A corrin is a polyaromatic ring related to the porphyrin ring in hemoglobin, consisting of 4 pyrrole subunits, joined on opposite sides by a C-CH3 methylene link, on one side by a C-H methylene link, and with the two of the pyrroles joined directly. Compared to a porphyrin, it is missing a bridging methylene group between a pair of pyrroles. This ring is central to the cobalt containing vitamin, vitamin B12, or cobalamin. Corrins have properties related to porphyrins, but:
- They are more flexible than porphyrins.
- They are not as flat as porphyrins.
- Because of the loss of the bridging methylene, they do not have a full aromatic character around the entire ring. Instead, the ring has a kind of "3/4" aromaticity.