The stretching frequency of the coordination bond between the heme Fe atom and proximal histidine (His93) Nεatom (νFe-His), and the NMR shift of the His93NδH proton (His93NδH shift) of the deoxy form of a hemoprotein have been used to determine the electronic nature of the His93 imidazole highly relevant to regulation of heme Fe reactivity. We determined the νFe-Hisvalues and His93NδH shifts of the deoxy forms of myoglobins reconstituted with artificial heme cofactors possessing strongly electron-withdrawing trifluoromethyl (CF3) group(s) as peripheral side chain(s). The study revealed that the bond between the heme Fe and His93 becomes stronger with increasing number of CF3substitutions due to an increase in acidity of the His93NδH hydrogen bonded to the carbonyl O atom of Leu89. Thus, the present study demonstrated that the electronic nature of the His93 imidazole in deoxy Mb is affected by electronic perturbation induced by chemical modification of the heme cofactor.
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