Mossbauer and EPR characterization of the S = 9/2 mixed-valence Fe(II)Fe(III) cluster in the cryoreduced R2 subunit of Escherichia coli ribonucleotide reductase

Carsten Krebs, Roman Davydov, Jeff Baldwin, Brian M. Hoffman, Joseph M. Bollinger, Jr., Boi Hanh Huynh

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Abstract

Low-temperature (77 K) radiolytic reduction of the diferric cluster in the met R2 subunit of Escherichia coli ribonucleotide reductase yields an antiferromagnetically coupled mixed-valence Fe(II)Fe(III) cluster ([R2(met)](mv1/2)). Annealing the radiolytically reduced sample at 180 K converts the mixed-valence cluster in [R2(met)](mv1/2) into a ferromagnetically coupled cluster having an S = 9/2 ground state (R2(mv9/2)). We have used Mossbauer and EPR spectroscopy to study the electronic and magnetic properties of R2(mv9/2). The Mossbauer data, recorded over wide ranges of temperature and applied field, indicate that the mixed- valence cluster in R2(mv9/2) is valence localized. The spectra can be deconvoluted into two spectral components, of which analysis yields parameters (δ = 1.25 mm/s, ΔE(Q) = -2.80 mm/s, η = 1.30, and a/g(n)β(n) = -(13.5, 10.8, 20.3) T for site 1; and δ = 0.53 mm/s, ΔE(Q) = -0.57 mm/s, η = -3, and a/g(n)β(n) = -(22.1, 22.0, 22.0) T for site 2) that are characteristic of high-spin ferrous (site 1) and high-spin ferric (site 2) ions with octahedral O/N coordination. The spin-spin interaction between the two valence localized iron sites is ferromagnetic and the effective exchange coupling constant (J(eff) in the exchange Hamiltonian J(eff)S1·S2) is estimated to be ca. -12 cm-1 from the high-temperature strong-field data. Taking into consideration the various factors that control the electronic properties of a mixed-valence Fe(II)Fe(III) compound and comparing the observed spectroscopic properties of R2(mv9/2) with those of model complexes, a core structure with two single-oxygen bridges is proposed for R2(mv9/2). It is suggested that conversion of [R2(met)](mv1/2) to R2(mv9/2) may involve a carboxylate shift of E238 from a monodentate terminal chelating mode to a monodentate bridging and chelating mode, in addition to protonation of the oxo bridge. R2(mv9/2) displays EPR signals at g = 14-15, 6.6, and 5.4. Analysis of the data indicates that these features can be properly simulated by assuming an S = 9/2 center with a central E/D of 0.05 and a distribution in E/D (σ(E/D) = 0.023). Effects of E/D distribution on the EPR spectrum are discussed.

Original languageEnglish (US)
Pages (from-to)5327-5336
Number of pages10
JournalJournal of the American Chemical Society
Volume122
Issue number22
DOIs
StatePublished - Jun 7 2000

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Ribonucleotide Reductases
Escherichia coli
Paramagnetic resonance
Chelation
Electronic properties
Temperature
Mossbauer Spectroscopy
Hamiltonians
Exchange coupling
Protonation
Ground state
Magnetic properties
Temperature distribution
Iron
Spectroscopy
Annealing
Ions
Oxygen
Oxidoreductases

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

@article{d2b0b07632d44e6089f83255e6c68714,
title = "Mossbauer and EPR characterization of the S = 9/2 mixed-valence Fe(II)Fe(III) cluster in the cryoreduced R2 subunit of Escherichia coli ribonucleotide reductase",
abstract = "Low-temperature (77 K) radiolytic reduction of the diferric cluster in the met R2 subunit of Escherichia coli ribonucleotide reductase yields an antiferromagnetically coupled mixed-valence Fe(II)Fe(III) cluster ([R2(met)](mv1/2)). Annealing the radiolytically reduced sample at 180 K converts the mixed-valence cluster in [R2(met)](mv1/2) into a ferromagnetically coupled cluster having an S = 9/2 ground state (R2(mv9/2)). We have used Mossbauer and EPR spectroscopy to study the electronic and magnetic properties of R2(mv9/2). The Mossbauer data, recorded over wide ranges of temperature and applied field, indicate that the mixed- valence cluster in R2(mv9/2) is valence localized. The spectra can be deconvoluted into two spectral components, of which analysis yields parameters (δ = 1.25 mm/s, ΔE(Q) = -2.80 mm/s, η = 1.30, and a/g(n)β(n) = -(13.5, 10.8, 20.3) T for site 1; and δ = 0.53 mm/s, ΔE(Q) = -0.57 mm/s, η = -3, and a/g(n)β(n) = -(22.1, 22.0, 22.0) T for site 2) that are characteristic of high-spin ferrous (site 1) and high-spin ferric (site 2) ions with octahedral O/N coordination. The spin-spin interaction between the two valence localized iron sites is ferromagnetic and the effective exchange coupling constant (J(eff) in the exchange Hamiltonian J(eff)S1·S2) is estimated to be ca. -12 cm-1 from the high-temperature strong-field data. Taking into consideration the various factors that control the electronic properties of a mixed-valence Fe(II)Fe(III) compound and comparing the observed spectroscopic properties of R2(mv9/2) with those of model complexes, a core structure with two single-oxygen bridges is proposed for R2(mv9/2). It is suggested that conversion of [R2(met)](mv1/2) to R2(mv9/2) may involve a carboxylate shift of E238 from a monodentate terminal chelating mode to a monodentate bridging and chelating mode, in addition to protonation of the oxo bridge. R2(mv9/2) displays EPR signals at g = 14-15, 6.6, and 5.4. Analysis of the data indicates that these features can be properly simulated by assuming an S = 9/2 center with a central E/D of 0.05 and a distribution in E/D (σ(E/D) = 0.023). Effects of E/D distribution on the EPR spectrum are discussed.",
author = "Carsten Krebs and Roman Davydov and Jeff Baldwin and Hoffman, {Brian M.} and {Bollinger, Jr.}, {Joseph M.} and Huynh, {Boi Hanh}",
year = "2000",
month = "6",
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pages = "5327--5336",
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Mossbauer and EPR characterization of the S = 9/2 mixed-valence Fe(II)Fe(III) cluster in the cryoreduced R2 subunit of Escherichia coli ribonucleotide reductase. / Krebs, Carsten; Davydov, Roman; Baldwin, Jeff; Hoffman, Brian M.; Bollinger, Jr., Joseph M.; Huynh, Boi Hanh.

In: Journal of the American Chemical Society, Vol. 122, No. 22, 07.06.2000, p. 5327-5336.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Mossbauer and EPR characterization of the S = 9/2 mixed-valence Fe(II)Fe(III) cluster in the cryoreduced R2 subunit of Escherichia coli ribonucleotide reductase

AU - Krebs, Carsten

AU - Davydov, Roman

AU - Baldwin, Jeff

AU - Hoffman, Brian M.

AU - Bollinger, Jr., Joseph M.

AU - Huynh, Boi Hanh

PY - 2000/6/7

Y1 - 2000/6/7

N2 - Low-temperature (77 K) radiolytic reduction of the diferric cluster in the met R2 subunit of Escherichia coli ribonucleotide reductase yields an antiferromagnetically coupled mixed-valence Fe(II)Fe(III) cluster ([R2(met)](mv1/2)). Annealing the radiolytically reduced sample at 180 K converts the mixed-valence cluster in [R2(met)](mv1/2) into a ferromagnetically coupled cluster having an S = 9/2 ground state (R2(mv9/2)). We have used Mossbauer and EPR spectroscopy to study the electronic and magnetic properties of R2(mv9/2). The Mossbauer data, recorded over wide ranges of temperature and applied field, indicate that the mixed- valence cluster in R2(mv9/2) is valence localized. The spectra can be deconvoluted into two spectral components, of which analysis yields parameters (δ = 1.25 mm/s, ΔE(Q) = -2.80 mm/s, η = 1.30, and a/g(n)β(n) = -(13.5, 10.8, 20.3) T for site 1; and δ = 0.53 mm/s, ΔE(Q) = -0.57 mm/s, η = -3, and a/g(n)β(n) = -(22.1, 22.0, 22.0) T for site 2) that are characteristic of high-spin ferrous (site 1) and high-spin ferric (site 2) ions with octahedral O/N coordination. The spin-spin interaction between the two valence localized iron sites is ferromagnetic and the effective exchange coupling constant (J(eff) in the exchange Hamiltonian J(eff)S1·S2) is estimated to be ca. -12 cm-1 from the high-temperature strong-field data. Taking into consideration the various factors that control the electronic properties of a mixed-valence Fe(II)Fe(III) compound and comparing the observed spectroscopic properties of R2(mv9/2) with those of model complexes, a core structure with two single-oxygen bridges is proposed for R2(mv9/2). It is suggested that conversion of [R2(met)](mv1/2) to R2(mv9/2) may involve a carboxylate shift of E238 from a monodentate terminal chelating mode to a monodentate bridging and chelating mode, in addition to protonation of the oxo bridge. R2(mv9/2) displays EPR signals at g = 14-15, 6.6, and 5.4. Analysis of the data indicates that these features can be properly simulated by assuming an S = 9/2 center with a central E/D of 0.05 and a distribution in E/D (σ(E/D) = 0.023). Effects of E/D distribution on the EPR spectrum are discussed.

AB - Low-temperature (77 K) radiolytic reduction of the diferric cluster in the met R2 subunit of Escherichia coli ribonucleotide reductase yields an antiferromagnetically coupled mixed-valence Fe(II)Fe(III) cluster ([R2(met)](mv1/2)). Annealing the radiolytically reduced sample at 180 K converts the mixed-valence cluster in [R2(met)](mv1/2) into a ferromagnetically coupled cluster having an S = 9/2 ground state (R2(mv9/2)). We have used Mossbauer and EPR spectroscopy to study the electronic and magnetic properties of R2(mv9/2). The Mossbauer data, recorded over wide ranges of temperature and applied field, indicate that the mixed- valence cluster in R2(mv9/2) is valence localized. The spectra can be deconvoluted into two spectral components, of which analysis yields parameters (δ = 1.25 mm/s, ΔE(Q) = -2.80 mm/s, η = 1.30, and a/g(n)β(n) = -(13.5, 10.8, 20.3) T for site 1; and δ = 0.53 mm/s, ΔE(Q) = -0.57 mm/s, η = -3, and a/g(n)β(n) = -(22.1, 22.0, 22.0) T for site 2) that are characteristic of high-spin ferrous (site 1) and high-spin ferric (site 2) ions with octahedral O/N coordination. The spin-spin interaction between the two valence localized iron sites is ferromagnetic and the effective exchange coupling constant (J(eff) in the exchange Hamiltonian J(eff)S1·S2) is estimated to be ca. -12 cm-1 from the high-temperature strong-field data. Taking into consideration the various factors that control the electronic properties of a mixed-valence Fe(II)Fe(III) compound and comparing the observed spectroscopic properties of R2(mv9/2) with those of model complexes, a core structure with two single-oxygen bridges is proposed for R2(mv9/2). It is suggested that conversion of [R2(met)](mv1/2) to R2(mv9/2) may involve a carboxylate shift of E238 from a monodentate terminal chelating mode to a monodentate bridging and chelating mode, in addition to protonation of the oxo bridge. R2(mv9/2) displays EPR signals at g = 14-15, 6.6, and 5.4. Analysis of the data indicates that these features can be properly simulated by assuming an S = 9/2 center with a central E/D of 0.05 and a distribution in E/D (σ(E/D) = 0.023). Effects of E/D distribution on the EPR spectrum are discussed.

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