Figure 1.

Stereochemical interpretation of the side path scheme for geminate recombination (from Gemini, the sign of twins—referring to rebinding of the photodissociated molecule before diffusional escape) and bimolecular binding of O₂from solvent.Upper panel, photodissociated ligands first move into the empty space toward the back of the distal pocket (state B). Some of the molecules move further into the protein toward the Xe4 and perhaps Xe1-binding sites (states C). O₂ moves back from these more remote positions into state B and either rebinds to form MbO₂ (state A) or escapes when the distal His64 gate is open. Lower panel, this reaction scheme is proposed, where k 1 = k bond, k 2 = k escape, and k 3 and k 4 equal the rates of movement into the secondary sites and back to the primary site or state B (9).