Abstract
Electron spin echo envelope spectroscopy was used to probe the two metal binding sites of Cu(II)-conalbumin. The echo envelope spectrum of Cu(II)-conalbumin-oxalate, with metal ion at either one or both of the binding sites, contains lines arising from the interaction of the electron spin of Cu(II) with bound imidazole, demonstrating histidine ligation to the metal ion. The 13C superhyperfine interaction of bound [13C]oxalate, obtained from the ratio of the electron spin echo envelopes of Cu(II)-conalbumin-[13C]oxalate to that of Cu(II)-conalbumin-[12C]oxalate, is about twice the free precession frequency and indicates a contact interaction between 13C and Cu(II). This study indicates that oxalate is directly coordinated to the metal ion. Over the pH range 7.0 to 10.0, where Cu(II)-conalbumin binds carbonate as an associated anion, the echo envelope spectrum indicates that at least one imidazole ligand is coordinated to Cu(II). Below pH 6.0 and above pH 11.0, imidazole coordination is not observed.
Highlights
In this present study, the Cu(I1) complexes of conalbumin were examined using electron spin echo spectroscopy
From a comparison of the echo envelopes of the oxalate complexes preoxalate is directly coordinated to the metal ion
Min binds carbonate as an associated anion, the echo envelope spectrum indicates that laeat st one imidazole ligand is coordinated to Cu(I1)
Summary
Electron spin echo envelope spectroscopy was used directly coordinated to Cu(I1) is not seen with this technique to probe the two metal bindinsgites of Cu(II)-conalbu- [5].(For a discussion of the manner in which the I4Nnuclear min. The superhyperfine interaction of bound [‘3C]~xalate, obtained from the ratioof the electron spin echeonvelopes of Cu(II)-~onalbumin-[’~C]oxalateto that of Cu(II)-c~nalbumin-[’~C]oxalatei,s about twice the free precession frequency and indicates a contact interactionbetween andCu(I1). This studyindicatesthat tion, spin echo spectroscopy can sometimes be used to identify 13C-containingligands to Cu(II) [9]. EPR spectral studies of Cu(I1)-conalbumin, either as the carbonate or oxalate complex, have demonstrated the presence of a nitrogen ligand at each of the two metal binding sites of the protein, while the pH behavior of the EPR specof the highest quality commercially obtainable. Anion binding site for this protein, but the EPR parameters for the A and B sites are changed when oxalate is substituted
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