A slightly smaller value of 3.7 Å can be found in case of PSII . The SP of the BRC features the smallest distance R ⊥ = 3.1 Å between the co-factors . Therefore, one might expect that the interaction energy between the co-factors is the largest for the It is also interesting to note that the BChl a macrocycles in the crystal structure of BRC are essentially planar, while a strong out-of-plane bent is present for both PSI and PSII inner pairs. These spatial arrangements of co-factors are largely governed by interactions with the surrounding protein pockets, which are often arranged in an asymmetrical way around the constituting inner pair molecules and interact differently with those. Thus, the inner co-factors of PSII interact strongly with the surrounding protein, whereas P A and P B are reported to be essentially undisturbed .