Photosynthetic proteins chlorophyll dimer

Fig. 4. High-field (left side) and X-band (right side) EPR spectra on two transient radicals created within the photocycle in photosynthetic proteins, a) P ) chlorophyll dimer cation radical of deuterated cyanobacteria, measured at 140 and at 9 GHz. b) Q semiquinone radical of bacterial reaction centres of Rhodobacter sphaeroides R26 measured at 95 and at 9 GHz,...

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

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