Chemistry of porphyrins in fossil plants and animals

Porphyrins are macrocyclic tetrapyrrole derivatives that are widely distributed in nature. They are often complexed with a metal ion located in the center of the ring system and may be modified by various substituents including additional rings, or by ring opening, which leads to a plethora of different functions. Due to their extended conjugated aromatic ring system, porphyrins absorb light in the visible range and therefore show characteristic colors. Well-known natural porphyrins include the red-colored heme present in hemoglobin, which is responsible for blood oxygen transport, and the chlorophylls in some bacteria and in plants which are utilized for photosynthesis. Porphyrins are mostly lipophilic pigments that display relatively high chemical stability. Therefore, they can even survive hundreds of millions of years. The present review article provides an overview of natural porphyrins, their chemical structures, and properties. A special focus is put on porphyrins discovered in the fossil record. Examples will be highlighted, and information on their chemical analysis will be provided. We anticipate that the development of novel analytical methods with increased sensitivity will prompt new discoveries of porphyrins in fossils.