Editing Corrin (section)

==Coordination chemistry==
{{main|vitamin B12|l1=Vitamin B<sub>12</sub>}}
Upon deprotonation, the corrinoid ring is capable of binding [[cobalt]].  In vitamin B<sub>12</sub>, the resulting complex also features a benzimidazole-derived ligand, and the sixth site on the octahedron serves as the catalytic center.

The corrin ring resembles the [[porphyrin]] ring.<ref>{{cite journal |doi=10.1021/cr030720z|title=Chemistry and Enzymology of Vitamin B12 |year=2005 |last1=Brown |first1=Kenneth L. |journal=Chemical Reviews |volume=105 |issue=6 |pages=2075–2150 |pmid=15941210 }}</ref>  Both feature four [[pyrrole]]-like subunits organized into rings.  Corrins have a central 15-membered {{chem2|C11N4}} ring whereas porphryins have an interior 16-membered {{chem2|C12N4}} ring. All four nitrogen centers are linked by [[Conjugated system|conjugation]] structure, with alternating double and single bonds.  In contrast to [[porphyrin]]s, corrins lack one of the carbon groups that link the pyrrole-like units into a fully conjugated structure.  With a conjugated system that extends only 3/4 of the way around the ring, and does not include any of the outer edge carbons, corrins have a number of non-conjugated sp<sup>3</sup> carbons, making them more flexible than porphyrins and not as flat.   A third closely related biological structure, the [[chlorin]] ring system found in [[chlorophyll]], is intermediate between porphyrin and corrin, having 20 carbons like the porphyrins and a conjugated structure extending all the way around the central atom, but with only 6 of the 8 edge carbons participating.
 
[[Corrole]]s (octadehydrocorrins) are fully [[aromatic]] derivatives of corrins.