Understanding the Energy Transfer Function of LHCII, the Major Light-Harvesting Complex of Green Plants

Abstract

Since the crystal structure of the major light-harvesting complex II (LHCII) of green plants was obtained by Ku ¨hlbrandt, Wang and Fujiyoshi (Nature 1994, 367, 614-621), this chlorophyll-containing trimeric membrane protein has been the subject of intensive investigation. The complex contains between 36 and 42 chlorophyll molecules per trimer (Chl a and Chl b) and 10 to 12 xanthophyll molecules (lutein, neoxanthin and violaxanthin). The protein displays a rich spectrum of interactions, both between pigments and between the pigments and the protein, and these interactions have been studied with a multitude of different techniques. In this article we present an overview of the most important experimental results that have become available over the past decade and relate these to the structural knowledge. Emphasis will be put on the pigment identities, their spectroscopic features, and the interactions between the pigments, which determine both steady-state (polarized) properties and singlet and triplet energy transfer dynamics. Remaining questions will be pinpointed and hopefully they can help direct research in the near future.

Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy

“…This DAS describes energy transfer, which is observed to be rather conservative in the BBY membranes, with loss of emission at 675 nm, a zero-crossing at 682 nm and a gain at 687 nm. This DAS is interpreted as energy transfer from LHCII to the PSII core complex (Andrizhiyevskaya et al 2005), where the transfer timescale is in agreement with the 32 ps migration time of excitations between LHCII trimers which was estimated at room temperature (Barzda et al 2001;Van Amerongen and Van Grondelle 2001). The non-conservativity is explained by trapping taking place on a similar timescale in PSII.…”

“…The DAS of the unstacked membranes, extends to the blue with respect to the native DAS, with its maximum at 680 nm, but moreover it shows a small rise of fluorescence around 730 nm. Energy transfer from LHCII trimers to other complexes is expected to occur on this time-range (Barzda et al 2001;Van Amerongen and Van Grondelle 2001) as well. Thus, the spectrum and amplitude of the 25 ps energy transfer component in the unstacked membranes with the positive feature at 675-680 nm and the negative feature above 720 nm and the absence of these characteristics in the native membranes can be explained by energy transfer from LHCII to PSI in presumably LHCII-LHCI-PSI complexes.…”

On the relation of protein dynamics and exciton relaxation in pigment–protein complexes: An estimation of the spectral density and a theory for the calculation of optical spectra

“…One can infer that for any multi-pigment system composed of pigment-protein subunits of structure similar to that of the B777-complex, significant non-Markovian effects in the exciton transfer dynamics can be expected only when exciton transfer occurs on a 100 fs ͑or shorter͒ time scale. Such ultrafast transfer times have been observed in a number of antennas, among them the LH2, 70 LH1, 70 LHCII 71 and the photosynthetic reaction centers of bacteria and green plants. 72 In that case a non-Markovian theory is expected to provide a more detailed understanding of such experiments.…”

“…Additional evidence arises from a comparison of the decay of the correlation function of the optically probed energy gap of the pigments and ultrafast exciton re-laxation times measured in different antennas. [70][71][72] The correlation function itself was estimated from fluorescence line narrowing spectra of B777-complexes.…”

A comparison of the three isoforms of the light-harvesting complex II using transient absorption and time-resolved fluorescence measurements

“…All the reconstituted samples exhibited an enhanced pump-probe signal between 655 and 665 nm (see Figures 5 and 6) compared to native complexes, which we ascribe to the larger absorption cross-section at those wavelengths observed in the steady-state absorption spectra of the reconstituted isoforms. The ESA observed in this wavelength region after equilibration appears to be an intrinsic property of the LHC II complexes (see also, Visser et al 1996;Gradinaru et al 1998), and it probably arises from excitonic interactions (Buck et al 1997;Amerongen et al 2000).…”