Resonance raman spectroscopy of binuclear iron centers. Hemerythrin, ribonucleotide reductase and iron phenanthroline complexes

Abstract

Binuclear iron centers are known to be present in the respiratory protein, hemerythrin [1], and in the enzyme, ribonucleotide reductase [2]. These centers are characterized by strong antiferromagnetic coupling (-J ≅ 100 cm −1) of the two ferric ions, and by one or more intense absorption bands between 320 and 380 nm (ϵ ≅ 4000 M −1 cm − per Fe atom). These properties have long been ascribed to the presence of a μ-oxo bridge between the iron atoms. Verification of such a bridge was obtained in the 2.2-Å resolution crystal structure of azidomethemerythrin [3]. Resonance Raman spectroscopy provides an additional valuable technique for the detection and characterization of binuclear iron centers. The FeOFe symmetric stretch, ν s(FeOFe), is Raman active and the intensity of this vibration may be enhanced by excitation within the FeOFe charge transfer band in the near ultraviolet. As a model system, we have investigated the resonance Raman spectra of binuclear 1,10-phenanthroline (phen) complexes of iron(III) [4]. The complex Fe 2O(phen) 4(NO 3) 4 · 7H 2O has a Raman peak at 395 cm −1 which can be assigned to ν s(FeOFe) on the basis of its frequency being appropriate to an FeOFe angle of 154°, its absence from the spectrum of mono-nuclear complex Fe(phen) 3(ClO 4) 3 · 3H 2O, and its intensity being dependent upon excitation wavelength. The perchlorate and chloride salts of [Fe 2O(phen) 4] 4+ have similar resonance-enhanced modes close to 400 cm −1, and in all three cases the enhancement is maximized using 363.8 nm excitation. Confirmatory evidence for an FeOFe vibration can be obtained from oxygen isotope exchange with H 18 2O solvent. For example, in the μ-oxo bridged dimer, [Fe 2O(Cl) 6] 2−, ν s(FeOFe) at 458 cm −1 shifts to 400 cm −1 in H 2 18O [5]. Similarly, the resonance-enhanced band at 507 cm −1 in the Raman spectrum of azidomethemerythrin shifts to 490 cm −1 for a sample which has been formed from oxyhemerythrin H 18 2O [6]. We have now found evidence for the corresponding FeOFe vibration at 489 cm −1 in the resonance Raman spectrum of oxyhemerythrin, through the use of near-ultraviolet excitation [7]. Furthermore, this band shows an isotope dependence on solvent (H 18 2O), and appears to be in resonance with the 360 nm electronic transition of oxyhemerythrin. The resonance enhancement of the ν s(FeOFe) peak intensity with ultraviolet excitation has also been observed for azidomethemerythrin [7] and for ribonucleotide reductase [2]. In contrast to hemerythrin, however, where the oxo group only exchanges during exogenous ligand replacement, the oxo group in ribonucleotide reductase undergoes facile exchange with solvent (k obs = 8.3 × 10 −4 s −1, indicating it is located in a more accessible site.