O₂ and CO Reactions with Heme Proteins: Quantum Yields and Geminate Recombination on Picosecond Time Scales

Picosecond time-resolved absorption spectroscopy and low-temperature studies have been undertaken in order to understand the nature of the intrinsic quantum yields and geminate recombination of carbon monoxide and oxygen to hemoglobin and myoglobin. We find that the photoproduct yields at 40 ps and long times (minutes) after photolysis at 8 K are similar; however, the yield of oxygen photoproducts is 0.4 ± 0.1 while the yield of carbon monoxide photoproducts is 1.0 ± 0.1 for both myoglobin and hemoglobin. Measurements in the Soret, near-infrared, and far-IR are used to quantitate the photoproduct yields. These results call into question previous cryogenic kinetic studies of O₂ recombination. Significant subnanosecond geminate recombination is observed in oxyhemoglobin down to 150 K, while below 100 K this geminate recombination disappears. The lower photo product yields for oxyheme protein complexes can be attributed to both subnanosecond and subpicosecond recombination events which are ligand and protein dynamics dependent.