The parent hydrocarbon of chlorophyll; differs from porphin (porphyrin) in the presence of an isocyclic ring formed by the addition of a two-carbon group bridging the 13 and 15 positions of porphin (porphyrin) and by saturation of the 17–18 double bond (with realignment of conjugated double bonds). Addition of hydrocarbon side chains in specific locations yields phorbins characterized by prefixes; e.g., phenophorbin.
Medical dictionary. 2011.