Carotenoid-to-chlorophyll energy transfer in recombinant major light-harvesting complex (LHCII) of higher plants.: I.: Femtosecond transient absorption measurements

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 similar to 50 and 75 fs for the two luteins, and similar to 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 Chi a. Neoxanthin almost exclusively transfers to Chi b. Due to various complex effects discussed in the paper, such as a specific coupling of Chi b and Chi 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. Muller, 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.