Energy transfer and primary charge separation upon selective femtosecond excitation at 810 nm in the reaction center complex from Heliobacterium modesticaldum

Abstract

The kinetics of excitation energy transfer and charge separation process in the photosynthetic reaction center of Heliobacterium modesticaldum were investigated with femtosecond transient absorption spectroscopy at room temperature. This is the first report using the highly purified reaction center isolated from heliobacteria. The bacteriochlorophyll (BChl) g pigment pool was selectively excited at its Qy band of 810 nm. The transient difference spectra were structured enough to be assigned to major pigment pools (Bmd787, Bmd800, and Bmd812), despite the broad absorption spectrum of the Qy region at room temperature. The spectral equilibration among three different spectral pools completed in less than 0.2 ps, indicating a very rapid energy transfer process because of the strong excitonic interaction between BChl gs. The decay-associated difference spectra (DADS) obtained by a global fit analysis from 450 nm to 900 nm revealed a very rapid energy transfer process and a subsequent trapping, with a time constant of 20 ps, to form the initial charge separation state of P800+A0–. The 20 ps component showed the bleaching Qy band at 816 nm and would derive from the excited red-BChl gs or might be due to the exciton coupling between P800 and a monomeric accessory BChl g. Furthermore, the non-decaying component spectrum exhibited a slightly broader peak centered at 792 nm, ascribed to the overlapping of two bleaching bands derived from both the P800 itself and an accessory BChl g excitonically coupled with the primary acceptor 81−OH-Chl aF. This was supported by the appearance of another bleaching band at 573 nm, which was detected in the Qx region of BChl gs.