Ultrafast Evolution of the Excited States in the Chlorophyll a/b Complex CP29 from Green Plants Studied by Energy-Selective Pump-Probe Spectroscopy †

The energy transfer process in the minor light-harvesting antenna complex CP29 of green plants was probed in multicolor transient absorption experiments at 77 K using selective subpicosecond excitation pulses at 640 and 650 nm. Energy flow from each of the chlorophyll (Chl) b molecules of the complex could thus be studied separately. The analysis of our data showed that the "blue" Chl b (absorption around 640 nm) transfers excitation to a "red" Chl a with a time constant of 350 ( 100 fs, while the 'red' Chl b (absorption at 650 nm) transfers on a picosecond time scale (2.2 ( 0.5 ps) toward a "blue" Chl a. Furthermore, both fast (280 ( 50 fs) and slow (10 -13 ps) equilibration processes among the Chl a molecules were observed, with rates and associated spectra very similar to those of the major antenna complex, LHC-II. Based on the protein sequence homology between CP29 and LHC-II, a basic modelling of the observed kinetics was performed using the LHC-II structure and the Forster theory of energy transfer. Thus, an assignment for the spectral properties and orientation of the two Chl's b, as well as for their closest Chl a neighbors, is put forward, and a comparison is made with the previous assignments and models for LHC-II and CP29.