A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate Complex—The Product of the Reaction of Nitrogen Monoxide (·NO_(g)) with a Ferric-Superoxide Species

Abstract

Peroxynitrite (−OON═O, PN) is a reactive nitrogen species (RNS) which can effect deleterious nitrative or oxidative (bio)chemistry. It may derive from reaction of superoxide anion (O2•−) with nitric oxide (·NO) and has been suggested to form an as-yet unobserved bound heme-iron-PN intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD) enzymes, which facilitate a ·NO homeostatic process, i.e., its oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin porphyrinate-FeIII complex [(PIm)-FeIII(−OON═O)] (3) (PIm; a porphyrin moiety with a covalently tethered imidazole axial “base” donor ligand) has been identified and characterized by various spectroscopies (UV–vis, NMR, EPR, XAS, resonance Raman) and DFT calculations, following its formation at −80 °C by addition of ·NO(g) to the heme-superoxo species, [(PIm)FeIII(O2•−)] (2). DFT calculations confirm that 3 is a six-coordinate low-spin species with the PN ligand coordinated to iron via its terminal peroxidic anionic O atom with the overall geometry being in a cis-configuration. Complex 3 thermally transforms to its isomeric low-spin nitrato form [(PIm)FeIII(NO3−)] (4a). While previous (bio)chemical studies show that phenolic substrates undergo nitration in the presence of PN or PN-metal complexes, in the present system, addition of 2,4-di-tert-butylphenol (2,4DTBP) to complex 3 does not lead to nitrated phenol; the nitrate complex 4a still forms. DFT calculations reveal that the phenolic H atom approaches the terminal PN O atom (farthest from the metal center and ring core), effecting O–O cleavage, giving nitrogen dioxide (·NO2) plus a ferryl compound [(PIm)FeIV═O] (7); this rebounds to give [(PIm)FeIII(NO3−)] (4a).The generation and characterization of the long sought after ferriheme peroxynitrite complex has been accomplished.

New transition metal complexes of 9,10‐phenanthrenequinone p‐toluyl hydrazone Schiff base: Synthesis, spectroscopy, DNA and HSA interactions, antimicrobial, DFT and docking studies

“…Full geometry optimization of ligand (HL), its copper and nickel complexes have been performed without imposing any constrain using Becke's three parameter hybrid exchange functional (B3) and the Lee‐Yang‐Parr correlation functional (LYP) (B3LYP) along with 6‐31G*(d) basis set. Then, to support the stability of compounds, we performed vibrational frequency calculations at B3LYP/6‐31G*(d) level . No imaginary frequency of the optimized geometries of ligand, Cu (II) and Ni (II) complexes presented that the optimized geometry exhibits stable structures in the potential energy surfaces (local minima) (Figure ).…”

Direct Resonance Raman Characterization of a Peroxynitrito Copper Complex Generated from O2 and NO and Mechanistic Insights into Metal‐Mediated Peroxynitrite Decomposition

“…UV/Vis quantification puts the yield of 2 at 95 AE 4%( Figure S15), and the presence of nitrite was independently quantified by aGriess assay to be 75 AE 2% ( Figure S16 and Table S1). [36,40,44,46,56] To confirm the generation of these radical species,a ddition of > 500 equiv excess of 2,4-di-tert-butylphenol (DTBP) was shown to result in the nearly quantitative formation of both 2 and the coupled bisphenol product, 3,3',5,5'-tetra-tert-butyl-[1,1'-biphenyl]-2,2'-diol ( Figures S18 and S19). This result rules against higher-order copper adducts participating in the rate-limiting step of peroxynitrite decay as suggested in the aqueous H + and CO 2 mediated decay pathways.…”

“…We postulate that in the absence of substrate,t he putative cupryl intermediate goes on to hydroxylate the solvent through aradical rebound pathway to give LCu I and R À OH (R = solvent) before LCu I reacts with NO 2 C to yield LCu II À(NO 2 À )a st he final product (Scheme S1). [36,40,44,46,56] To confirm the generation of these radical species,a ddition of > 500 equiv excess of 2,4-di-tert-butylphenol (DTBP) was shown to result in the nearly quantitative formation of both 2 and the coupled bisphenol product, 3,3',5,5'-tetra-tert-butyl-[1,1'-biphenyl]-2,2'-diol ( Figures S18 and S19). Thec oupling of phenoxyl species to give the observed diol can be rationalized by successive hydrogen atom abstractions by LCu II ÀOC and NO 2 C (a strong oxidant [57,58] )t og ive LCu II À OH and HNO 2 before ligand exchange to give LCu II À NO 2 À and H 2 O(Scheme 3).…”

“…[27,28] Metal-coordinated peroxynitrites are of interest because of the intimate role that metals play in the regulation and processing of both NOC and O 2 C À , [3,[29][30][31][32] which may implicate metals as important factors in the physiological formation and transformation of peroxynitrite (Scheme 1). [33][34][35][36][37][38][39][40][41][42] Examples of stable peroxynitrite complexes are scarce, [43,44] and those containing vibrational (i.e., IR spectroscopic) information about the peroxynitrite ligand even more so. [33][34][35][36][37][38][39][40][41][42] Examples of stable peroxynitrite complexes are scarce, [43,44] and those containing vibrational (i.e., IR spectroscopic) information about the peroxynitrite ligand even more so.…”

“…While progress has been made to implicate metal ions in the decay of peroxynitrite,adiscrete peroxynitrite coordination complex has rarely been observed. [33][34][35][36][37][38][39][40][41][42] Examples of stable peroxynitrite complexes are scarce, [43,44] and those containing vibrational (i.e., IR spectroscopic) information about the peroxynitrite ligand even more so. [45][46][47][48] In ap revious report, we have described the formation of ac upric peroxynitrite complex from the addition of NOC to [Cu II (TMG 3 tren)(O 2 C À )] + (1)atÀ80 8 8Ctogenerate the cupric peroxynitrito complex [Cu II (TMG 3 tren)(k 2 -O,O'-OONO)] + (PN2;S cheme 2).…”