The primary electron donor of photosystem II of the cyanobacterium Acaryochloris marina is a chlorophyll d and the water oxidation is driven by a chlorophyll a/chlorophyll d heterodimer.

We present a theoretical analysis of the flash-induced absorbance difference spectrum assigned to the formation of the secondary radical pair P(+)Q(A)(-) in photosystem II of the chlorophyll d-containing cyanobacterium Acaryochloris marina. An exciton Hamiltonian determined previously for chlorophyll a-containing photosystem II complexes is modified to take into account the occupancy of certain binding sites by chlorophyll d instead of chlorophyll a. Different assignments of the reaction center pigments to chlorophyll a or d from the literature are investigated in the calculation of the absorbance difference spectrum. A quantitative explanation of the experimental data requires one chlorophyll a molecule per reaction center, located at the site of P(D1). The remaining sites are occupied by chlorophyll d and pheophytin a. By far, the lowest site energy is found for the accessory chlorophyll of the D1 branch, Chl(D1), which represents the sink of excitation energy and therefore the primary electron donor. The cationic state is stabilized at the chlorophyll a, which drives the oxidation of water.