A corrin is a macrocycle relate to the porphyrin ring in hemoglobin, consisting of 4 pyrrole subunits, joined on opposite sides by a C-CH3methylene 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: