Magnetic resonance studies of Mn(II)-, Mn(III)-, and Fe(III)-conalbumin complexes

Abstract

Apoconalbumin binds Mn(II) at two sites with association constants of K 1 = 7(±1) × 10 4 and K 2 = 0.4(±0.25) × 10 4 M −1. The binding is tighter in the presence of excess bicarbonate resulting in K 1 = 1.8(±0.2) × 10 5 and K 2 = 3(±2) × 10 4 M −1. The electron paramagnetic resonance spectrum (at both 9 and 35 GHz) of Mn(II) bound at the tight site reveals a rhombic distortion (λ = E/D ≅ 0.25–0.31) in the protein ligand environment of the metal ion. An evaluation of the 1/ pT 1 p , paramagnetic contribution to the longitudinal relaxation rate of solvent protons with Mn(II)-, Mn(III)-, and Fe(III)-derivatives of conalbumin revealed that the metal ion in each site of conalbumin is accessible to one water molecule. For Mn(II)-conalbumin and Mn(III)-conalbumin species, inner coordination sphere protons are rapidly exchanging with the bulk solvent, while slow exchange conditions prevail for Fe(III)-conalbumin.