Letter to the Editor Regarding the Article âReaction of CO2 with ONOO

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Letter to the Editor Regarding the Article "Reaction of CO2 with ONOO–: One Molecule of CO2 Is Not Enough" by Koppenol and coworkers., 2018 Hans-Gert Korth, and Michael Kirsch Chem. Res. Toxicol., Just Accepted Manuscript • DOI: 10.1021/acs.chemrestox.8b00364 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Chemical Research in Toxicology

Letter to the Editor Regarding the Article "Reaction of CO2 with ONOO–: One Molecule of CO2 Is Not Enough" by Koppenol and coworkers, 2018

Hans-Gert Korth*,† and Michael Kirsch‡ †Institut

für Organische Chemie, Universität Duisburg-Essen, Universitätsstr. 7, 45117 Essen,

Germany ‡Institut

für Physiologische Chemie, Universitätsklinikum Essen, Hufelandstr. 5, 45112 Essen,

Germany

ACS Paragon Plus Environment

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To the Editor: Nitrosoperoxocarbonate, ONOOCO2−, the elusive addition product of peroxynitrite, ONOO–, to carbon dioxide is believed as an important oxidative intermediate in living systems, causing oxidative stress not only by direct action but also via its established (fractional) homolytic cleavage to NO2 and CO3− radicals (see Supporting Information, p. S2). In a recent article in Chemical Research in Toxicology, Koppenol and coworkers reported on a kinetic (re)investigation of the decay of ONOOCO2−.1 The kinetic experiments led to the conclusion that a second CO2 molecule is involved in the main decay path of ONOOCO2−. The mechanism presented in ref 1 based primarily on the observation of a transient, broad (composite) UV/Vis absorption in the 500−700 nm range (i.e. a blue color) and a much stronger absorption at 300 nm, both being produced on rapid addition of ONOO– to CO2 in aqueous solution as well as on reaction of solid tetraammoniun peroxynitrite, Me4N+ONOO–, with gaseous CO2. We here show that the spectral assignments in ref 1 are unsupported by chemical knowledge and theory. The 500−700 nm absorption has already been reported in two previous papers from this group to be partly due to ONOOCO2−.2,3 The lower-wavelength section (max ≈ 600 nm) of the visible region has unequivocally been identified to be due to the carbonate radical anion, CO3−, as proven by ESR spectrometry2,4 and comparison with independently generated UV/Vis (see Table S1, entry 8) and ESR spectra. The crucial matter in ref 1 is the assignment of the longer-wavelength section (max = 630–680 nm) and the strong absorption around 300 nm to ONOOCO2−. It should be emphasized that to date no other sound experimental (spectroscopic) evidence is available to support this assignment. (Yields of the final reaction products nitrate and nitrite and kinetics are inadequate to indicate a specific reactive nitrogen-oxygen intermediate).


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When we became aware of ref 1, it occurred to us that non-radical (closed-shell) nitrito (O=N–O–) and carbonate (–O2CO–) compounds having absorptions in the 600–700 nm range, i.e. exhibiting a green to blue color, would be unprecedented. Such type of compounds are generally colorless or yellow, i.e. having max ≤ 400 nm. A conjugative combination of both groups by a peroxo bridge would not be sufficient to shift the absorption by about 300 nm to longer wavelengths. We were particularly puzzled by the fact that in refs 1–3, as well as in a criticizing paper,5 the presence of possible (families of) nitrogen-oxygen compounds well known for their blue color, that is, (N–N-bonded) dinitrogen trioxide (N2O3), nitrate radical (NO3), and C-nitroso compounds (R–N=O), were not considered.6 In order to see whether the proposed blue color for ONOOCO2− would be supported by theory, we performed quantumchemical computations employing the accurate and reliable equation-of-motion coupled-cluster (EOM-CCSD) method (see SI, p. S5). Aqueous solution was modelled by the continuum PCM method. A number of studies (see SI, p. S8) have shown that EOM-CCSD generally overestimates electronic excitation energies, by max 0.2 eV in the gas phase and by max 0.4 eV in PCM aqueous solution computations. At 300 and 700 nm, these upper margins correspond to wavelength differences exp–calc of about 15 and 80 nm the gas phase and 30 nm and 170 nm in water, respectively. Computed absorption maxima for both conformers of ONOOCO2−, the blue compounds (N2O3, NO3 R–N=O, CO3−, and some related compounds are collected in Table S1.7 Available experimental data are also given. As can be seen (Table S1, entries 6, 7), the longest-wavelength absorption of ONOOCO2− is predicted to be in the range 340–360 nm. This absorption is very weak, its oscillator strength being similar to the 340–360 nm absorption of cis-ONOO−. The strongest absorption of ONOOCO2− is at 220–240 nm with about 2/3 the oscillator strength of ONOO− at 300 nm. Even


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allowing for the upper error limits of EOM-CCSD, this max should not be >270 nm. Thus, in solution a strong absorption of ONOOCO2− at max ≈ 300 nm is unlikely. Note that the experimental absorptions of all other compounds are reproduced well within the error margins. Hence, ONOOCO2− certainly does not have a blue color. It is colorless or (faintly) yellow at most. On comparing the 500–700 nm "ONOOCO2−" UV/Vis spectra displayed in refs 1–3 with a variety of published ones of N2O3, NO3 and alkyl nitroso (R–N=O) compounds (Figure 1) it appears that any of these three species might be present. However, a striking similarity with the spectrum of NO3 is evident. In solution (Figure 1A), NO3 shows a characteristic vibrational three-band pattern (gas phase: 662, 623, 589 nm8) with ca. 40 nm peak separation (trace 3). A ~40 nm peak distance is in fact discernible in Figure 2 of ref 1 (trace 1). Note that the authors have even fitted this spectrum to three overlapping Gaussian curves. The solution spectrum of N2O3 (trace 4) is broad and vibrationally unstructured. Its max is quite strongly dependent on the solvent (see SI, refs 11–18 to Table S1). Anyway, N2O3 cannot be excluded to contribute to traces 1 and 2. C-nitroso compounds also exhibit some vibrational structure, but with smaller peak separations and unresolved in aqueous solution (trace 5). However, being excellent spin traps and easily to oxidize, nitroso compounds are expected to be rapidly consumed under the reaction conditions.


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 / nm Figure 1. (A) Electronic absorption spectra in aqueous solution. 1, Me4N+ONOO– + CO2 with the CO3− spectrum subtracted (open circles; solid line is a spline fit);1 2, Me4N+ONOO– + CO2 (blue line);3 3, NO3 in HNO3 (red line);9 4, N2O3 (green line);10 5, EtNO (magenta line).11 (B) Electronic absorption spectra in the solid state. 6, solid Me4N+ONOO– + CO2 (black line);1 7, solid Me4N+ONOO– + CO2 (blue line);3 8, NO3 in acetonitrile solution for comparison (green


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line, original data shifted by +10 nm);12 9, CO3− + NO3 in beryl crystal (red line, original data shifted by –15 nm).13 The spectra were reconstructed from digitized traces displayed in the cited references. Amplitudes were scaled arbitrarily. The three-band structure is also visible in the solid state (Figure 1B). Noteworthy, strikingly similar spectral patterns in the 550–750 nm range have been observed from -irradiated or electrolytically colored beryl (gemstone) crystals (trace 9), identified to be due to both CO3− and NO3 deriving from carbonate and nitrate impurities.13,14 The authors of ref 1 support their spectral assignment of ONOOCO2− by referring (refs 35– 38) to a blue color reported for peroxodicarbonate, –O2COOCO2−, salts. However, an extensive literature search revealed that pure (and aqueous solutions of) peroxodicarbonate salts are colorless (even mentioned in ref 38: "colorless to light blue").15 This is confirmed by our computations, yielding  ≤ 240 nm (Table S1, entry 12) for –O2COOCO2− (Note, peroxodicarbonate alkyl diesters are generally colorless). A "faint/light/weak/pale" sky blue color of solid –O2COOCO2− salts has only been reported for material produced from carbonate solutions by electrocrystallization, i.e. derived from inclusion of the initial one-electron oxidation product CO3−. Similarly, –O2COOCO2− salts produced by reaction of H2O2 with CO2 in strong alkaline solution may exhibit a variably intense orange color (max ≈ 460 nm) from inclusion of the ozonide radical anion, O3− (Table S1, entry 13).15,16 The presence of such radicals has been confirmed by ESR spectrometry.16 Hence, it would have been extremely enlightening if the authors had performed ESR measurements (detection of NO3), at least on their solid material. Another issue not considered in ref 1: It seems, all graphically displayed UV/Vis and kinetic data had been obtained from Me4N+ONOO– as peroxynitrite source (same in refs 2,3).


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However, the involvement of the Me4N+ cation has not been discussed, obviously considered "innocent". But, as an organic compound providing 12 C–H bonds per molecule, Me4N+ must be expected to be a target under the given reaction conditions. For example, hydrogen atom abstraction from Me4N+ might occur by the highly reactive CO3−. In such case, the thus formed distonic Me3N+CH2 radical cation would rapidly be converted to various oxygenated and/or nitrogenated products (e.g. entry 11 in Table S1), thereby giving rise to additional UV/Vis absorptions and thus affecting the kinetics. We could not find in the literature any 1H or 13C NMR spectrometric data on the Me4N+ONOO– salt, neither prior to nor after its decomposition, which would have revealed the presence of organic impurities and/or reaction products. In conclusion, whatever species might be responsible for the 600–700 nm absorption (NO3 is the most likely candidate), it is not ONOOCO2−.

ASSOCIATED CONTENT Supporting Information. A table of (PCM)EOM-CCSD-computed and experimental UV/Vis absorption data with references, computational details, and references to related 15N CIDNP studies. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Hans-Gert Korth: 0000-0001-6056-1748


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REFERENCES (1) Serrano-Luginbuehl, S., Kissner, R., and Koppenol, W. H. (2018) Reaction of CO2 with ONOO– : One Molecule of CO2 Is Not Enough. Chem. Res. Toxicol. 31, 721–730. (2) Meli, R., Nauser, T., and Koppenol, W. H. (1999) Direct observation of intermediates in the reaction of peroxynitrite with carbon dioxide. Helv. Chim. Acta 82, 722–725. (3) Meli, R., Nauser, T., Latal, P., and Koppenol, W. H. (2002) Reaction of peroxynitrite with carbon dioxide: Intermediates and determination of the yield of CO3●− and NO2●. J. Biol. Inorg. Chem. 7, 31–36. (4) Bonini, M. G., Radi, R., Ferrer-Sueta, G., Ferreira, A. M. D. C., and Augusto, O. (1999) Direct EPR detection of the carbonate radical anion produced from peroxynitrite and carbon dioxide. J. Biol. Chem. 274, 10802-10806. (5) Goldstein, S., Czapski, G., Lind, J., and Merenyi, G. (2001) Carbonate Radical Ion Is the Only Observable Intermediate in the Reaction of Peroxynitrite with CO2. Chem. Res. Toxicol. 14, 1273–1276. (6) From a historical point of view it is interesting to note that already in 1921 Wieland pointed out the different colors of nitrito (colorless) versus C-nitroso compounds (blue): Wieland, H. (1921) Einige Beiträge zur Kenntnis des Stickstoffdioxyds. Ber. Dtsch. Chem. Ges. B 54, 1776–1784. (7) Since for conformationally flexible molecules the electronic absorption might strongly differ for individual conformers, we explored the conformer space of the relevant molecules. Only two ground-state conformers for ONOOCO2− (entries 6,7) and just one conformer for –O

−

2COOCO2

(entry 12) could be located in the gas phase as well as in aqueous phase.


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(8) Stanton, J. F. (2007) On the vibronic level structure in the NO3 radical. I. The ground electronic state. J. Chem. Phys. 126, 134309/134301–134309/134320. (9) Balcerzyk, A., El Omar, A. K., Schmidhammer, U., Pernot, P., and Mostafavi, M. (2012) Picosecond Pulse Radiolysis Study of Highly Concentrated Nitric Acid Solutions: Formation Mechanism of NO3 Radical. J. Phys. Chem. A 116, 7302–7307. (10) Mason, J. (1959) Dinitrogen trioxide: some observations on the electronic spectrum and structure. J. Chem. Soc., 1288–1295. (11) Fornstedt, L., Hakman, M., and Lindquist, S. E. (1980) Kinetics of reactions of monomeric nitrosoethane induced by flash photolysis. II. Aqueous solutions. Chem. Scr. 15, 97–101. (12) McGarvey, D. J., Mulroy, L., Robertson, A. B., Truscott, T. G., Parker, A. W., Tavender, S., King, M., Biggs, P., Canosamas, C., and Wayne, R. P. (1997) A TR3 study of the nitrate radical with organic molecules in solution. CLF Ann. Rep. 1996/97, 128–129. (13) Chen, W., Gu, H., Jia, L., Wang, F., Ma, D., and Zhu, R. (2010) Electrolytic coloration and spectral properties of natural beryl crystals. Phys. B (Amsterdam, Neth.) 405, 331–334. (14) Pinheiro, M. V. B., Krambrock, K., Guedes, K. J., and Spaeth, J. M. (2007) Opticallydetected magnetic resonance of molecular color centers CO3– and NO3 in gamma-irradiated beryl. Phys. Status Solidi C 4, 1293–1296. (15) Roberts, J. L., Jr., Calderwood, T. S., and Sawyer, D. T. (1984) Nucleophilic oxygenation of carbon dioxide by superoxide ion in aprotic media to form the peroxydicarbonate(2–) ion species. J. Am. Chem. Soc. 106, 4667–4670. (16) Jones, D. P., and Griffith, W. P. (1980) Alkali metal peroxocarbonates, M2[CO3].nH2O2, M2[C2O6], M[HCO4].nH2O, and Li2[CO4].H2O. J. Chem. Soc., Dalton Trans., 2526–2532.


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Figure Caption

Figure 1. (A) Electronic absorption spectra in aqueous solution. 1, Me4N+ONOO– + CO2 with the CO3− spectrum subtracted (open circles; solid line is a spline fit);1 2, Me4N+ONOO– + CO2 (blue line);3 3, NO3 in HNO3 (red line);9 4, N2O3 (green line);10 5, EtNO (magenta line).11 (B) Electronic absorption spectra in the solid state. 6, solid Me4N+ONOO– + CO2 (black line);1 7, solid Me4N+ONOO– + CO2 (blue line);3 8, NO3 in acetonitrile solution for comparison (green line, original data shifted by +10 nm);12 9, CO3− + NO3 in beryl crystal (red line, original data shifted by –15 nm).13 The spectra were reconstructed from digitized traces displayed in the cited references. Amplitudes were scaled arbitrarily.


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Letter to the Editor Regarding the Article âReaction of CO2 with ONOO

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Letter to the Editor Regarding the Article âReaction of CO2 with ONOO