Electrochemistry of myoglobins reconstituted with azahemes and mesohemes

The iron complexes of alpha-azamesoporphyrin XIII and beta, delta-diazameso porphyrin III were incorporated into apomyoglobin to obtain mono- and di-az aheme reconstituted myoglobins and their electrochemical behavior was inves tigated. To understand precisely the effect of introduced nitrogen atom(s) to the porphyrin backbone on electrochemical behavior of reconstituted myog lobins, iron complexes of mesoporphyrin XIII and mesoporphyrin III as well as myoglobins reconstituted with these mesohemes were also prepared and the ir electrochemical properties were examined. Significant positive shifts in redox potential of azahemes compared with the corresponding mesoporphyrins were observed. However, the redox potentials of azaheme reconstituted myog lobins showed no or much less positive shift compared with those of mesohem e reconstituted myoglobins, suggesting the heme environment including axial ligands of the redox center would be more important than the structure of porphyrin ring itself for determining the redox potential of reconstituted myoglobins. Azaheme reconstituted myoglobins showed larger affinity with cy anide ion than the corresponding mesoheme reconstituted myoglobins. Electro n transfer kinetics measured both on an electrode and by chemical reduction with dithionite correlated well to each other and depended on the structur es of hemes. Azaheme reconstituted myoglobins showed larger electron transf er than the corresponding mesoheme reconstituted myoglobins, which would be explained in terms of the change in spin-state of the heme iron for azahem e reconstituted myoglobins. Also, electron transfer reaction at an In2O3 el ectrode showed the largest kinetics at pH 6.5 for myoglobins having aquomet type redox centers. (C) 1998 Elsevier Science S.A. All rights reserved.