Sanakis Yiannis, Power Philip P, Stubna Audria, Münck Eckard
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, USA.
Inorg Chem. 2002 May 20;41(10):2690-6. doi: 10.1021/ic0111278.
The cofactor (M-center) of the MoFe protein of nitrogenase, a MoFe(7)S(9):homocitrate cluster, contains six Fe sites with a (distorted) trigonal sulfido coordination. These sites exhibit unusually small quadrupole splittings, Delta E(Q) approximately 0.7 mm/s, and isomer shifts, delta approximately 0.41 mm/s. Mössbauer and ENDOR studies have provided the magnetic hyperfine tensors of all iron sites in the S = 3/2 state M(N). To assess the intrinsic zero-field splittings and hyperfine parameters of the cofactor sites, we have studied with Mössbauer spectroscopy two salts of the three-coordinated Fe(II) thiolate complex Fe(SR)(3) (R = C(6)H(2)-2,4,6-tBu(3)). One of the salts, [Ph(4)P][Fe(SR)(3)] x 2MeCN x C(7)H(8), 1, has a planar geometry with idealized C(3h) symmetry. This S = 2 complex has an axial zero-field splitting with D = +10.2 cm(-1). The magnetic hyperfine tensor components A(x) = A(y) = -7.5 MHz and A(z) = -29.5 MHz reflect an orbital ground state with d(z(2)) symmetry. A(iso) = (A(x) +A(y) +A(z))/3 = -14.9 MHz, which includes the contact interaction (kappa P = -21.9 MHz) and an orbital contribution (+7 MHz), which is substantially smaller than A(iso) approximately -22 MHz of the tetrahedral Fe(II)(S-R)(4) sites of both rubredoxin and PPh(4)[Fe(II)(SPh)(4)]. The largest component of the electric field gradient (EFG) tensor is negative, as expected for a d(z(2)) orbital. However, Delta E(Q) = -0.83 mm/s, which is smaller than expected for a high-spin ferrous site. This reduction can be attributed to a ligand contribution, which in planar complexes provides a large positive EFG component perpendicular to the ligand plane. The isomer shift of 1, delta = 0.56 mm/s, approaches the delta-values reported for the six trigonal cofactor sites. The parameters of 1 and their importance for the cofactor cluster of nitrogenase are discussed.
固氮酶钼铁蛋白的辅因子（M中心），即MoFe(7)S(9):高柠檬酸簇，包含六个具有（扭曲）三角硫配位的铁位点。这些位点表现出异常小的四极分裂，ΔE(Q)约为0.7 mm/s，以及同质异能位移，δ约为0.41 mm/s。穆斯堡尔谱和电子核双共振研究提供了S = 3/2态M(N)中所有铁位点的磁超精细张量。为了评估辅因子位点的本征零场分裂和超精细参数，我们用穆斯堡尔谱研究了三配位铁硫醇盐配合物Fe(SR)(3)（R = C(6)H(2)-2,4,6-tBu(3)）的两种盐。其中一种盐，[Ph(4)P][Fe(SR)(3)]·2MeCN·C(7)H(8)，1，具有理想C(3h)对称性的平面几何结构。这个S = 2的配合物具有轴向零场分裂，D = +10.2 cm(-1)。磁超精细张量分量A(x) = A(y) = -7.5 MHz，A(z) = -29.5 MHz反映了具有d(z(2))对称性的轨道基态。A(iso) = (A(x)+A(y)+A(z))/3 = -14.9 MHz，其中包括接触相互作用（κP = -21.9 MHz）和一个轨道贡献（+7 MHz），该轨道贡献明显小于铁氧化还原蛋白和PPh(4)[Fe(II)(SPh)(4)]的四面体Fe(II)(S-R)(4)位点的A(iso)约-22 MHz。电场梯度（EFG）张量的最大分量为负，这对于d(z(2))轨道是预期的。然而，ΔE(Q) = -0.83 mm/s，这比高自旋亚铁位点预期的值要小。这种降低可归因于配体贡献，在平面配合物中，配体贡献提供了一个垂直于配体平面的大的正EFG分量。1的同质异能位移，δ = 0.56 mm/s，接近六个三角辅因子位点报道的δ值。讨论了1的参数及其对固氮酶辅因子簇的重要性。
