Energy transfer in the CP43 and CP47 complexes of photosystem II

Abstract

The pigment-protein complexes CP43 and CP47 are part of the photosynthetic apparatus of green plants and cyanobacteria. In the core of photosystem II (PSII), their main function is to funnel excitation energy from antennas outside the core towards the photochemical reaction center (RC), where a charge separation takes place. A crystal structure at 4Å resolution of the core (RC-CP43-CP47) was recently published [1], showing the structural organization of the bound chlorophyll a (Chl) pigments. Here, we present difference absorption spectra excited and probed in the region of the lowest Chl electronic state in isolated CP43 and CP47 proteins, measured at 77 K and with ~120 fs time resolution. Excitation energy transfer in both pigment-protein complexes appeared to be very similar, with main lifetimes of 0.2-0.4 and 2-3 ps. The observed bands in the transient spectra could be related to (excitonic) states calculated from the structure. The observed rates agree well with preliminary calculations of transfer times between weakly coupled Chls. From our experiments and simulations, we propose a model for the flow of excitation energy towards the reaction center. Most transfer from antenna to RC proceeds via Chls on the stromal side of the membrane towards the reaction center pheophytins. Because the dynamics observed in CP43 and CP47 is fast, antenna to RC transfer most likely contributes significantly to the well-known slow trapping kinetics in the PSII core.