Influence of ring substituents on the binding of nitrosobenzene by ferrohemoglobin

The affinity of a substituted nitrosobenzene for ferrohemoglobin and the visible absorption spectrum of the resulting compound was influenced by the nature, position, and number of substituents on its benzene ring. Alkyl ring substituents inhibited the binding of nitrosobenzene to ferrohemoglobin, and binding was blocked by an ortho tertbutyl group or by a pair of ortho methyl groups. A single halogen atom increased binding affinity except that iodine decreased affinity, more at the ortho than at the para position. Binding occurred with a pair of ortho fluorine atoms but not with a pair of ortho chlorine or bromine atoms. These results favor a model of nitrosoarene ferrohemoglobin in which the iron of ferroheme is bonded to the nitrogen atom of the nitroso group since a bond to the oxygen atom would not be hindered by ortho substituents. The presence of a carboxylate substituent resulted in prevention of binding, which was reversed by esterification of the group. Large neutral para substituents, which cannot directly affect formation of the Fe-NO bond, inhibited binding, although not to the same degree as ortho substituents. It thus appears that the affinity of a substituted nitrosobenzene for ferrohemoglobin depends on interactions of the nonbonding part of the ligand molecule with the heme crevice as well as on the ability of its nitroso group to form a bond with the iron of heme. Nitrosoarenes also exhibited in varying degrees the property of removing an electron from ferrohemoglobin to form ferrihemoglobin.