Protoporphyrin IX is an organic compound, classified as a porphyrin, that plays an important role in living organisms as a precursor to other critical compounds like heme (hemoglobin) and chlorophyll. It is a deeply colored solid that is not soluble in water. The name is often abbreviated as PPIX. Protoporphyrin IX contains a porphine core, a tetrapyrrole macrocycle with a marked aromatic character. Protoporphyrin IX is essentially planar, except for the N-H bonds that are bent out of the plane of the rings, in opposite (trans) directions. Nomenclature The general term protoporphyrin refers to porphine derivatives that have the outer hydrogen atoms in the four pyrrole rings replaced by other functional groups. The prefix proto often means 'first' in science nomenclature (such as carbon protoxide), hence Hans Fischer is thought to have coined the name protoporphyrin as the first class of porphyrins. Fischer described iron-deprived heme becoming the "proto-" porphyrin, particularly in reference to Hugo Kammerer's porphyrin. In modern times, 'proto-' specifies a porphyrin species bearing methyl, vinyl, and carboxyethyl/propionate side groups. Fischer also generated the Roman numeral naming system which includes 15 protoporphyrin analogs, the naming system is not systematic however. An alternative name for heme is iron protoporphyrin IX (iron PPIX). PPIX contains four methyl groups (M), two vinyl groups (V), and two propionic acid groups (P). The suffix "IX" indicates that these chains occur in the circular order MV-MV-MP-PM around the outer cycle at the following respective positions: c2,c3-c7,c8-c12,c13-c17,c18. The methine bridges of PPIX are named alpha (c5), beta (c10), gamma (c15), and delta (c20). In the context of heme, metabolic biotransformation by heme oxygenase results in the selective opening of the alpha-methine bridge to form biliverdin/bilirubin. In this case, the resulting bilin carries the suffix IXα which indicates the parent molecule was protoporphyrin IX cleaved at the alpha position. Non-enzymatic oxidation may result in the ring opening at other bridge positions. The use of Greek letters in this context originates from the pioneering work of Georg Barkan in 1932. Properties When UV light is shone on the compound, it fluoresces with a bright red color. It Is also the component in egg shells that give them their characteristic brown color. Natural occurrence The compound is encountered in nature in the form of complexes where the two inner hydrogen atoms are replaced by a divalent metal cation. When complexed with an iron(II) (ferrous) cation , the molecule