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Porphyrins (pronounce) are tetrapyrroles. They consist of four weakly aromatic pyrrole (pronounce) rings joined by methene bridges.

  • Porphyrin rings are numbered with Roman numerals I through IV, starting at the top and proceeding clockwise.
  • The methene bridges are lettered with Greek letters, alpha through delta, again proceeding clockwise.
  • Positions at which substituents could be attached are numbered 1-8, starting with the I ring, and proceeding clockwise.
  • The structure is often represented in a cross-shaped shorthand form. The common substituents are often abbreviated as follows:
    • A = acetic acid (or CM = carboxymethyl)
    • P = propionic acid (or CE = carboxyethyl)
    • M = methyl
    • V = vinyl

Here is the of porphin, the parent porphyrin, showing these features

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