Energy transfer among CP29 chlorophylls: Calculated Forster rates and experimental transient absorption at room temperature

The energy transfer rates between chlorophylls in the light harvesting comp lex CP29 of higher plants at room temperature were calculated ab initio acc ording to the Forster mechanism (Forster T. 1948, Ann. Physik. 2:55-67). Re cently, the transition moment orientation of CP29 chlorophylls was determin ed by differential linear dichroism and absorption spectroscopy of wild-typ e versus mutant proteins in which single chromophores were missing (Simonet to R., Crimi M., Sandona D., Croce R., Cinque G., Breton J., and Bassi R. 1 999. Biochemistry. 38:12974-12983). In this way the Q(y) transition energy and chlorophyll a/b affinity of each binding site was obtained and their ch aracteristics supported by reconstruction of steady-state linear dichroism and absorption spectra at room temperature. in this study, the spectral for m of individual chlorophyll a and b ligands within the protein environment was experimentally determined, and their extinction coefficients were also used to evaluate the absolute overlap integral between donors and accepters employing the Stepanov relation for both the emission spectrum and the Sto kes shift. This information was used to calculate the time-dependent excita tion redistribution among CP29 chlorophylls on solving numerically the Paul i master equation of the complex: transient absorption measurements in the (sub)picosecond time scale were simulated and compared to pump-and-probe ex perimental data in the Q(y) region on the native CP29 at room temperature u pon selective excitation of chlorophylls b at 640 or 650 nm. The kinetic mo del indicates a bidirectional excitation transfer over all CP29 chlorophyll s a species, which is particularly rapid between the pure sites A1-A2 and A 4-A5. Chlorophylls b in mixed sites act mostly as energy donors for chlorop hylls a, whereas site B5 shows high and bidirectional coupling independent of the pigment hosted.