X-ray Magnetic Circular Dichroism of Pseudomonas aeruginosa Nickel(II) Azurin

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L3-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) μ_B. Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d_x^2-y^2 and 3d_z^2 are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d_x^2_(-y)^2 having much greater ligand character

Thin films of non-stoichiometric perovskites as potential oxygen sensors

Thin films of non-stoichiometric perovskites of general formula ABO\u2082.\u2085 + x have been grown, on sapphire substrates, by the technique of pulsed laser ablation/deposition. The structural properties of the films upon exposure to atmospheres of various oxygen pressures, at elevated temperatures, have been determined by X-ray diffraction; they show changes which correlate with those found for bulk powder samples. For the material SrFeO\u2082.\u2085 + x, with 0 64 x 64 0.5, pressure-composition isotherms reveal several pressure ranges over which structural changes occur. By monitoring the changes in physical properties that accompany the bulk chemical changes, this system can be exploited to provide a thin-film sensor, the structural changes of which are specific to oxygen.Peer reviewed: YesNRC publication: Ye

π covalency in the halogen bond

Current models of halogen bonding describe the σ-symmetric component of this interaction but do not contemplate the possibility of π-covalency. Here the authors provide experimental and computational evidence of π-covalency in halogen bonds involving radical cation halogen bond donors

Spectroscopy of Non-Heme Iron Thiolate Complexes: Insight into the Electronic Structure of the Low-Spin Active Site of Nitrile Hydratase

Detailed spectroscopic and computational studies of the low-spin iron complexes [FeIII(S2Me2N3(Pr,Pr))(N3)] (1) and [FeIII(S2Me2N3(Pr,Pr))]1+ (2) were performed to investigate the unique electronic features of these species and their relation to the low-spin ferric active sites of nitrile hydratases. Low-temperature UV/vis/NIR and MCD spectra of 1 and 2 reflect electronic structures that are dominated by antibonding interactions of the Fe 3d manifold and the equatorial thiolate S 3p orbitals. The six-coordinate complex 1 exhibits a low-energy Sπ → Fe 3dxy (∼13000 cm-1) charge-transfer transition that results predominantly from the low energy of the singly occupied Fe 3dxy orbital, due to pure π interactions between this acceptor orbital and both thiolate donor ligands in the equatorial plane. The 3dπ → 3dσ ligand-field transitions in this species occur at higher energies (>15000 cm-1), reflecting its near-octahedral symmetry. The Fe 3dxz,yz → Fe 3dxy (dπ → dπ) transition occurs in the near-IR and probes the FeIII−S π-donor bond; this transition reveals vibronic structure that reflects the strength of this bond (νe ≈ 340 cm-1). In contrast, the ligand-field transitions of the five-coordinate complex 2 are generally at low energy, and the Sπ → Fe charge-transfer transitions occur at much higher energies relative to those in 1. This reflects changes in thiolate bonding in the equatorial plane involving the Fe 3dxy and Fe 3dx2-y2 orbitals. The spectroscopic data lead to a simple bonding model that focuses on the σ and π interactions between the ferric ion and the equatorial thiolate ligands, which depend on the S−Fe−S bond angle in each of the complexes. These electronic descriptions provide insight into the unusual S = 1/2 ground spin state of these complexes:  the orientation of the thiolate ligands in these complexes restricts their π-donor interactions to the equatorial plane and enforces a low-spin state. These anisotropic orbital considerations provide some intriguing insights into the possible electronic interactions at the active site of nitrile hydratases and form the foundation for further studies into these low-spin ferric enzymes

Evidence for halogen bond covalency in acyclic and interlocked halogen-bonding receptor anion recognition.

The synthesis and anion binding properties of novel halogen-bonding (XB) bis-iodotriazole-pyridinium-containing acyclic and [2]catenane anion host systems are described. The XB acyclic receptor displays selectivity for acetate over halides with enhanced anion recognition properties compared to the analogous hydrogen-bonding (HB) acyclic receptor. A reversal in halide selectivity is observed in the XB [2]catenane, in comparison to the acyclic XB receptor, due to the interlocked host's unique three-dimensional binding cavity, and no binding is observed for oxoanions. Notable halide anion association constant values determined for the [2]catenane in competitive organic-aqueous solvent mixtures demonstrate considerable enhancement of anion recognition as compared to the HB catenane analogue. X-ray crystallographic analysis of a series of halide catenane complexes reveal strong XB interactions in the solid state. These interactions were studied using Cl and Br K-edge X-ray Absorption Spectroscopy (XAS) indicating intense pre-edge features characteristic of charge transfer from the halide to its bonding partner (σ(AX←X(-))(*) ← X1s), and providing a direct measure of the degree of covalency in the halogen bond(s). The data reveal that the degree of covalency is similar to that which is observed in transition metal coordinate covalent bonds. These results are supported by DFT results, which correlate well with the experimental data