Corrin

  A corrin is a macrocycle 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. Also, the pyrrole rings in corrin have fully saturated edge-carbons, that is, two hydrogens attached compared to only one in porphyrins. 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, and partly saturated pyrroles, they do not have a full conjugated character around the entire ring. Instead, the ring has a kind of "3/4" conjugation.

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The related corroles are fully aromatic.