EPR studies on the photoinduced intermediates of NO complexes in recombinant ferric-Mb trapped at low temperatures

The nitrosyl complex of ferric myoglobin is EPR-silent. Upon photolysis at low temperatures, the photoinduced intermediates trapped in the distal heme cavity exhibit new EPR spectra due to the interaction between the photodis sociated NO (S=1/2) and the ferric high spin heme (S=5/2). In order to eluc idate the effect of distal E7 (His64) and E11 (Val68) mutations upon the el ectronic structure of the metal center, its immediate environment, and its interaction with the photodissociated NO, EPR spectra of the photoproducts of the NO complexes of recombinant ferric Mb mutants were measured at 5 K. EPR spectra of the photoproducts were closely related to the size and/or th e polarity of the distal pocket residues. The distal pocket of the E7 mutan ts seemed to be sterically crowded, even decreasing the side chain volume o r changing its hydrophobicity by replacing amino acid at position 64. We ha ve found that the mobility of the photodissociated NO molecule in the dista l heme pocket was strongly governed by the nature of the amino acid residue at E11 position. (C) 2000 Elsevier Science B.V. All rights reserved.