Carotenoid-to-Chlorophyll Energy Transfer in Recombinant Major Light-Harvesting Complex (LHCII) of Higher Plants. I. Femtosecond Transient Absorption Measurements

Abstract

The energy transfer kinetics from carotenoids to chlorophylls and among chlorophylls has been measured by femtosecond transient absorption kinetics in a monomeric unit of the major light-harvesting complex (LHCII) from higher plants. The samples were reconstituted complexes with different carotenoid contents. The kinetics was measured both in the carotenoid absorption region and in the chlorophyll Q(y) region using two different excitation wavelengths suitable for selective excitation of the carotenoids. Analysis of the data shows that the overwhelming part of the energy transfer from the carotenoids occurs directly from the initially excited S(2) state of the carotenoids. Only a small part (<20%) may possibly take an S(1) pathway. All the S(2) energy transfer from carotenoids to chlorophylls occurs with time constants <100 fs. We have been able to differentiate among the three carotenoids, two luteins and neoxanthin, which have transfer times of approximately 50 and 75 fs for the two luteins, and approximately 90 fs for neoxanthin. About 50% of the energy absorbed by carotenoids is initially transferred directly to chlorophyll b (Chl b), while the rest is transferred to Chl a. Neoxanthin almost exclusively transfers to Chl b. Due to various complex effects discussed in the paper, such as a specific coupling of Chl b and Chl a excited states, the percentage of direct Chl b transfer thus is somewhat lower than estimated by us previously for LHCII from Arabidopsis thaliana. (Connelly, J. P., M. G. Müller, R. Bassi, R. Croce, and A. R. Holzwarth. 1997. Biochemistry. 36:281). We can distinguish three different Chls b receiving energy directly from carotenoids. We propose as a new mechanism that the carotenoid-to-Chl b transfer occurs to a large part via the B(x) state of Chl b and to the Q(x) state, while the transfer to Chl a occurs only via the Q(x) state. We find no compelling evidence in favor of a substantial S(1) transfer path of the carotenoids, although some transfer via the S(1) state of neoxanthin can not be entirely excluded. The S(1) lifetimes of the two luteins were determined to be 15 ps and 3.9 ps. A detailed quantitative analysis and kinetic model of the processes described here will be presented in a separate paper.

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

“…Slow (ps) Chl b fi Chl a energy transfer has also been reported for native complexes in the literature (Bittner et al 1994(Bittner et al , 1995Visser et al 1996;Connelly et al 1997;). The obtained transfer times for Chl b fi Chl a transfer in Lhcb1 (see Table 2) are somewhat different from those obtained by Croce et al (2001). They reported transfer times of 0.13, 0.33, 1 and 7 ps.…”

“…at around 660 nm Visser et al 1996Visser et al , 1997Gradinaru et al 1998), but at the same time an ultrafast process has been reported, although it was not unambiguously ascribed to Chl fi Chl energy transfer due to the long pulses used in the experiments (200-250 fs) (Visser et al 1996;Gradinaru et al 1998). Recently, Croce et al (2001) performed a detailed transient absorption (TA) study on one of the reconstituted isoforms of LHC II, namely Lhcb1. They reported for both carotenoid to chlorophyll and chlorophyll to chlorophyll similar transfer times and efficiencies to those of native LHC II complexes.…”

Excitation Decay Pathways of Lhca Proteins: A Time-Resolved Fluorescence Study

“…50 It has also been found in the case of monomeric systems and actually shows up more pronounced in studies with reconstituted systems. 47,48,51 However, as stated above, in the case of LHCI proteins, a similar component was found in both native and reconstituted systems. It is tempting to consider this loss channel to have the same origin as that found in the native PSI-LHCI particles.…”

Energy Transfer, Excited-State Deactivation, and Exciplex Formation in Artificial Caroteno-Phthalocyanine Light-Harvesting Antennas

“…The carotenoid to Pc energy transfer in dyad 1 is reminiscent of several natural light-harvesting antennas where high carotenoid to chlorophyll energy transfer efficiency is obtained by employing a multiphasic Car to Chl energy transfer. 2,11,21,[31][32][33] The final EADS (magenta line) appears after 2 ns and represents the component that does not decay on the time scale of the experiment. It features the typical carotenoid triplet ESA in the 475-550 nm region as well as a bleach/band-shift-like signal in the Pc Q region.…”