Impact of structural sampling, coupling scheme and state of interest on the energy transfer in CP29

Abstract

The Qy and Bx excitation energy transfer (EET) in the minor light harvesting complex CP29 (LHCII B4.1) antenna complex of Pisum sativum was characterized using a computational approach. We applied Förster theory (FRET) and the transition density cube (TDC) method estimating the Coulombic coupling, based on a combination of classical molecular dynamics and QM/MM calculations. Employing TDC instead of FRET mostly affects the EET between chlorophylls (Chls) and carotenoids (Crts), as expected due to the Crts being spatially more challenging for FRET. Only between Chls, effects are found to be small (about only 0.1 EET efficiency change when introducing TDC instead of FRET). Effects of structural sampling were found to be small, illustrated by a small average standard deviation for the Qy state coupling elements (FRET/TDC: 0.97/0.94 cm-1). Due to the higher flexibility of the Bx state, the corresponding deviations are 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 depending on FRET or TDC, resulting in a minute effect on EET acceptor preference. In contrast, the coupling in the B band spectral region is found to be more affected. Here, the S2 (1Bu) states of the spatially challenging Crts may act as acceptors in addition to the Chl B states. Depending on 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 Crts vary however strongly with the chosen coupling scheme (e.g., up to 0.29/0.99 FRET/TDC efficiency for the Chl a604/neoxanthin pair). Thus, the choice of coupling scheme must involve a consideration of the state of interest.

First-principles simulation of excitation energy transfer and transient absorption spectroscopy in the CP29 light-harvesting complex

“…The largest couplings (∼170 and 130 cm −1 ) are found for the a611-a612 and a603-a609 pairs, while the largest a/b coupling is a604-b606 (111 cm −1 ). These couplings are slightly larger compared to previous work, 36,63,64 although they are similar to the QM/MMPol couplings by Jurinovich et al 63 To characterize the exciton structure in the disordered ensemble, we show in Figure 2c the vertical exciton state pigment distribution functions (eq. 7).…”

Excitation Energy Transfer between Higher Excited States of Photosynthetic Pigments: 1. Carotenoids Intercept and Remove B Band Excitations

“…19,21−23 In a recent contribution, we have shown that for an ensemble of pigments, such as the LHC studied here, TDC and FRET will, on average, provide nearly identical results. 24 Hence, for a proof of concept, FRET suffices since more elaborately predicted rates should be generally on the same order of magnitude as the rates predicted by FRET. 18,24 FRET further has the distinct advantage of being easily comparable with experimental data.…”

“…24 Hence, for a proof of concept, FRET suffices since more elaborately predicted rates should be generally on the same order of magnitude as the rates predicted by FRET. 18,24 FRET further has the distinct advantage of being easily comparable with experimental data. Coherent transport for strongly coupled pigment pairs is also possible but will not be explored here; its occurrence would, however, only strengthen our arguments since it is an additional transport mechanism on a likely ultrafast timescale.…”

Excitation Energy Transfer between Higher Excited States of Photosynthetic Pigments: 2. Chlorophyll b is a B Band Excitation Trap

“…39 We have, however, recently presented evidence that higher level incoherent EET models perform equivalently to FRET when considering ensembles such as the antenna studied here. 40 Neglecting B band EET for Chls relies on the assumption that Chl B → Q IC outpaces all other processes. The presence of Crt absorption in the same spectral region poses an additional experimental obstacle.…”