Accessibility and selective stabilization of the principal spin states of iron by pyridyl versus phenolic ketimines: modulation of the 6A1 ↔ 2T2 ground-state transformation of the [FeN4O2]+ chromophore.

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2'-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH(2)CH(2)NH(2), Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe(II)(apRen)(2)]X(2) (R = H, Me; X(-) = ClO(4)(-), BPh(4)(-), PF(6)(-)) and [Fe(III)(hapRen)(2)]X (R = Me, Et; X(-) = ClO(4)(-), BPh(4)(-)). Single-crystal X-ray analyses of [Fe(II)(apRen)(2)](ClO(4))(2) (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe(II)-N bond distances (1.896-2.041 Å) pointing to the (1)A(1) (d(π)(6)) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe(III)(hapMen)(2)]ClO(4), determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the (2)T(2) (d(π)(5)) ground state. Magnetic susceptibility measurements along with EPR and Mössbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe(III)N(4)O(2)](+) chromophore was modulated with alkyl substituents to afford two-step and one-step (6)A(1) ↔ (2)T(2) transformations in [Fe(III)(hapMen)(2)]ClO(4) and [Fe(III)(hapEen)(2)]ClO(4), respectively. Previously, none of the X-salRen- and X-sal(2)trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d(π) → p(π)* and p(π) → d(π) CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H(2)salen-type quadridentate ketimine H(2)hapen complexed with an equivalent amount of iron(III) to afford the µ-oxo-monobridged dinuclear complex [{Fe(III)(hapen)}(2)(µ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ -99 cm(-1)).

Coordination versatility of tridentate pyridyl aroylhydrazones towards iron: tracking down the elusive aroylhydrazono-based ferric spin-crossover molecular materials.

The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL(1)) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL(2)) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe(II)(HL)(2)](ClO(4))(2), [Fe(II)L(2)] and [Fe(III)L(2)]ClO(4) (HL = HL(1) or HL(2)) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe(III)(L(1))(2)]ClO(4) is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe(III)(L(1))(2)]ClO(4) and [Fe(III)(L(2))(2)]ClO(4) are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the (6)A(1)↔(2)T(2) transition curve is sigmoidal with T(½)∼280 K and the SCO is virtually complete. As regards [Fe(III)(L(1))(2)]ClO(4), Mössbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.

Nickel coordination is regulated by the DNA-bound state of NikR.

The uptake of nickel in Escherichia coli and other microorganisms is transcriptionally regulated by the NikR repressor or its homologs. Here we report the structure of the high-affinity nickel-binding site in NikR and show that it responds dramatically to DNA binding. X-ray absorption spectroscopy reveals that nickel in the holo-NikR protein is bound in a novel four-coordinate planar site consisting of two histidines, one additional O- or N-donor ligand and one S-donor ligand. Site-directed mutation of His87, His89, Cys95 or Glu97 in NikR to alanine eliminates high-affinity nickel binding and abolishes DNA binding but maintains stable protein folding. An unanticipated feature of the NikR structure is that the nickel coordination responds to DNA binding. A six-coordinate nickel site composed of O- or N-donor ligands, but lacking cysteine, forms when NikR binds to operator DNA. Because nickel binding and DNA binding are mediated by different domains within NikR, a communication link between the two domains is implicated, consistent with the finding that the nickel-binding site in a fragment corresponding to the C-terminal domain of NikR is structurally distinct from that found in holo-NikR.

Use of XAS for the elucidation of metal structure and function: applications to nickel biochemistry, molecular toxicology, and carcinogenesis.

Nickel has been shown to be an essential trace element involved in the metabolism of several species of bacteria, archea, and plants. In these organisms, nickel is involved in enzymes that catalyze both non-redox (e.g., urease, glyoxalase I) and redox (e.g., hydrogenase, carbon monoxide dehydrogenase, superoxide dismutase) reactions, and proteins involved in the transport, storage, metallocenter assembly, and regulation of nickel concentration have evolved. Studies of structure/function relationships in nickel biochemistry reveal that cysteine ligands are used to stabilize the Ni(III/II) redox couple. Certain nickel compounds have also been shown to be potent human carcinogens. A likely target for carcinogenic nickel is nuclear histone proteins. Here we present X-ray absorption spectroscopic studies of a model Ni peptide designed to help characterize the structure of the nickel complexes formed with histones and place them in the context of nickel structure/function relationships, to gain insights into the molecular mechanism of nickel carcinogenesis.

XAS investigation of the structure and function of Ni in acireductone dioxygenase.

Acireductone dioxygenases (ARDs) are enzymes involved in the methionine recycle pathway, which regulates aspects of the cell cycle. Klebsiella pneumoniae produces two enzymes that share a common polypeptide sequence and differ only in the metal ion present. Reaction of acireductone (1,2-dihydroxy-3-keto-5-methylthiopentene) with Fe-ARD and dioxygen produces formate and 2-keto-4-methylthiobutanoic acid, the alpha-ketoacid precursor of methionine. Ni-ARD reacts with acireductone and dioxygen to produce methylthiopropionate, CO, and formate and does not lie on the methionine recycle pathway. An X-ray absorption spectroscopy (XAS) study of the structure of the catalytic Ni center in resting Ni-ARD enzyme and the enzyme-substrate complex is reported. This study establishes the structure of the Ni site in resting Ni-ARD as containing a six coordinate Ni site composed of O/N-donor ligands including 3-4 histidine residues, demonstrates that the substrate binds to the Ni center in a bidentate fashion by displacing two ligands, at least one of which is a histidine ligand, and provides insight into the mechanism of catalysis employed by a Ni-containing dioxygenase. Efficiently relaxed and hyperfine-shifted resonances are observed in the (1)H nuclear magnetic resonance spectrum of Ni-ARD that can be attributed to the His imidazoles ligating the paramagnetic Ni ion and are consistent with the XAS results regarding His ligation. These resonances show significant perturbation in the presence of substrate, confirming that the metal ion interacts directly with the substrate.