Monte Carlo simulations of excitation and electron transfer in grana membranes

Abstract

Time-resolved fluorescence measurements on grana membranes with instrumental response function of 3ps reveal faster excitation dynamics (120ps) than those reported previously. A possible reason for the faster decay may be a relatively low amount of “extra” LHCII trimers per reaction center of Photosystem II. Monte Carlo modeling of excitation dynamics in C2S2M2 form of PSII–LHCII supercomplexes has been performed using a coarse grained model of this complex, constituting a large majority of proteins in grana membranes. The main factor responsible for the fast fluorescence decay reported in this work was the deep trap constituted by the primary charge separated state in the reaction center (950–1090cm−1). This value is critical for a good fit, whereas typical hopping times between antenna polypeptides (from ~4.5 to ~10.5ps) and reversible primary charge separation times (from ~4 to ~1.5ps, respectively) are less critical. Consequently, respective mean migration times of excitation from anywhere in the PSII–LHCII supercomplexes to reaction center range from ~30 to ~80ps. Thus 1/4–2/3 of the ~120-ps average excitation lifetime is necessary for the diffusion of excitation to reaction center, whereas the remaining time is due to the bottle-neck effect of the trap. Removal of 27% of the Lhcb6 apoprotein pool by mutagenesis of DEG5 gene caused the acceleration of the excitation decay from ~120 to ~100ps. This effect may be due to the detachment of LHCII-M trimers from PSII–LHCII supercomplexes, accompanied by deepening of the reaction center trap.