Temperature-Dependent Proton NMR Investigation of the Electronic Structure of the Trinuclear Iron Cluster of the Oxidized Desulfovibrio gigas Ferredoxin II

Proton NMR sprectra of the native (oxidized) Desulfovibrio gigas ferredoxin II (a protein which contains only a [3Fe-4S] cluster as its prosthetic group) have been recorded at 300 and 500 MHz. Proton resonances are observed between 6 and 30 ppm, and their temperature dependences are determined over the range 293–308 K. On the basis of the nuclear Overhauser effect and relaxation measurements, the two most downfield-shifted signals (29.0 and 24.4 ppm at 303 K) are assigned to a pair of β-CH ₂ protons of one of the cysteinyl residues coordinated to the [3Fe-4S] cluster. Except for these two most downfield shifted signals, which show Curie type temperature dependence, the others show anti-Curie dependence. To analyze the temperature dependence of these resonances a Heisenberg-Dirac-Van Vleck spin-coupling model is introduced. It is found that the observed different types of temperature dependence can be explained by the presence of different exchange coupling interactions between the three iron sites; assuming J ₁₃ = J ₂₃ = J and J ₁₂ = J + ΔJ (ΔJ > 0), the β-CH ₂ protons of the cysteine ligated to iron site 3 show Curie behavior, while protons of cysteines bound to iron sites 1 and 2 show anti-Curie behavior. Detailed analysis futher suggests that J ≈ 300 cm ⁻¹ and that the coupling constants between the iron sites are slightly different: ΔJ/J ≈ 0.02. Using the available X-ray crystallographic coordinates together with the pattern observed for the two most downfield shifts, we have tentatively assigned them to the methylene protons of Cys 50.