Picosecond and nanosecond geminate recombination of myoglobin with CO, O sub(2), NO, and isocyanides

With use of an amplified colliding-pulse mode-locked ring dye laser, the recombination kinetics of small ligands binding to sperm whale and elephant myoglobins have been studied on the picosecond time scale. Nanosecond experiments were performed as well. On the picosecond time scale, all ligands investigated except CO show a substantial amount of rebinding. Picosecond rebinding of the isocyanides to iron is surprising as they are isoelectronic with CO and might be expected to show the same or similar recombination kinetics; the difference may be due to the greater Lewis basicity of the isocyanides. The authors propose that the geminate pair exists in two very different configurations: the contact pair, or fast-reacting form, in which the ligand remains within a few angstroms of the iron atom and the separated pair, or slow-reacting form, in which the ligand wanders throughout the so called "pocket".