Energy-transfer dynamics in the LHCII complex of higher plants: Modified redfield approach

We have modeled energy-transfer dynamics in the peripheral plant light-harvesting complex LHCII using both standard Redfield theory and its modification for the case of strong exciton-phonon coupling (Zhang, W. M.; Meier, T.; Chernyak, V.; Mukamel, S. J. Chem. Phys. 1998, 108, 7763). A quantitative simultaneous fit of the absorption (OD), linear dichroism (LD), steady-state fluorescence (FL) spectra at 7-293 K, and transient absorption (TA) kinetics at 77 and 293 K has been obtained using the experimental exciton-phonon spectral density to model the temperature-dependent line shape. We use configurations of the antenna (i.e., chlorophyll (Chi) a/b identities, orientations, and site energies) close to those proposed in our previous study (Novoderezhkin, V.; Salverda, J. M.; van Amerongen, H.; van Grondelle, R. J. Phys. Chem. B 2003, 107, 1893). These configurations have been further adjusted from the fit with the modified Redfield approach. The new (adjusted) models allow a better quantitative explanation of the spectral shapes. A combination of fast (femtosecond) interband energy transfer and slow (picosecond) intraband equilibration can be better reproduced as well. We paid special attention to unravel the origins of the slow components preliminarily assigned to localized states in the previous work. These "bottleneck" states have been directly visualized in this study via selective femtosecond excitation and probing at different wavelengths. In our modeling, these states are determined by two or three (depending on the model) monomeric Chls a or b shifted to the spectral region of 655-670 nm between the main absorption peaks of Chi b (650 nm) and Chl a (675 nm). In all configurations we have found these energy-shifted Chls to be bound at mixed sites (i.e., A(3), A(6), A(7), or B-3). Experiments and self-consistent modeling using the modified Redfield theory allow us to explore the participation of these states in the overall energy-transfer dynamics. This has led to a more complete and physically adequate model for the energy-transfer dynamics in LHCII.