pyrrole nucleus

pyr·role nu·cle·us

of porphyrins, a cyclic tetrapyrrole; four pyrrole groups joined into an anular structure by way of -CH= (methylidyne) bridges between the α (2) position of one pyrrole and the α' (5) position of another pyrrole, the fourth pyrrole being joined to the first.
See also: porphin, porphyrin.
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