Oxygen Binding to a Model for the Active Site in Cobalt-Substituted Hemoglobin

Cobalt(II) porphyrin complexes, which are models for the active site in cobalt-substituted hemoglobin, CoHb, have been synthesized and characterized by NMR and visible spectroscopy. The model compounds are based on the tetratolylporphyrin ring system and have covalently attached chains which terminate in a pyridyl ring. The cobalt(II) complexes of these “looping-over porphyrins” react reversibly with dioxygen at low temperatures. The oxygen affinities of these compounds have been measured and the results compared with those obtained with other porphyrins and the protein. Our results indicate that there is negligible enhancement of the oxygen affinity of a cobalt(II) porphyrin when an axial base is covalently attached.