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Porphyrins are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent porphyrin is porphin, and substituted porphines are called porphyrins. The porphyrin macrocycle has 26 (delocalized) pi electrons in total, therefore by Hückel's rule it is aromatic, possessing 4n+2 π-electrons (n=4, for the shortest cyclic path). Thus porphyrin macrocycles are highly conjugated systems and consequently they typically have very intense absorption bands in the visible region and may be deeply colored; the name "porphyrin" comes from the Greek word πορφύρα (''porphyra''), meaning ''purple''. Many porphyrins are naturally occurring; one of the best-known porphyrins is heme, the pigment in red blood cells, a cofactor of the protein hemoglobin. (The specific porphyrin in heme B (see Figure) is called protoporphyrin IX and has 4 methyl, two vinyl, and two propionic acid substituents at the indicated positions.) ==Complexes of porphyrins and related molecules== Porphyrins are the conjugate acids of ligands that bind metals to form complexes. The metal ion usually has a charge of 2+ or 3+. A schematic equation for these syntheses is shown: :H2porphyrin + ()2+ → M(porphyrinate)Ln−4 + 4 L + 2 H+ where M = metal ion and L = a ligand A porphyrin without a metal-ion in its cavity is a ''free base''. Some iron-containing porphyrins are called hemes. Heme-containing proteins, or ''hemoproteins'', are found extensively in nature. Hemoglobin and myoglobin are two O2-binding proteins that contain iron porphyrins. Various cytochromes are also hemoproteins. Several other heterocycles are related to porphyrins. These include corrins, chlorins, bacteriochlorophylls, and corphins. Chlorins (2,3-dihydroporphyrin) are more reduced, contain more hydrogen than porphyrins, and feature a pyrroline subunit. This structure occurs in a chlorophyll molecule. Replacement of two of the four pyrrolic subunits with pyrrolinic subunits results in either a bacteriochlorin (as found in some photosynthetic bacteria) or an isobacteriochlorin, depending on the relative positions of the reduced rings. Some porphyrin derivatives follow Hückel's rule, but most do not. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Porphyrin」の詳細全文を読む スポンサード リンク
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