Structural interpretation of low-temperature heme-ligand recombination rates in myoglobin

The nonexponential recombination of photodissociated heme-CO and heme-O2 in myoglobin, which is geminate at T less than 180 K, is interpreted as being due to a narrow, random distribution of ligand transfer distances in the heme pocket. This permits evaluation of the most probable recombination rate which is shown to be consistent with ligand tunneling.