Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Am Chem Soc. 2010 Mar 24;132(11):4006-15. doi: 10.1021/ja9090098.
Most bacteria and fungi produce low-molecular-weight iron chelators called siderophores. Although different siderophore structures have been characterized, the iron-binding moieties often contain catecholate or hydroxamate groups. Siderophores function because of their extraordinarily high stability constants (K(STAB) = 10(30)-10(49)) and selectivity for Fe(III), yet the origin of these high stability constants has been difficult to quantify experimentally. Herein, we utilize Fe L-edge X-ray absorption spectroscopy to determine the differential orbital covalency (i.e., the differences in the mixing of the metal d-orbitals with ligand valence orbitals) of a series of siderophore model compounds. The results enable evaluation of the electronic structure contributions to their high stability constants in terms of sigma- and pi-donor covalent bonding, ionic bonding, and solvent effects. The results indicate substantial differences in the covalent contributions to stability constants of hydroxamate and catecholate complexes and show that increased sigma as well as pi bonding contributes to the high stability constants of catecholate complexes.
大多数细菌和真菌会产生称为铁载体的低分子量铁螯合剂。虽然已经对不同的铁载体结构进行了表征，但铁结合部分通常含有儿茶酚或羟肟酸基团。铁载体之所以具有功能，是因为它们具有极高的稳定常数（K(STAB) = 10(30)-10(49)）和对 Fe(III)的选择性，但这些高稳定常数的起源很难通过实验进行量化。在此，我们利用 Fe L 边 X 射线吸收光谱来确定一系列铁载体模型化合物的差分轨道共价性（即金属 d 轨道与配体价轨道混合的差异）。结果可根据 sigma- 和 pi-供体共价键合、离子键合和溶剂效应来评估对其高稳定常数的电子结构贡献。结果表明，羟肟酸和儿茶酚配合物的稳定常数的共价贡献存在显著差异，并且表明增加的 sigma 以及 pi 键合有助于儿茶酚配合物的高稳定常数。
