Nanosecond Flash Photolysis Study of Carbon monoxide Binding to the β Chain of Hemoglobin Zürich [β 63(E7)His→ Arg]

Binding of carbon monoxide to β chains of hemoglobin Zürich has been studied by flash photolysis over the time range of nanoseconds to seconds at temperatures from 20 to 300 K. From 20 to 200 K a single rebinding process (process I) is seen, characterized by a distribution of barrier heights with a peak enthalpy of 2.3 kJ/mol. Above 200 K some ligands escape from the pocket into the matrix, and above 260 K recombination from the solvent sets in. Process I is visible up to 300 K, but above 200 K its rate remains essentially constant at about 4 × 108s-1. Above about 250 K, process I is exponential in time, indicating rapid conformational relaxation. The results are discussed within the framework of a sequential model for ligand binding.