Cobalt(II) nitrosyl cation radicals of porphyrins, chlorins, and isobacteriochlorins. Models for nitrite and sulfite reductases and implications for A1u heme radicals

Etsuko Fujita, C. K. Chang, Jack Fajer

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Abstract

Oxidation of cobalt(II) nitrosyl complexes of porphyrins (P), chlorins (C), and isobacteriochlorins (iBC) yields CoIINO π-cation radicals. The radicals are stable and recyclable to the parent compound without loss of NO. The oxidized species have been characterized by electrochemistry, visible and infrared spectroscopy, and electron spin resonance (ESR). Interest in these intermediates is prompted by the presence of iron Cs or iBCs in enzymes that catalyze the reductions of nitrite to ammonia or nitric oxide (nitrite reductases) and the recent identification of cobalt iBC in sulfite reductases, enzymes which are also capable of nitrite reduction. The optical spectra of the CoIINO radicals resemble those previously obtained on oxidation of FeIINOC and iBC and lend support to the assignment of those ESR-silent species as FeIINOC+ and iBC+ π-cation radicals. As found in other series with the same macrocycles (FeIINO, FeIIICl, Zn, H2), the CoIINO complexes become progressively easier to oxidize as the porphyrin is saturated (P <C <iBC, easiest). In all three species, the Co-NO bonds remain bent, suggestive of some Co-NO- character. The C and iBC radicals exhibit ESR features characteristic of "a1u" radicals that include spin delocalization onto the metal. Similar spin profiles may rationalize the ESR spectra of presumed "a1u" porphyrin radicals in oxidized chloroperoxidase, catalase, and cytochrome P450.

Original languageEnglish
Pages (from-to)7665-7669
Number of pages5
JournalJournal of the American Chemical Society
Volume107
Issue number25
Publication statusPublished - 1985

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Oxidoreductases Acting on Sulfur Group Donors
Nitrite Reductases
Porphyrins
Cobalt
Heme
Paramagnetic resonance
Cations
Positive ions
Electron Spin Resonance Spectroscopy
Enzymes
Nitrites
Oxidation
Nitric oxide
Electrochemistry
Chloride Peroxidase
Infrared spectroscopy
Ammonia
Iron
Catalase
Cytochrome P-450 Enzyme System

ASJC Scopus subject areas

  • Chemistry(all)

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title = "Cobalt(II) nitrosyl cation radicals of porphyrins, chlorins, and isobacteriochlorins. Models for nitrite and sulfite reductases and implications for A1u heme radicals",
abstract = "Oxidation of cobalt(II) nitrosyl complexes of porphyrins (P), chlorins (C), and isobacteriochlorins (iBC) yields CoIINO π-cation radicals. The radicals are stable and recyclable to the parent compound without loss of NO. The oxidized species have been characterized by electrochemistry, visible and infrared spectroscopy, and electron spin resonance (ESR). Interest in these intermediates is prompted by the presence of iron Cs or iBCs in enzymes that catalyze the reductions of nitrite to ammonia or nitric oxide (nitrite reductases) and the recent identification of cobalt iBC in sulfite reductases, enzymes which are also capable of nitrite reduction. The optical spectra of the CoIINO radicals resemble those previously obtained on oxidation of FeIINOC and iBC and lend support to the assignment of those ESR-silent species as FeIINOC+ and iBC+ π-cation radicals. As found in other series with the same macrocycles (FeIINO, FeIIICl, Zn, H2), the CoIINO complexes become progressively easier to oxidize as the porphyrin is saturated (P <C - character. The C and iBC radicals exhibit ESR features characteristic of {"}a1u{"} radicals that include spin delocalization onto the metal. Similar spin profiles may rationalize the ESR spectra of presumed {"}a1u{"} porphyrin radicals in oxidized chloroperoxidase, catalase, and cytochrome P450.",
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T1 - Cobalt(II) nitrosyl cation radicals of porphyrins, chlorins, and isobacteriochlorins. Models for nitrite and sulfite reductases and implications for A1u heme radicals

AU - Fujita, Etsuko

AU - Chang, C. K.

AU - Fajer, Jack

PY - 1985

Y1 - 1985

N2 - Oxidation of cobalt(II) nitrosyl complexes of porphyrins (P), chlorins (C), and isobacteriochlorins (iBC) yields CoIINO π-cation radicals. The radicals are stable and recyclable to the parent compound without loss of NO. The oxidized species have been characterized by electrochemistry, visible and infrared spectroscopy, and electron spin resonance (ESR). Interest in these intermediates is prompted by the presence of iron Cs or iBCs in enzymes that catalyze the reductions of nitrite to ammonia or nitric oxide (nitrite reductases) and the recent identification of cobalt iBC in sulfite reductases, enzymes which are also capable of nitrite reduction. The optical spectra of the CoIINO radicals resemble those previously obtained on oxidation of FeIINOC and iBC and lend support to the assignment of those ESR-silent species as FeIINOC+ and iBC+ π-cation radicals. As found in other series with the same macrocycles (FeIINO, FeIIICl, Zn, H2), the CoIINO complexes become progressively easier to oxidize as the porphyrin is saturated (P <C - character. The C and iBC radicals exhibit ESR features characteristic of "a1u" radicals that include spin delocalization onto the metal. Similar spin profiles may rationalize the ESR spectra of presumed "a1u" porphyrin radicals in oxidized chloroperoxidase, catalase, and cytochrome P450.

AB - Oxidation of cobalt(II) nitrosyl complexes of porphyrins (P), chlorins (C), and isobacteriochlorins (iBC) yields CoIINO π-cation radicals. The radicals are stable and recyclable to the parent compound without loss of NO. The oxidized species have been characterized by electrochemistry, visible and infrared spectroscopy, and electron spin resonance (ESR). Interest in these intermediates is prompted by the presence of iron Cs or iBCs in enzymes that catalyze the reductions of nitrite to ammonia or nitric oxide (nitrite reductases) and the recent identification of cobalt iBC in sulfite reductases, enzymes which are also capable of nitrite reduction. The optical spectra of the CoIINO radicals resemble those previously obtained on oxidation of FeIINOC and iBC and lend support to the assignment of those ESR-silent species as FeIINOC+ and iBC+ π-cation radicals. As found in other series with the same macrocycles (FeIINO, FeIIICl, Zn, H2), the CoIINO complexes become progressively easier to oxidize as the porphyrin is saturated (P <C - character. The C and iBC radicals exhibit ESR features characteristic of "a1u" radicals that include spin delocalization onto the metal. Similar spin profiles may rationalize the ESR spectra of presumed "a1u" porphyrin radicals in oxidized chloroperoxidase, catalase, and cytochrome P450.

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