Chemistry Reference
In-Depth Information
The same bands were resolved in the resonance Raman spectra for the PSII membranes (Ruban
et al., unpublished). Therefore, this method, for example, can be used to assess whether the LHCII
trimers are intact
in vivo
at various physiological conditions.
Various spectroscopic approaches applied to the 510 nm transition indicate an unusual envi-
ronment for the redshifted lutein (Figures 7.5 and 7.7a). Interaction with the Chl
a
603 could force
lutein 2 molecule to adopt a twisted coni guration. In addition, strong interaction with a number
of aromatic residues, in particular tryptophan and phenylalanine, which possess relatively large
surface areas, could further promote this distortion. It is reasonable to assume that the energy
required to produce this distortion comes from the forces involved in the stabilization of LHCII
trimers.
Recently, a detailed structural analysis of the luteins in the LHCII has provided further evidence
to support our proposal that lutein 2 is in a twisted coni guration (Yan et al., 2007). This xanthophyll
appears to be more distorted along the carbon backbone than lutein 1 (Figure 7.7b). The fact that the
distortion can be seen at a 2.72 Å resolution suggests its relatively large magnitude. The calculation
of the local energy strain proi les reveals that lutein 2 contains more atoms with bonds affected by
distortion than lutein 1 (Figure 7.7b).
7.6.2 N
EOXANTHIN
D
ISTORTION
UPON
A
GGREGATION
AND
C
RYSTALLIZATION
OF
LHCII
AND
I
N
V
IVO
Early resonance Raman experiments on trimeric and aggregated LHCII revealed small but specii c
differences in their spectra (Ruban et al., 1995). It has been noticed that the n
4
region is the only
one, which is affected by aggregation. A major band at 950 cm
−1
accompanied by a group of minor
upshifted bands appears in the spectrum of aggregated LHCII (Figure 7.8). The difference between
the spectra of aggregates and the trimers reveals that the shape of the aggregation-associated change
is different from the twisting i ngerprint of lutein 2, described above. There, the 950 cm
−1
band is
strongly dominated by 955 and 965 cm
−1
peaks (Figure 7.8). The variation in the excitation reso-
nance Raman proi les of the amplitude of the 950 cm
−1
band have revealed remarkable similarity to
the n
1
frequency dependence for neoxanthin in trimeric LHCII (Ruban et al., 2000). This was a clear
indication that the enhancement of the 950 cm
−1
transition originates from the twisted coni guration
of neoxanthin.
The amplitude of the neoxanthin distortion band is always in good correlation with the extent
of the chlorophyll
a
l uorescence quenching in aggregates of LHCII. Figure 7.9 shows a series
of resonance Raman spectra of the n
4
region for neoxanthin of aggregated LHCII with differing
extents of l uorescence quenching, calculated as the nonradiative constant, k
D
. The amplitude of the
950 cm
−1
band increases with the enhancement in the amount of nonradiative energy dissipation.
This relationship is highly nonlinear (Ruban et al., 2007). This is likely due to a gradual increase in
the interacting pigment domain size resulting from the aggregation process, which also causes the
change in effectiveness of the quencher (Barzda et al., 2001).
Neoxanthin seems to be bound only by the
cis
-end within the LHCII complex, the opposite
allene end of the molecule protruding into the environment (Figure 7.3). Therefore, in aggregates,
neoxanthin may be easily affected by the close proximity of neighboring proteins exerting distor-
tion forces. On the other hand, if the intrinsic conformational change takes place within the LHCII
monomer it may cause a strain upon the thoroughly embedded 9-
cis
side. A critical experiment to
test which of these two possibilities takes place was designed. LHCII crystals used for structural
analysis were subjected to the rigorous photophysical investigation. The 77K l uorescence spectra
and l uorescence lifetime analysis reveal that the complex possesses characteristics of the quenched
antenna state F700 band and a decreased lifetime (Pascal et al., 2005). Therefore, it was concluded
that the known LHCII structure must correspond to the dissipative or photoprotective antenna state.
Remarkably, the crystals possessed a very pronounced neoxanthin twisting Raman i ngerprint.
Search WWH ::
Custom Search