Identification of Carotenoids in Photosynthetic Proteins: Xanthophylls of the Light Harvesting Antenna - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 125

It remains unclear how lutein 1 becomes engaged as a quencher in aggregated LHCII. Recently,
structural studies performed on crystals of this complex in various states of quenching led to the
suggestion that this xanthophyll molecule can move toward the terminal emitter pigments due to
structural alterations in the D-helix and its environment (Yan et al., 2007). Such a movement would
alter not only the interpigment distances, but also the mutual orientation and conformation of the
pigments in lutein 1 locus—these factors could be important in creating an efi cient and reversible
energy trap.

7.6 DISTINGUISHING CONFIGUR ATIONAL VARIATIONS
IN XANTHOPHYLLS
The resonance Raman spectra are very rich in information. They carry not only a i ngerprint of a
type of carotenoid and its conformation, but also the information about molecular distortion. Even
though the geometric changes are relatively small, resonance Raman can be very useful for the
identii cation and the probing properties of the xanthophyll binding loci.

7.6.1 L UTEIN 2 T WISTING C ONFIGURATION IN T RIMERIC LHCII
The 510 nm absorbing species in LHCII has previously revealed a strong negative CD band, imply-
ing that the molecule could be in a deformed coni guration resulting from an interaction from its
environment. In addition, Stark measurements showed that this species possesses a very large
dipole moment of more than 14 D (Palacios et al., 2003). This is also an indicator of very spe-
cii c surroundings exerting a polarization effect. Resonance Raman was yet another approach to
explore the coni guration of this molecule (Ruban et al., 2000). Measurements of the resonance
Raman spectra excited at 501.7 as well as 514.5 nm revealed four clearly dei ned and pronounced
bands in the n 4 region (Ruban et al., 2000, 2001). They are i ngerprints of a twisted carotenoid
coni guration, which can be completely abolished by the monomerization of trimers (Figure 7.8).

0.4

Lutein 2 Neoxanthin

0.6
0.5
0.4
0.3
0.2
0.1
0.0

0.3

0.2

Trimer

0.1

0.0

Monomer

Zeaxanthin Violaxanthin

0.2 0.2

0.1 0.1

0.0 0.0

920 930 940 950 960 970 980 920 930 940 950 960 970 980

Wavenumber (cm -1 )

FIGURE 7.8 n 4 resonance Raman spectra of all four LHCII xanthophylls.