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, 39 (12), 3057-64

The Iron-Site Structure of [Fe]-hydrogenase and Model Systems: An X-ray Absorption Near Edge Spectroscopy Study

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The Iron-Site Structure of [Fe]-hydrogenase and Model Systems: An X-ray Absorption Near Edge Spectroscopy Study

Marco Salomone-Stagni et al. Dalton Trans.

Abstract

The [Fe]-hydrogenase is an ideal system for studying the electronic properties of the low spin iron site that is common to the catalytic centres of all hydrogenases. Because they have no auxiliary iron-sulfur clusters and possess a cofactor containing a single iron centre, the [Fe]-hydrogenases are well suited for spectroscopic analysis of those factors required for the activation of molecular hydrogen. Specifically, in this study we shed light on the electronic and molecular structure of the iron centre by XAS analysis of [Fe]-hydrogenase from Methanocaldococcus jannashii and five model complexes (Fe(ethanedithiolate)(CO)(2)(PMe(3))(2), [K(18-crown-6)](2)[Fe(CN)(2)(CO)(3)], K[Fe(CN)(CO)(4)], K(3)[Fe(III)(CN)(6)], K(4)[Fe(II)(CN)(6)]). The different electron donors have a strong influence on the iron absorption K-edge energy position, which is frequently used to determine the metal oxidation state. Our results demonstrate that the K-edges of Fe(II) complexes, achieved with low-spin ferrous thiolates, are consistent with a ferrous centre in the [Fe]-hydrogenase from Methanocaldococcus jannashii. The metal geometry also strongly influences the XANES and thus the electronic structure. Using in silico simulation, we were able to reproduce the main features of the XANES spectra and describe the effects of individual donor contributions on the spectra. Thereby, we reveal the essential role of an unusual carbon donor coming from an acyl group of the cofactor in the determination of the electronic structure required for the activity of the enzyme.

Figures

Fig. 1
Fig. 1
On the left: structure of the [Fe]-hydrogenase octahedral metal binding site as modelled by EXAFS analysis. The iron is coordinated to: the Cys176-sulfur, two CO and the pyridinol-sp2-hybridized nitrogen and the acyl-carbon of the cofactor. An “unknown” donor, here represented as X, is associated to the iron trans to the acyl carbon. This is considered the vacant position ready for H2 binding. The model complexes are represented in A–E: Fe(ii)(edt)(CO)2(PMe3)2 (A); K2[Fe(0)(CN)2(CO)3] (B); K[Fe(0)(CN)(CO)4] (C); K3[Fe(iii)(CN)6] (D); K4[Fe(ii)(CN)6] (E).
Fig. 2
Fig. 2
Comparison among the experimental XANES of the model systems and jHmd-wt. Black line: jHmd-wt; black broken line: jHmd-CN; red line: Fe(ii)(edt)(CO)2(PMe3)2 (A); blue line: [K(18-crown-6)]2[Fe(0)(CN)2(CO)3] (B); green line: K[Fe(0)(CN)(CO)4] (C); orange line: K3[Fe(iii)(CN)6] (D); purple line: K4[Fe(ii)(CN)6] (E).
Fig. 3
Fig. 3
Comparison between experimental and simulated spectra of jHmd-wt. (A) the XANES and in (B) its 1st derivative is shown. The full lines represent the experimental data, while the broken lines represent the simulations.
Fig. 4
Fig. 4
Comparison between the edge positions of the model compounds, jHmd-wt and jHmd-CN. Black line: jHmd-wt; black broken line: jHmd-CN; red line: Fe(ii)(edt)(CO)2(PMe3)2 (A); blue line: [K(18-crown-6)]2[Fe(0)(CN)2(CO)3] (B); green line: K[Fe(0)(CN)(CO)4] (C); orange line: K3[Fe(iii)(CN)6] (D); purple line: K4[Fe(ii)(CN)6] (E). For each sample, the edge position—first maximum of the first derivative in the rising edge—is indicated.
Fig. 5
Fig. 5
Comparison among the simulated XANES of the model systems and jHmd-wt. Black line: jHmd-wt; red line Fe(ii)(edt)(CO)2(PMe3)2 (A); blue line: [K(18-crown-6)]2[Fe(0)(CN)2(CO)3] (B); green line: K[Fe(0)(CN)(CO)4] (C); orange line: K3[Fe(iii)(CN)6] (D); purple line: K4[Fe(ii)(CN)6] (E).
Fig. 6
Fig. 6
Comparison between the first derivative of jHmd-wt XANES simulation in the full coordination (dotted lines) and simulations omitting one donor at the time (full lines): (A) CO1 omitted, (B) CO2 omitted, (C) acyl-C of the cofactor omitted, (D) pyridol-N of the cofactor omitted, (E) Cys176-S omitted, (F) oxygen omitted. The comparisons for the XANES simulations are shown in Fig. S4.‡

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