Energy Transfer in LHCII Monomers at 77K Studied by Sub-Picosecond Transient Absorption Spectroscopy

Abstract

Energy transfer from chlorophyll b (Chl b) to chlorophyll a (Chl a) in monomeric preparations of light-harvesting complex II (LHCII) from spinach was studied at 77 K using pump-probe experiments. Sub-picosecond excitation pulses centered at 650 nm were used to excite preferentially Chl b and difference absorption spectra were detected from 630 to 700 nm. Two distinct Chl b to Chl a transfer times, ∼200 fs and 3 ps, were found. A clearly distinguishable energy transfer process between Chl a molecules occurred with a time constant of 18 ps. The LHCII monomer data are compared to previously obtained LHCII trimer data, and both data sets are fitted simultaneously using a global analysis fitting routine. Both sets could be described with the following time constants: 140 fs, 600 fs, 8 ps, 20 ps, and 2.9 ns. In both monomers and trimers 50% of the Chl b to Chl a transfer is ultrafast (<200 fs). However, for monomers this transfer occurs to Chl a molecules that absorb significantly more toward shorter wavelengths than for trimers. Part of the transfer from Chl b to Chl a that occurs with a time constant of 600 fs in trimers is slowed down to several picoseconds in monomers. However, it is argued that observed differences between monomers and trimers should be ascribed to the loss of some Chl a upon monomerization or a shift of the absorption maximum of one or several Chl a molecules. It is concluded that Chl b to Chl a transfer occurs only within monomeric subunits of the trimers and not between different subunits.Light-harvesting complex II (LHCII) 1 is the most abundant light-harvesting antenna pigment-protein complex of green plants. In nature it is believed to be organized in trimeric units and it binds 50% of the chlorophyll (CHl) present in the chloroplast (1). HPLC analysis has shown that each monomeric subunit binds 5-6 Chl b, 7-8 Chl a, and several xanthophyll (Xan) molecules: 2 luteins (Lut), 1 neoxanthin (Neo), and varying, substoichiometric amounts of violaxanthin (Vio) [see also Table 1, which is taken from (2)].The structure of the trimeric LHCII has been resolved to 3.4 Å using electron microscopy and electron diffraction of two-dimensional crystals (3). A significant part of the 26 kDa monomeric LHCII protein and the positions of 12 Chls and two luteins in the center of the protein were determined. However, the resolution was not high enough to distinguish between Chl a and Chl b, and the orientation of the lowestenergy electronic transition dipole moment (Q y ) of the individual chlorophylls cannot yet be assigned. Center to center distances between Chl a and Chl b within a monomer range from 8.3 to 10.5 Å. The role of the xanthophylls is, apart from light harvesting and structural stabilization of the complex, to quench Chl triplets to prevent the formation of singlet oxygen. This is only possible when the distance between the Chls and the Xans is small (4). On the basis of the idea that excitons are transferred very fast from Chl b to Chl a, so that Chl triplets are only formed on Chl a, a ...

The Functional Significance of the Monomeric and Trimeric States of the Photosystem II Light Harvesting Complexes

“…previously described (39), modified as in ref 40. The pigment composition of isolated samples was determined by reversephase HPLC according to the method of Faber et al (41).…”

Principles of light harvesting from single photosynthetic complexes

“…LHCII is trimeric, and each monomer contains 14 chlorophyll (Chl), six Chl-b and eight Chl-a, and four carotenoids surrounded by a protein matrix [24]. Energy rapidly transfers to the Chl-a, and fluorescence occurs out of the Chl-a band [41][42][43][44][45][46][47]. On-going efforts are focused on characterizing the changes in the excited state manifold of LHCII under conditions that mimic high light.…”

Selective Interaction between Xanthophylls and Chlorophylls in LHCII Probed by Femtosecond Transient Absorption Spectroscopy

“…7 The former one was assigned to neoxanthin, and the peak position is close to the one for reconstituted monomeric LHCII. 33 Also, Ruban et al 5 found that the maximum of neoxanthin is located at 486 nm for trimeric LHCII from spinach. The 510 nm peak is the weakest one of the three, and it was originally assigned to violaxanthin because the amount of violaxanthin was significantly less than that of the other xanthophylls.…”