Enhanced Stability of the FeII/MnII 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 MII binding to the apoprotein. We report here equilibrium studies of FeII/MnII 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 = [FeII2(F-HXTA)­(H2O)4]−; 2 = [MnII2(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 [FeIIMnII(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 MII(aq) at various metal ratios, and equilibrium constants for FeII/MnII discrimination were calculated from these data. FeII is preferred over MnII 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 MnII → FeII replacement. The relevance to FeII/MnII discrimination in R2c and R2lox proteins is discussed.