The first important difference to be mentioned is the spatial gap between the co-factors composing the inner pair. If we define this gap through the distance R ⊥ between parallel planes going through the aromatic macrocycles of the co-factors (for a more robust definition of these planes, see Sec. 4), then the largest separation between inner (B)Chl molecules can be found in the crystal structure of PSI and is equal to about 3.9 Å.