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

Abstract

In this article we report the characterization of the energy transfer process in the reconstituted isoforms of the plant light-harvesting complex II. Homotrimers of recombinant Lhcb1 and Lhcb2 and monomers of Lhcb3 were compared to native trimeric complexes. We used low-intensity femtosecond transient absorption (TA) and time-resolved fluorescence measurements at 77 K and at room temperature, respectively, to excite the complexes selectively in the chlorophyll b absorption band at 650 nm with 80 fs pulses and on the high-energy side of the chlorophyll a absorption band at 662 nm with 180 fs pulses. The subsequent kinetics was probed at 30-35 different wavelengths in the region from 635 to 700 nm. The rate constants for energy transfer were very similar, indicating that structurally the three isoforms are highly homologous and that probably none of them play a more significant role in light-harvesting and energy transfer. No signature has been found in the transient absorption measurements at 77 K for Lhcb3 which might suggest that this protein acts as a relative energy sink of the excitations in heterotrimers of Lhcb1/Lhcb2/Lhcb3. Minor differences in the amplitudes of some of the rate constants and in the absorption and fluorescence properties of some pigments were observed, which are ascribed to slight variations in the environment surrounding some of the chromophores depending on the isoform. The decay of the fluorescence was also similar for the three isoforms and multi-exponential, characterized by two major components in the ns regime and a minor one in the ps regime. In agreement with previous transient absorption measurements on native LHC II complexes, Chl b fi Chl a energy transfer exhibited very fast channels but at the same time a slow component (ps). The Chls absorbing at around 660 nm exhibited both fast energy transfer which we ascribe to transfer from 'red' Chl b towards 'red' Chl a and slow transfer from 'blue' Chl a towards 'red' Chl a. The results are discussed in the context of the new available atomic models for LHC II.

Light-harvesting and structural organization of Photosystem II: From individual complexes to thylakoid membrane

“…In another recent study by Schlau-Cohen et al [115] a sub-110 fs relaxation process through spatially overlapping states was detected that had not been observed before but all the slower processes that were observed were in excellent agreement with previous results. It should be noticed that although all theoretical models are based on the structural model of Lhcb1, the (sub)ps transient absorption kinetics of three gene products forming LHCII, Lhcb1, Lhcb2 and Lhcb3, are identical [116].…”

Femtosecond pump probe spectroscopy for the study of energy transfer of light-harvesting complexes from extractions of spinach leaves

“…The time constant of the major component of energy transfer from chlorophyll b to chlorophyll a was reported in the literature to be about 600 fs. 3,[45][46][47][48] Time constants obtained for 542 nm are 4.76 ± 1 ps and 43 ± 6 ps, again the latter value is less accurate due to the limited range of the experimental measurement. This broadband feature, from 500 nm to 600 nm (with peak at 542 nm), is related to the relaxation of the vibrational energy of the excited state of the chlorophyll molecules.…”

“…Several studies on the energy transfer between the chlorophylls have shown that the transfer usually takes place from chlorophyll b to chlorophyll a. 3,45 In addition to the transfer from chlorophyll b to chlorophyll a, there is also energy equilibration that takes place from chlorophyll a to chlorophyll a and from chlorophyll b to chlorophyll b. The LHC II monomer is composed of 7-8 chlorophyll a molecules, 4-5 chlorophyll b molecules and 3 carotenoids, at a chlorophyll a:b ratio of 1.6:2.…”

“…The energy transfer from chlorophyll b to chlorophyll a, within this monomer, exhibits three components: a very fast component that occurs at about 100-150 fs and two other components that have lifetimes of 600-800 fs and 5-7 ps. 45 In order to obtain complete information on the five bands observed in Fig. 12, a contour graph can be plotted (Fig.…”

“…These values compare well with the values of 600 fs and 5-7 ps reported in the literature. 3,45,47,48 The excitation of the sample at 680 nm gave us time constants of about 0.35 ± 0.1 ps and 6.90 ± 0.3 ps. These values are correlated to the chlorophyll a to chlorophyll a energy equilibration and are in agreement with the values reported in the literature, 48 which are 300 fs and 6 ps.…”