Modeling the Signatures of Hydrides in Metalloenzymes: ENDOR Analysis of a Di-iron Fe(μ-NH)(μ-H)Fe Core

The application of 35 GHz pulsed EPR and ENDOR spectroscopies
has established that the biomimetic model complex L3Fe­(μ-NH)­(μ-H)­FeL3 (L3 = [PhB­(CH2PPh2)3]−) complex, 3, is a novel S = 1/2 type-III mixed-valence di-iron
II/III species, in which the unpaired electron is shared equally between
the two iron centers. 1,2H and 14,15N ENDOR
measurements of the bridging imide are consistent with an allyl radical
molecular orbital model for the two bridging ligands. Both the (μ-H)
and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a ‘bridging’
hydride that is essentially equidistant between two ‘anchor’
metal ions: a rhombic dipolar interaction tensor, T ≈
[T, –T, 0]. The point-dipole
model for describing the anisotropic interaction of a bridging H as
the sum of the point-dipole couplings to the ‘anchor’
metal ions reproduces this signature with high accuracy, as well as
the axial tensor of a terminal hydride, T ≈ [−T, –T, 2T], thus
validating both the model and the signatures. This validation in turn
lends strong support to the assignment, based on such a point-dipole
analysis, that the molybdenum–iron cofactor of nitrogenase
contains two [Fe–H––Fe] bridging-hydride
fragments in the catalytic intermediate that has accumulated four
reducing equivalents (E4). Analysis further reveals a complementary
similarity between the isotropic hyperfine couplings for the bridging
hydrides in 3 and E4. This study provides
a foundation for spectroscopic study of hydrides in a variety of reducing
metalloenzymes in addition to nitrogenase.

本研究借助35吉赫脉冲电子顺磁共振（pulsed EPR）与电子核双共振（ENDOR）光谱技术，确证仿生模型配合物L3Fe(μ-NH)(μ-H)FeL3（其中L3为[PhB(CH2PPh2)3]⁻，即化合物3）属于新型自旋S=1/2的III型混合价二铁(II/III)物种，其未成对电子在两个铁中心间均匀分布。针对桥连酰亚胺开展的1,2H与14,15N ENDOR测量结果，与两种桥连配体的烯丙基自由基分子轨道模型相符。经晶体学表征的化合物3中，(μ-H)与(μ-NH)的质子均呈现出所提出的“桥连氢化物”特征信号：该氢化物与两个“锚定”金属离子基本等距，对应菱形偶极相互作用张量T≈[T, –T, 0]。将桥连氢的各向异性相互作用描述为其与“锚定”金属离子的点偶极耦合（point-dipole coupling）之和的点偶极模型（point-dipole model），不仅可高精度复现该特征信号，还能复现端基氢化物的轴向张量T≈[−T, –T, 2T]，由此验证了该模型与特征信号的合理性。这一验证进一步为基于该点偶极分析的指认工作提供了有力支撑：固氮酶的钼铁辅因子在积累了四个还原当量的催化中间体（E4）中，存在两个[Fe–H––Fe]桥连氢化物片段。分析还发现，化合物3与E4中的桥连氢化物的各向同性超精细耦合（isotropic hyperfine coupling）具有互补相似性。本研究为除固氮酶外各类还原性金属酶中氢化物的光谱学研究奠定了重要基础。