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

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.