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.

异金属铌铁辅因子存在于Ic类核糖核苷酸还原酶（R2c）和R2样配体结合氧化酶（R2lox）的R2亚基中。辅因子的选择性组装至少部分归因于MII与脱辅酶蛋白的热力学相互作用。本研究报道了在生物模拟模型体系H5(F-HXTA)（5-氟-2-羟基-1,3-二甲苯-α,α′-二胺-N,N,N′,N′-四乙酸）中FeII/MnII区分的平衡研究。同金属F-HXTA复合物[Fe(H2O)6]2·14H2O和[Mn(H2O)6]2·14H2O（1 = [FeII2(F-HXTA)(H2O)4]−；2 = [MnII2(F-HXTA)(H2O)4]−）通过单晶X射线衍射进行了表征。核磁共振数据表明，1在溶液中保持其结构（2为核磁共振沉默）。金属交换过程简单易行，异金属复合物[FeIIMnII(F-HXTA)(H2O)4]−（3）由1和2的混合物形成。19F核磁共振被用于在过量MII(aq)存在下量化1和3，并从这些数据中计算了FeII/MnII区分的平衡常数。FeII相对于MnII更受青睐，完全取代的平衡常数为K1 = 182 ± 13（2 ⇌ 1）。这种相对温和的偏好归因于二价阳离子与多羧酸配体之间的硬-软酸-碱不匹配。取代的逐步常数为K2 = 20.1 ± 1.3（2 ⇌ 3）和K3 = 9.1 ± 1.1（3 ⇌ 1）。K2 > K3表明异金属态的稳定性增强，超出了简单的MnII → FeII取代的预期。讨论了其在R2c和R2lox蛋白中FeII/MnII区分的相关性。