ABSTRACT
The all-ferric [Fe(4)S(4)](4+) cluster [Fe(4)S(4){N(SiMe(3))(2)}(4)] 1 and its one-electron reduced form [1](-) serve as convenient precursors for the synthesis of 31-site differentiated [Fe(4)S(4)] clusters and high-potential iron-sulfur protein (HiPIP) model clusters. The reaction of 1 with four equivalents (equiv) of the bulky thiol HSDmp (Dmp = 2,6-(mesityl)(2)C(6)H(3), mesityl = 2,4,6-Me(3)C(6)H(2)) followed by treatment with tetrahydrofuran (THF) resulted in the isolation of [Fe(4)S(4)(SDmp)(3)(THF)(3)] 2. Cluster 2 contains an octahedral iron atom with three THF ligands, and its Fe(S)(3)(O)(3) coordination environment is relevant to that in the active site of substrate-bound aconitase. An analogous reaction of [1](-) with four equiv of HSDmp gave [Fe(4)S(4)(SDmp)(4)](-) 3, which models the oxidized form of HiPIP. The THF ligands in 2 can be replaced by tetramethyl-imidazole (Me(4)Im) to give [Fe(4)S(4)(SDmp)(3)(Me(4)Im)] 4 modeling the [Fe(4)S(4)(Cys)(3)(His)] cluster in hydrogenases, and its one-electron reduced form [4](-) was synthesized from the reaction of 3 with Me(4)Im. The reversible redox couple between 3 and [3](-) was observed at E(1/2) = -820 mV vs. Ag/Ag(+), and the corresponding reversible couple for 4 and [4](-) is positively shifted by +440 mV. The cyclic voltammogram of 3 also exhibited a reversible oxidation couple, which indicates generation of the all-ferric [Fe(4)S(4)](4+) cluster, [Fe(4)S(4)(SDmp)(4)].
Subject(s)
Bacterial Proteins/metabolism , Ferric Compounds/chemistry , Ferrous Compounds/chemistry , Hydrogenase/metabolism , Iron-Sulfur Proteins/metabolism , Photosynthetic Reaction Center Complex Proteins/metabolism , Bacterial Proteins/chemistry , Crystallography, X-Ray , Ferrous Compounds/chemical synthesis , Furans/pharmacology , Hydrogenase/chemistry , Iron-Sulfur Proteins/chemistry , Models, Chemical , Models, Molecular , Molecular Structure , Oxidation-Reduction , Photosynthetic Reaction Center Complex Proteins/chemistryABSTRACT
High-yield synthesis of the iron-sulfur cluster [{N(SiMe(3))(2)}{SC(NMe(2))(2)}Fe(4)S(3)](2)(mu(6)-S) {mu-N(SiMe(3))(2)}(2) (1), which reproduces the [8Fe-7S] core structure of the nitrogenase P(N)-cluster, has been achieved via two pathways: (1) Fe{N(SiMe(3))(2)}(2) + HSTip (Tip = 2,4,6-(i)Pr(3)C(6)H(2)) + tetramethylthiourea (SC(NMe(2))(2)) + elemental sulfur (S(8)); and (2) Fe(3){N(SiMe(3))(2)}(2)(mu-STip)(4) (2) + HSTip + SC(NMe(2))(2) + S(8). The thiourea and terminal amide ligands of 1 were found to be replaceable by thiolate ligands upon treatment with thiolate anions and thiols at -40 degrees C, respectively, and a series of [8Fe-7S] clusters bearing two to four thiolate ligands have been synthesized and their structures were determined by X-ray analysis. The structures of these model [8Fe-7S] clusters all closely resemble that of the reduced form of P-cluster (P(N)) having 8Fe(II) centers, while their 6Fe(II)-2Fe(III) oxidation states correspond to the oxidized form of P-cluster (P(OX)). The cyclic voltammograms of the [8Fe-7S] clusters reveal two quasi-reversible one-electron reduction processes, leading to the 8Fe(II) state that is the same as the P(N)-cluster, and the synthetic models demonstrate the redox behavior between the two major oxidation states of the native P-cluster. Replacement of the SC(NMe(2))(2) ligands in 1 with thiolate anions led to more negative reduction potentials, while a slight positive shift occurred upon replacement of the terminal amide ligands with thiolates. The clusters 1, (NEt(4))(2)[{N(SiMe(3))(2)}(SC(6)H(4)-4-Me)Fe(4)S(3)](2)(mu(6)-S){mu-N(SiMe(3))(2)}(2) (3a), and [(SBtp){SC(NMe(2))(2)}Fe(4)S(3)](2)(mu(6)-S){mu-N(SiMe(3))(2)}(2) (5; Btp = 2,6-(SiMe(3))(2)C(6)H(3)) are EPR silent at 4-100 K, and their temperature-dependent magnetic moments indicate a singlet ground state with antiferromagnetic couplings among the iron centers. The (57)Fe Mössbauer spectra of these clusters are consistent with the 6Fe(II)-2Fe(III) oxidation state, each exhibiting two doublets with an intensity ratio of ca. 1:3, which are assignable to Fe(III) and Fe(II), respectively. Comparison of the quadrupole splittings for 1, 3a, and 5 has led to the conclusion that two Fe(III) sites of the clusters are the peripheral iron atoms.
Subject(s)
Bacterial Proteins/chemistry , Nitrogenase/chemistry , Electrochemistry , Electrons , Models, Molecular , Molecular StructureABSTRACT
A C-H bond of Cp*(2)Co was found to be cleaved by a [Fe(8)S(7)] cluster model of the nitrogenase P-cluster. This is the first example of C-H bond activation mediated by a biologically relevant Fe/S cluster. The reaction mechanism probably consists of electron transfer from Cp*(2)Co to the [Fe(8)S(7)] cluster and subsequent proton abstraction by the reduced form of the cluster.
