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Synthetic Iron Porphyrins for Probing the Differences in the Electronic Structures of Heme a3, Heme d, and Heme d1.

A variety of heme derivatives are pervasive in nature, having different architectures that are complementary to their function. Herein, we report the synthesis of a series of iron porphyrinoids, which bear electron-withdrawing groups and/or are saturated at the β-pyrrolic position, mimicking the structural variation of naturally occurring hemes. The effects of the aforementioned factors were systematically studied using a combination of electrochemistry, spectroscopy, and theoretical calculations with the carbon monoxide (CO) and nitric oxide (NO) adducts of these iron porphyinoids. The reduction potentials of iron porphyrinoids vary over several hundreds of millivolts, and the X-O (X = C, N) vibrations of the adducts vary over 10-15 cm-1. Density functional theory calculations indicate that the presence of electron-withdrawing groups and saturation of the pyrrole ring lowers the π*-acceptor orbital energies of the macrocycle, which, in turn, attenuates the bonding of iron to CO and NO. A hypothesis has been presented as to why cytochrome c containing nitrite reductases and cytochrome cd1 containing nitrite reductases follow different mechanistic pathways of nitrite reduction. This study also helps to rationalize the choice of heme a3 and not the most abundant heme b cofactor in cytochrome c oxidase.