Chlorophyll b to chlorophyll a energy transfer kinetics in the CP29 antenna complex: a comparative femtosecond absorption study between native and reconstituted proteins.

The energy transfer processes between Chls b and Chls a have been studied in the minor antenna complex CP29 by femtosecond transient absorption spectroscopy. Two samples were analyzed: the native CP29, purified from higher plants, and the recombinant one, reconstituted in vitro with the full pigment complement. The measurements indicate that the transfer kinetics in the two samples are virtually identical, confirming that the reconstituted CP29 has the same spectroscopic properties as the native one. In particular, three lifetimes (150 fs, 1.2 ps, and 5-6 ps) were identified for Chl b-652 nm to Chl a energy transfer and at least one for Chl b-640 nm (600-800 fs). Considering that the complexes bind two Chls b per polypeptide, the observation of more than two lifetimes for the Chl b to Chl a energy transfer, in both samples, clearly indicates the presence of the so-called mixed Chl binding sites--sites which are not selective for Chl a or Chl b, but can accommodate either species. The kinetic components and spectra are assigned to specific Chl binding sites in the complex, which provides further information on the structural organization.

Energy transfer pathways in the minor antenna complex CP29 of photosystem II: a femtosecond study of carotenoid to chlorophyll transfer on mutant and WT complexes.

The energy transfer processes between carotenoids and Chls have been studied by femtosecond transient absorption in the CP29-WT complex, which contains only two carotenoids per polypeptide located in the L1 and L2 sites, and in the CP29-E166V mutant in which only the L1 site is occupied. The comparison of these two samples allowed us to discriminate between the energy transfer pathways from the two carotenoid binding sites and thus to obtain detailed information on the Chl organization in CP29 and to assign the acceptor chlorophylls. For both samples, the main transfer occurs from the S(2) state of the carotenoid. In the case of the L1 site the energy acceptor is the Chl a 680 nm (A2), whereas the Chl a 675 nm (A4-A5) and the Chl b 652 nm (B6) are the acceptors from the xanthophyll in the L2 site. These transfers occur with lifetimes of 80-130 fs. Two additional transfers are observed with 700-fs and 8- to 20-ps lifetimes. Both these transfers originate from the carotenoid S(1) states. The faster lifetime is due to energy transfer from a vibrationally unrelaxed S(1) state, whereas the 8- to 20-ps component is due to a transfer from the S(1,0) state of violaxanthin and/or neoxanthin located in site L2. A comparison between the carotenoid to Chl energy transfer pathways in CP29 and LHCII is presented and differences in the structural organization in the two complexes are discussed.