Single-Crystal Electron Paramagnetic Resonance Studies of Photolyzed Oxy- and Nitric Oxide-Cobalt Myoglobins

Low-temperature photodissociation of oxygen from oxy-cobalt myoglobin was studied by single-crystal electron paramagnetic resonance (EPR) spectroscopy at 5 K. The photolyzed oxy-cobalt myoglobin exhibited an EPR spectrum consisting of two nonequivalent sets (species I and II) of the principal values and eigenvectors of the g tensors: g₁^I = 3.55, g₂^I = 3.47, and g₃^I = 2.26 for species I, and g₁^II = 2.04, g₂^II = 1.93, and g₃^II = 1.86 for species II, which resembled neither the deoxy nor the oxy form. Possible models of the photo-dissociated state of oxy-cobalt myoglobin are proposed by comparison with cobalt porphyrin complexes. The photolyzed product of nitric oxide-cobalt myoglobin exhibited new EPR signals at g = 4.3 and a very broad signal at around g = 2. The principal g values have been determined from the single-crystal EPR measurements: g₁ = 4.39, g₂ = 4.27, and g₃ = 4.00. Analysis of another EPR signal around g = 2 was difficult due to its broadness. Magnetic interactions were observed. An isotropic EPR signal at g = 4.3 suggested a weakly spin-coupled system between cobaltous spin (S = ½ or ½) and nitric oxide spin (S = ½).