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Relative to the standard provided by an unconstrained five-coordinate imidazoleiron(II) porphyrin, the axial separation (Nε ...Pμ ) in deoxyhemoglobin of the complexed histidine-nitrogen (Nε ) atom from the mean plane (Pμ ) of the protoporphyrin is stretched by [unk] 0.30 Å. Retention of the same globin framework (quaternary structure) in deoxycobaltohemoglobin implies an axial connection with the cobalt protoporphyrin that carries enhanced tension, but a somewhat smaller value of Nε ...Pμ than the 2.90 Å observed in deoxyhemoglobin. Structural constraints imposed on and by the globin in support of tension in the axial linkage are conducive to doming of the entire 24-atom porphine skeleton of the protoporphyrin toward the metal (M) atom. Furthermore, the presence of an odd electron in the 3dz 2 orbital of the metal atom is responsible for an easily stretched M-Nε bond. It then appears that moderate doming of the porphinato core in combination with modest stretch in the Co-Nε bond can readily lead to an Nε ...Pμ distance approaching 2.90 Å in deoxycobaltohemoglobin and, consequently, to compatibility with Perutz's postulated trigger for the initiation of cooperative interaction of the subunits during the reversible oxygenation of hemoglobin.

Stereochemical Trigger for Initiating Cooperative Interaction of the Subunits During the Oxygenation of Cobaltohemoglobin