LHCII aggregation model to explain the mechanism
of NPQ (Horton et al., 1991, 2005). According to this model acidii cation of LHCII amino acid resi-
dues, resulting from the establishment of the transmembrane proton gradient, leads to the induction
of a conformational change in this complex and promotion of protein-protein interactions (aggrega-
tion). Indeed, isolated aggregated LHCII has been shown to possess a very low l uorescence yield
and a short excited state lifetime in comparison to the trimeric or monomeric complex (Mullineaux
et al., 1992; Ruban and Horton, 1992).
Recently, pump-probe femtosecond transient absorption spectroscopy has been employed in
order to search for a possible cause of the decrease in the excited state lifetime (Ruban et al.,
2007). The use of diode array detection allowed us to record the spectral evolution of changes fol-
lowing energy equilibrium, transfer, and dissipation in LHCII. It was found that in the aggregated
complex the dramatic reduction in the chlorophyll excited state lifetime is caused by a new energy
transfer path to one of the xanthophylls absorbing in 490-495 nm region (Ruban et al., 2007).
Since the lutein 1 absorption is found to be consistent with 495 nm, as described above, this i nd-
ing implied that this xanthophyll is likely to be the quencher of the chlorophyll
a
excited states in
aggregated LHCII. Lutein 1 is located near the three chlorophyll
a
molecules, Chl
a
610,
a
611, a nd
a
612 (Figure 7.5), which together form the terminal emitter cluster, possessing the highest exciton
density in LHCII (van Grondelle and Novoderezhkin, 2006). Therefore, Lutein 1 is ideally situated
for the quenching of excitation normally localized on this cluster of pigments for a period of more
than 4 ns.
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