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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
J Am Chem Soc. Author manuscript; available in PMC 2010 August 19.
Published in final edited form as:
PMCID: PMC2747244

Heme/O2/•NO Nitric Oxide Dioxygenase (NOD) Reactivity: Phenolic Nitration via a Putative Heme-Peroxynitrite Intermediate


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An oxy-heme complex, the heme-superoxo species (tetrahydrofuran)(F8)FeIII-(O2·−) (2) (F8 = anortho-difluoro substituted tetraarylporphyrinate), reacts with nitrogen monoxide (·NO; nitric oxide) to produce a nitrato-iron(III) compound (F8)FeIII-(NO3) (3) (X-ray). The chemistry mimics the action of ·NO Dioxygenases (NODs), microbial and mammalian heme proteins which facilitate ·NO detoxification/homeostasis. A peroxynitrite intermediate complex is implicated; if 2,4-di-t-butylphenol is added prior to ·NO reaction with 2, o-nitration occurs giving 2,4-di-t-butyl-6-nitrophenol. The iron product is (F8)FeIII-(OH) (4). The results suggest that heme/O2/·NO chemistry may lead to peroxynitrite leakage and/or exogenous substrate oxidative/nitrative reactivity.

The reaction of nitric oxide (·NO; nitrogen monoxide) with oxygenated heme proteins is of considerable biological interest. Nitric oxide is generated in vivo by the oxidation of l-arginine to l-citrulline mediated by the enzyme Nitric Oxide Synthase (NOS).1 Nitric oxide itself plays an important role in a number of physiological processes that include as a signaling agent leading to smooth muscle vasodilation, platelet disaggregation, neurotransmission, and immune response to bacterial infection.2

Overproduction of ·NO can lead to toxicological processes that include DNA damage, protein nitration leading to cell death, and formation of peroxynitrite (OON=O) with the latter's further chemistry leading to highly reactive free radicals.3 It effects oxidation/nitration of biomolecules; most evident is the nitration of tyrosine.3cd,4 To maintain proper ·NO levels, microbial heme protein ·NO Dioxygenases (NODs) catalyze the reaction of O2 and ·NO to yield the biologically benign nitrate anion (NO3).5

[center dot]NO+O2+eNODNO3

Hemoglobin (Hb) and myoglobin (Mb) are also major targets/sinks of ·NO in mammals, and they exhibit NOD activity.6 High fidelity incorporation of both 18O-labeled oxygen atoms into the nitrate (NO3) product occurs when ·NO reacts with red blood cell oxyHb [Hb(FeIII-18O2·−)], sperm whale oxyMb [Mb(FeIII-18O2·−)] and E. coli oxyflavohemoglobin [flavoHb(FeIII-18O2·−)].7 The generally stated mechanism of ·NO dioxygenase (Scheme 1) involves direct reaction of the FeIII-(O2·–) oxy complex with ·NO, giving a peroxynitrite intermediate. Subsequent homolytic O-O bond cleavage produces an oxo-ferryl (FeIV=O) species and the free radical nitrogen dioxide (·NO2); the latter attacks the ferryl O-atom to produce an N–O bond giving nitrate.3d,8-12

There have been some reports on the detection of a peroxynitrite intermediate: (i) Under alkaline conditions, Herold and co-workers obtained UV-vis spectroscopic evidence for a high-spin FeIII intermediate for both the Hb and Mb systems.13 (ii) Olson and co-workers suggested EPR evidence for a high-spin FeIII species (g = 6) using rapid-freezing techniques.8 However, based on thermokinetic14 and theoretical10b arguments, the peroxynitrite-iron intermediate would be too short-lived to detect. This is consistent with a very recent report employing rapid-freeze quench resonance Raman spectroscopy using Mb. This shows that the high-spin FeIII species detected was not the proposed peroxynitrite intermediate, but rather an iron-bound nitrate complex formed prior to its decay to metMb.9

We report here the chemistry of a synthetic oxy-heme which exhibits NOD reactivity, where the intermediacy of a peroxynitrite species is implicated.15 As previously described,16 (F8)FeII (1) {F8 = tetrakis(2,6-difluoro-phenyl)porphyrinate(2-)} {λmax = 422, 542 nm} reacts reversibly with dioxygen to give a diamagnetic iron(III)-superoxo species (S)(F8)FeIII-(O2·−) (2) {λmax = 416, 535 nm}, which is stable in solution below −40 °C in coordinating solvents (S) such as tetrahydrofuran (THF), acetone, or propionitrile (Figure 1).16 Subsequent addition of 1 equiv ·NO (at −80 °C) produces the five-coordinate nitrato heme complex (F8)FeIII-(NO3) (3) {λmax = 393, 505, 572, 634 nm (Figures 1, ,2)}2)} in near quantitative yield;17, 18 its structure (X-ray) is shown in Figure 2. In these benchtop experiments, where 10-20 s are required to add ·NO(g) in THF by syringe and record a spectrum, 3 has already fully formed. No hint of an intermediate is observed, even if the chemistry is carried out in 2-methyl-THF at −120 °C.18

Figure 1
UV-vis spectra (THF solvent) of (F8)FeII (1) (Blue), (thf)(F8)FeIII-(O2·−) (2) (Red) generated from 1 + O2 (g) and (F8)FeIII-(NO3) (3) (Green) generated from 2 + ·NO (g)
Figure 2
Reaction of (thf)(F8)FeIII-(O2·−) (2) (ArF = 2,6-diflurophenyl) + ·NO(g) produces the nitrato complex (F8)FeIII-(NO3) (3) (X-ray).17

Thus, we sought chemical evidence which might suggest the formation of a peroxynitrite species. When ·NO(g) reaction with (thf)(F8)FeIII-(O2·−) (2) is followed by addition of the tyrosine mimic 2,4-di-t-butylphenol (DTBP), (F8)FeIII-(NO3) (3) is never-the-less formed and unreacted DTBP is recovered.18 However, we do observe effective nitration chemistry when DTBP (≥ 1 equiv) is added prior to addition of 1 equiv ·NO(g) to 2 (Scheme 2).18 Workup of the reaction solution18 reveals that the ferric hydroxo product (F8)FeIII-OH (4) forms (~ 85% yield) along with high yields (> 82%) of 2,4-di-t-butyl-6-nitrophenol (NO2DTBP).

The following control experiments indicate that a new heme-NOx intermediate forms and is able to effect a phenol nitration reaction faster than its own isomerization to the nitrate complex 3: (i) Use of excess ·NO(g) bubbled into solution had no effect on the products or their relative yields. (ii) Bubbling excess ·NO(g) into a solution containing 1 at −80 °C in the presence of DTBP and subsequent warming yielded less than 2% of the NO2DTBP and no other products besides the starting DTBP and the iron-nitrosyl (F8)FeII-(NO) (5); excess ·NO(g) is not solely responsible for significant DTBP nitration. (iii) Warming 2 from −80 °C to RT in the presence of DTBP yielded the oxidatively coupled phenol 3,3′,5,5′-tetra-t-butyl-(1,1′-biphenyl)-2,2′-diol (~ 10%), unreacted DTBP (~ 90%) and 4; ·NO(g) is needed for DTBP nitration, 4 is formed from thermal decomposition of 2.16a (iv) Warming a −80 °C solution of (F8)FeIII-(NO3) (3) in the presence of DTBP gives no reaction of any kind; 3 itself won't nitrate DTBP.19

As said, these studies implicate a heme-peroxynitrite intermediate (F8)FeIII-(OON=O) (2a), formed from (thf)(F8)FeIII-(O2·−) (2) (no excess O2 present) plus one equiv ·NO(g). Following the literature suggestions, we can hypothesize that 2a undergoes homolysis to give a ferryl + ·NO2 (2b) (caged?), which however can be captured by a phenol which is already present in solution (Scheme 2). The ferryl would oxidize the phenol to a phenoxyl radical which will react with ·NO2 to give the nitrophenol. The very stable heme-FeIII-hydroxo complex 4 is formed by the wayside, as we observe.

Another reaction mechanism that should be considered is that (F8)FeIII-(OON=O) (2a) undergoes heterolytic O-O bond cleavage, producing nitronium (NO2+) ion which is an effective phenol nitrating agent.20 Generation of NO2+ in reactions of copper-zinc superoxide dismutase (CuZnSOD) with peroxynitrite was in fact suggested by Beckman,21 and other literature reports suggest this chemistry may be preferred when metal ions are present.4c,20 However, for metalloporphyrins, O-O heterolytic cleavage to give NO2+ seems to have been ruled out or not considered.22

In summary, we have here described a heme complex that acts as a nitrogen monoxide dioxygenase, facilitating the reaction of O2 and ·NO to yield the nitrate anion NO3. Generation of a heme-peroxynitrite species is implicated; it can be trapped by a phenolic substrate, leading to o-nitration. The results lead to the suggestion that in heme proteins peroxynitrite may leak and effect nitration of nearby residues or exogenous substrates. While Herold observed essentially no peroxynitrite leakage in oxyMb reactions with ·NO,13 other studies show that addition of peroxynitrite to various metal complexes and metalloproteins leads to protein tyrosine (including for Mb) or exogenous phenol nitration.23 Other examples include MnSOD and CuZnSOD.3d,24 Perhaps there are subtle differences in reactions of metal ion reduced centers with O2 followed by ·NO(g), compared to conditions involving addition of peroxynitrite reagent to oxidized metal ion centers. Further studies which address these issues and the mechanism of the reaction observed here are in progress.

Supplementary Material




This work was supported by NIH grant GM 60353 (K.D.K.).


Supporting Information Available: Details concerning synthesis, spectroscopy, reactivity and CIF file. This material is available free of charge on the Internet at


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