Fig. 2.

Structures of biological important variants of heme [17] and the dioxo-isobacteriochlorin heme d ₁. a Ligands at different residues on the heme macrocycle are donated R ₁, R ₂, and R ₃ and listed on the left. In archaea, the side chains at R₁ of heme a and o can have longer groups [171]. The pyrrole rings are labeled A–D in a clockwise direction, the bridging carbon atoms are numbered 5, 10, 15 and 20, and the nitrogens 21–24 according to IUPAC nomenclature. b In comparison, the dioxo-isobacteriochlorin heme d ₁