Enhanced Stability of the Fe^II/Mn^II State in a Synthetic Model of Heterobimetallic Cofactor Assembly

Heterobimetallic Mn/Fe cofactors are found in the R2 subunit of class Ic ribonucleotide reductases (R2c) and R2-like ligand binding oxidases (R2lox). Selective cofactor assembly is due at least in part to the thermodynamics of M^II binding to the apoprotein. We report here equilibrium studies of Fe^II/Mn^II discrimination in the biomimetic model system H₅(F-HXTA) (5-fluoro-2-hydroxy-1,3-xylene-α,α′-diamine-N,N,N′,N′-tetraacetic acid). The homobimetallic F-HXTA complexes [Fe­(H₂O)₆]­[1]₂·14H₂O and [Mn­(H₂O)₆]­[2]₂·14H₂O (1 = [Fe^II₂(F-HXTA)­(H₂O)₄]⁻; 2 = [Mn^II₂(F-HXTA)­(H₂O)₄]⁻) were characterized by single crystal X-ray diffraction. NMR data show that 1 retains its structure in solution (2 is NMR silent). Metal exchange is facile, and the heterobimetallic complex [Fe^IIMn^II(F-HXTA)­(H₂O)₄]⁻ (3) is formed from mixtures of 1 and 2. ¹⁹F NMR was used to quantify 1 and 3 in the presence of excess M^II(aq) at various metal ratios, and equilibrium constants for Fe^II/Mn^II discrimination were calculated from these data. Fe^II is preferred over Mn^II with K₁ = 182 ± 13 for complete replacement (2 ⇌ 1). This relatively modest preference is attributed to a hard–soft acid–base mismatch between the divalent cations and the polycarboxylate ligand. The stepwise constants for replacement are K₂ = 20.1 ± 1.3 (2 ⇌ 3) and K₃ = 9.1 ± 1.1 (3 ⇌ 1). K₂ > K₃ demonstrates enhanced stability of the heterobimetallic state beyond what is expected for simple Mn^II → Fe^II replacement. The relevance to Fe^II/Mn^II discrimination in R2c and R2lox proteins is discussed.