Comparison of the magnetic properties of deoxy- and photodissociated myoglobin.

The magnetic susceptibility of photodissociated carbon monoxy myoglobin has been measured over the temperature range from 1.7 to 25 K at 10 and 50 kG with a superconducting susceptometer. The spin and the crystal field parameters of the iron ion were extracted by a spin Hamiltonian approach. Under equivalent conditions the magnetic susceptibility of deoxy myoglobin was measured. In both experiments the CO-bound protein was used as a diamagnetic reference. Above about 5 K the metastable photolysed state and the equilibrium deoxy form of myoglobin are magnetically indistinguishable and can be fitted with S = 2 and g = 2. The transition from spin 0 to spin 2 and the conformational changes known to accompany the electronic change thus also occur after photolysis at low temperature. At temperatures below 5 K, differences become apparent, indicating a somewhat smaller zero-field splitting in the photoproduct as compared to the ligand-free state at equilibrium. In qualitative agreement with observations made by other techniques, the data imply that even at 1.7 K substantial structural relaxation occurs in the heme region of myoglobin after photodissociation. The results are important for the interpretation of the ligand binding kinetics after flash photolysis at low temperature and contribute to the understanding of the relationship between electronic structure and function in heme proteins.