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The Role of Porphyrin Peripheral Substituents in Determining the Reactivities of Ferrous Nitrosyl Species

Abstract

Ferrous nitrosyl {FeNO}7 species are intermediates common to the catalytic cycles of Cd1NiR and CcNiR, two heme-based nitrite reductases (NiR), and its reactivity vary dramatically in these enzymes. The former reduces NO2- to NO in the denitrification pathway while the latter reduces NO2- to NH4+ in dissimilatory nitrite reduction. With very similar electron transfer partners and heme based active sites, the origin of this difference in reactivity has remained unexplained. Differences in the structure of the heme d1 (Cd1NiR), which bears electron-withdrawing groups and has saturated pyrroles, relative to heme c (CcNiR) is often invoked to explain these reactivities. A series of iron porphyrinoids, designed to model the electron-withdrawing peripheral substitution as well as the saturation present in heme d1 in Cd1NiR, and their NO adducts are synthesized and their properties are investigated. The data clearly show that the presence of electron-withdrawing groups (EWG) and saturated pyrroles together in a synthetic porphyrinoid (FeDEsC) weakens the Fe-NO bond in {FeNO}7 adducts along with decreasing the bond dissociation free energies (BDFENH) of the {FeHNO}8 species. The EWG raises the E0 of {FeNO}7/8 process, making the electron transfer (ET) facile, but decreases the pKa of {FeNO}8 species, making protonation (PT) difficult, while saturation has the opposite effect. The weakening of the Fe-NO bonding biases the {FeNO}7 species of FeDEsC for NO dissociation, as in Cd1NiR, which is otherwise set-up for a proton coupled electron transfer (PCET) to form a {FeHNO}8 species eventually leading to its further reduction to NH4+.

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Article information


Submitted
19 Mar 2020
Accepted
07 May 2020
First published
07 May 2020

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2020, Accepted Manuscript
Article type
Edge Article

The Role of Porphyrin Peripheral Substituents in Determining the Reactivities of Ferrous Nitrosyl Species

S. Amanullah and A. Dey, Chem. Sci., 2020, Accepted Manuscript , DOI: 10.1039/D0SC01625J

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