The RC contains six Chl a molecules, of which four are believed to participate in the electron-transfer processes: i) the inner P D1 and P D2 co-factors composing the inner pair structurally similar to the SP in the BRC (depicted in green in Fig. As seen from Fig. , PSI (just like PSII) also contains two protein sub-units, which are nearly C 2 -symmetric. Its inner pair (shown in green) is formed by one Chl a molecule and a C-13 2 -epimer of Chl a (often referred to as Chl a ), which are denoted as P B and P A , respectively. It is well-known that P A forms several hydrogen bonds with amino acids from the surrounding protein environment, while P B does not . The two accessory pigments are Chl a molecules and called A -1A and A -1B (blue). Similar to the case of PSII, the magnesium atoms of the inner co-factors P A and P B are coordinated by His molecules, whereas the magnesium atoms of accessory pigments A -1A and A -1B are coordinated by water molecules. Instead of Phe a molecules, the RC of PSI contains two Chl a co-factors, which are referred to as A 0A and A 0B (red) and feature the central magnesium atoms being coordinated by sulfur atoms of methionine residues. In addition to the above-mentioned pigments, this RC also includes two phylloquinone co-factors A 1A and A 1B (violet) and an [4Fe-4S] cluster F x (brown).