Impact of Structure, Coupling Scheme, and State of Interest on the Energy Transfer in CP29

The Q(y) and B-x excitation energy transfer (EET) in the minor light-harvestingcomplexCP29 (LHCII B4.1) antenna complex of Pisum sativum was characterized using a computational approach. We applied Fo''rsterresonance energy transfer (FRET) and the transition density cube (TDC)method to estimate the Coulombic coupling, based on a combinationof classical molecular dynamics and quantum mechanics/molecular mechanicscalculations. Employing TDC instead of FRET mostly affects the EETbetween chlorophylls (Chls) and carotenoids (Crts), as expected dueto the Crts being spatially more challenging for FRET. Only betweenChls, effects are found to be small (about only 0.1 EET efficiencychange when introducing TDC instead of FRET). Effects of structuralsampling were found to be small, illustrated by a small average standarddeviation for the Q(y) state coupling elements(FRET/TDC: 0.97/0.94 cm(-1)). Due to the higher flexibilityof the B-x state, the corresponding deviationsare larger (FRET/TDC between Chl-Chl pairs: 17.58/22.67 cm(-1), between Crt-Chl pairs: 62.58/31.63 cm(-1)). In summary, it was found for the Q band that the coupling between Chls varies only slightly dependingon FRET or TDC, resulting in a minute effect on EET acceptor preference.In contrast, the coupling in the B band spectralregion is found to be more affected. Here, the S-2 (1B(u) ) states of the spatially challenging Crtsmay act as acceptors in addition to the B states of the Chls. Dependingon FRET or TDC, several Chls show different Chl-to-Crt couplings.Interestingly, the EET between Chls or Crts in the B band is found to often outcompete the corresponding decay processes.The individual efficiencies for B band EET to Crtsvary however strongly with the chosen coupling scheme (e.g., up to0.29/0.99 FRET/TDC efficiency for the Chl a604/neoxanthinpair). Thus, the choice of the coupling scheme must involve a considerationof the state of interest.