NMR-STUDIES OF THE DINUCLEAR IRON SITE IN REDUCED UTEROFERRIN AND ITS OXOANION COMPLEXES

The chemical shifts and relaxation behavior of paramagnetically shifted proton NMR resonances of reduced uteroferrin (Ufr) and its XO4n- complexes (n = 2: X = Mo, W; n = 3: X = P, As) are reported. Integrations with respect to an internal standard show that downfield resonances (δ > 14 ppm vs TMS) arise from 10 protons on ligands to the paramagnetic dinuclear iron center of Ufr, and upfield resonances (δ >-10 ppm vs TMS) arise from at least two protons. Two downfield and one upfield resonance are lost in D2O. Binding of molybdate, tungstate, or arsenate of Ufr at pH 4.9 causes minor changes in δ, whereas anaerobic addition of phosphate results in loss of interpretable NMR signals outside of the-3 to 15 ppm range. UfrPO4 at pH 3 exhibits downfield-shifted resonances somewhat larger than those of the other oxoanion complexes at pH 4.9, which suggests weaker antiferromagnetic coupling in UfrPO4. Primarily on the basis of δ and T1, values, seven of the paramagnetically shifted resonances are assigned to a single tyrosine bound to iron(III) and to a histidine bound to each of the two iron atoms. Probable assignments are given for other paramagnetically shifted peaks. © 1990, American Chemical Society. All rights reserved.