EPR studies on the photoproducts of manganese(II) protoporphyrin-IX substituted myoglobin nitrosyl complexes trapped at low temperature: effects of site-specific chemical modification of the distal histidine on ligand-binding structures.

Manganese(II) protoporphyrin-IX substituted myoglobin with site-specifically cyanated or N-tetrazolated distal histidine (His) was prepared and low-temperature photolysis of nitric oxide (NO) from their nitrosyl complexes was examined by electron paramagnetic resonance (EPR) spectroscopy in order to elucidate the steric crowding of the distal heme moiety. The photoproduct of NO complex of the tetrazolated Mn(II)Mb (tetrazole-Mn(II)Mb) exhibited widespread absorption in the magnetic field from zero to 0.4 T due to a spin-coupled interaction between the high-spin Mn(II) center (S = 5/2) and the photodissociated NO (S = 1/2) trapped adjacent to the metal center. This indicates that the NO complex of tetrazole-Mn(II)Mb has sterically restricted distal heme pocket. On the other hand, the photoproduct of NO complex of cyanated Mn(II)Mb (BrCN-Mn(II)Mb) exhibited only the broad g = 6 absorption due to the magnetic dipole-dipole interaction between the photodissociated NO and the high-spin Mn(II) center. A drastic conformational change in the heme-ligand moiety, in which the distal histidine side chain is pushed toward the outside of the heme pocket, leaving an open space in the distal heme pocket, can be suggested.