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.