 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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