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, for example, phenophorbin.