Monte Carlo simulation of energy transfer rates: application to downward energy transfer within the 825 nm absorption band of the FMO complex of Prosthechochloris aestuarii

Splitting of the individual subunit absorptions of the FMO complex of Prosthechochloris aestuarii due to disorder is determined by Monte Carlo simulation. The simulations accurately reproduce the 825 nm absorption band and indicate an upper limit for the inter-subunit coupling of 10 cm−1. From the simulations, rates for downward energy transfer between subunits can be determined using the spectral density determined from hole burning (J. Phys. Chem. B 104 (2000) 9564). On average the splitting between adjacent energy levels is ∼25 cm−1 and the energy transfer rate for relaxation between such levels is equal. Relaxation from the top to lowest level is significantly slower. This is in contrast with the experimental finding that relaxation from the top to bottom levels occurs at about twice the rate for the other two relaxations. Further, the rates determined do not agree with the observed variation of zero-phonon hole width and photon echo dephasing times with frequency. The possible involvement of Herzberg–Teller-like dark modes with a peak near 50 cm−1 is discussed.