Imidazole-containing (N3S)-Ni(II) complexes relating to nickel containing biomolecules

A Series of Ni Complexes Based on a Versatile ATCUN-Like Tripeptide Scaffold to Decipher Key Parameters for Superoxide Dismutase Activity

The cellular level of reactive oxygen species (ROS) has to be controlled to avoid some pathologies, especially those linked to oxidative stress. One strategy for designing antioxidants consists of modeling natural enzymes involved in ROS degradation. Among them, nickel superoxide dismutase (NiSOD) catalyzes the dismutation of the superoxide radical anion, O₂^•-, into O₂ and H₂O₂. We report here Ni complexes with tripeptides derived from the amino-terminal Cu^II- and Ni^II-binding (ATCUN) motif that mimics some structural features found in the active site of the NiSOD. A series of six mononuclear Ni^II complexes were investigated in water at physiological pH with different first coordination spheres, from compounds with a N3S to N2S2 set, and also complexes that are in equilibrium between the N-coordination (N3S) and S-coordination (N2S2). They were fully characterized by a combination of spectroscopic techniques, including ¹H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations and their redox properties studied by cyclic voltammetry. They all display SOD-like activity, with a k_cat ranging between 0.5 and 2.0 × 10⁶ M^-1 s^-1. The complexes in which the two coordination modes are in equilibrium are the most efficient, suggesting a beneficial effect of a nearby proton relay.

The crystal structure of hexakis(1-ethylimidazole-κ1 N)nickel(II) dichloride – 1-ethylimidazole (1/2), C40H64Cl2NiN16

C40H64Cl2NiN16, monoclinic, P21/c (no. 14), a = 8.2320(9) Å, b = 16.2902(16) Å, c = 17.3843(17) Å, α = 90°, β = 101.773(3)°, γ = 90°, V = 2282.2(4) Å3, Z = 2, R gt (F) = 0.0448, wR ref (F 2) = 0.1181, T = 298(2) K.

Thiolate Coordination vs C-S Bond Cleavage of Thiolates in Dinickel(II) Complexes

A detailed study for the synthesis of dinickel(II)-thiolate and dinickel(II)-hydrosulfide complexes and the complete characterization of the relevant intermediates involved in the C-S bond cleavage of thiolates are presented. Hydrated Ni(II) salts mediate the hydrolytic C-S bond cleavage of thiolates (NaSR/RSH; R = Me, Et, ⁿBu, ^tBu), albeit inefficiently, to yield a mixture of a dinickel(II)-hydrosulfide complex, [Ni₂(BPMP)(μ-SH)(DMF)₂]²⁺ (1), and the corresponding dinickel(II)-thiolate complexes, such as [Ni₂(BPMP)(μ-SEt)(ClO₄)]¹⁺ (2) (HBPMP is 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol). A systematic study for the reactivity of thiolates with Ni(II) was therefore pursued which finally yielded 1 as a pure product which has been characterized in comparison with the dinickel(II)-dichloride complex, [Ni₂(BPMP)(Cl)₂(MeOH)₂]¹⁺ (3). While the reaction of thiolates with anhydrous Ni(OTf)₂ in dry conditions could only yield [Ni₂(BPMP)(OTf)₂]¹⁺ (5) instead of the expected dinickel(II)-thiolate compound, the C-S bond cleavage could be suppressed by the use of a chelating thiol, such as PhCOSH, to yield [Ni₂(BPMP)(SCOPh)₂]¹⁺ (6). Finally, with the suitable choice of a monodentate thiol, a dinickel(II)-monothiolate complex, [Ni₂(BPMP)(SPh)(DMF)(MeOH)(H₂O)]²⁺ (7), was isolated as a pure product within 1 h of reaction, which after a longer time of reaction yielded 1 and PhOH. Complex 7 may thus be regarded as the intermediate that precedes the C-S bond cleavage and is generated by the reaction of a thiolate with an initially formed dinickel(II)-solvento complex, [Ni₂(BPMP)(MeOH)₂(H₂O)₂]³⁺(4). Selected dinickel(II) complexes were explored further for the scope of substitution reactions, and the results include the isolation of a dinickel(II)-bis(thiolate) complex, [Ni₂(BPMP)(μ-SPh)₂]¹⁺ (8).

A bioinspired thiolate-bridged dinickel complex with a pendant amine: synthesis, structure and electrocatalytic properties

By employing X(CH₂CH₂S^-)₂ (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel complexes supported by phosphine ligands, [(dppe)Ni(μ-1SSS':2SS-tpdt)Ni(dppe)][PF₆]₂ (1[PF₆]₂, dppe = Ph₂P(CH₂)₂PPh₂), [(dppe)Ni(μ-1SSN:2SS-npdt)Ni(dppe)][PF₆]₂ (2[PF₆]₂) and [(dppe)Ni(t-Cl)(μ-1SSX:2SS-C₄H₈S₂X)Ni(dppe)][PF₆] (3[PF₆], X = S; 4[PF₆], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF₆]₂, elongation of the Ni-N bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two nickel centers in solution, which is evidenced by ³¹P{¹H} NMR spectroscopic results. Furthermore, these thiolate-bridged dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF₆]₂ featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at a relatively low overpotential.

Mononuclear Ni(II) Complexes with a S3O Coordination Sphere Based on a Tripodal Cysteine-Rich Ligand: pH Tuning of the Superoxide Dismutase Activity

The superoxide dismutase (SOD) activity of mononuclear Ni^II complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide ligand, derived from nitrilotriacetic acid (NTA), where the three acid functions are grafted with cysteines (L^3S). Two mononuclear complexes, which exist in pH-dependent proportions, have been fully characterized by a combination of spectroscopic techniques including ¹H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations. They display similar square-planar S3O coordination, with the three thiolates of the three cysteine moieties from L^3S coordinated to the Ni^II ion, together with either a water molecule at physiological pH, as [NiL^3S(OH₂)]^-, or a hydroxo ion in more basic conditions, as [NiL^3S(OH)]^2-. The ¹H NMR study has revealed that contrary to the hydroxo ligand, the bound water molecule is labile. The cyclic voltammogram of both complexes displays an irreversible one-electron oxidation process assigned to the Ni^II/Ni^III redox system with E_pa = 0.48 and 0.31 V versus SCE for NiL^3S(OH₂) and NiL^3S(OH), respectively. The SOD activity of both complexes has been tested. On the basis of the xanthine oxidase assay, an IC₅₀ of about 1 μM has been measured at pH 7.4, where NiL^3S(OH₂) is mainly present (93% of the Ni^II species), while the IC₅₀ is larger than 100 μM at pH 9.6, where NiL^3S(OH) is the major species (92% of the Ni^II species). Interestingly, only NiL^3S(OH₂) displays SOD activity, suggesting that the presence of a labile ligand is required. The SOD activity has been also evaluated under catalytic conditions at pH 7.75, where the ratio between NiL^3S(OH₂)/ NiL^3S(OH) is about (86:14), and a rate constant, k_cat = 1.8 × 10⁵ M^-1 s^-1, has been measured. NiL^3S(OH₂) is thus the first low-molecular weight, synthetic, bioinspired Ni complex that displays catalytic SOD activity in water at physiological pH, although it does not contain any N-donor ligand in its first coordination sphere, as in the NiSOD. Overall, the data show that a key structural feature is the presence of a labile ligand in the coordination sphere of the Ni^II ion.

pH Dependent Reversible Formation of a Binuclear Ni2 Metal-Center Within a Peptide Scaffold

A disulfide-bridged peptide containing two Ni2+ binding sites based on the nickel superoxide dismutase protein, {Ni2(SODmds)}, has been prepared. At physiological pH (7.4) it was found that the metal sites are mononuclear with a square planar NOS2 coordination environment with the two sulfur-based ligands derived from cysteinate residues, the nitrogen ligand derived from the amide backbone and a water ligand. Furthermore, S K-edge X-ray absorption spectroscopy indicated that the two cysteinate sulfur atoms ligated to nickel are each protonated. Elevation of the pH to 9.6 results in the deprotonation of the cysteinate sulfur atoms, and yields a binuclear, cysteinate bridged Ni22+ center with each nickel contained in a distorted square planar geometry. At both pH = 7.4 and 9.6 the nickel sites are moderately air sensitive, yielding intractable oxidation products. However, at pH = 9.6 {Ni2(SODmds)} reacts with O2 at an ~3.5-fold faster rate than at pH = 7.4. Electronic structure calculations indicate the reduced reactivity at pH = 7.4 is a result of a reduction in S(3p) character and deactivation of the nucleophilic frontier molecular orbitals upon cysteinate sulfur protonation.

(2-{[2-(Dimethylamino)ethyl]iminomethyl}benzenethiolato-κ3 N,N′,S)(4-methoxybenzenethiolato-κS)nickel(II)

In the title compound, [Ni(C11H15N2S)(C7H7OS)] or [Ni(NNImS)(4-OCH3PhS)] (NNImS = 2-{[2-(dimethylamino)ethyl]iminomethyl}benzenethiolato), the NiII cation is coordinated by a tridentate NNImS ligand and a monodentate thiolate ligand giving an N2S2 coordination set defining an almost square-planar environment. The Ni—Namine bond in the coordination plane is approximately 0.1 Å longer than the Ni—Nimine bond.

High-affinity metal binding by the Escherichia coli [NiFe]-hydrogenase accessory protein HypB is selectively modulated by SlyD

[NiFe]-hydrogenase, which catalyzes the reversible conversion between hydrogen gas and protons, is a vital component of the metabolism of many pathogens. Maturation of [NiFe]-hydrogenase requires selective nickel insertion that is completed, in part, by the metallochaperones SlyD and HypB. Escherichia coli HypB binds nickel with sub-picomolar affinity, and the formation of the HypB-SlyD complex activates nickel release from the high-affinity site (HAS) of HypB. In this study, the metal selectivity of this process was investigated. Biochemical experiments revealed that the HAS of full length HypB can bind stoichiometric zinc. Moreover, in contrast to the acceleration of metal release observed with nickel-loaded HypB, SlyD blocks the release of zinc from the HypB HAS. X-ray absorption spectroscopy (XAS) demonstrated that SlyD does not impact the primary coordination sphere of nickel or zinc bound to the HAS of HypB. Instead, computational modeling and XAS of HypB loaded with nickel or zinc indicated that zinc binds to HypB with a different coordination sphere than nickel. The data suggested that Glu9, which is not a nickel ligand, directly coordinates zinc. These results were confirmed through the characterization of E9A-HypB, which afforded weakened zinc affinity compared to wild-type HypB but similar nickel affinity. This mutant HypB fully supports the production of [NiFe]-hydrogenase in E. coli. Altogether, these results are consistent with the model that the HAS of HypB functions as a nickel site during [NiFe]-hydrogenase enzyme maturation and that the metal selectivity is controlled by activation of metal release by SlyD.

Steric Enforcement about One Thiolate Donor Leads to New Oxidation Chemistry in a NiSOD Model Complex

Ni-containing superoxide dismutase (NiSOD) represents an unusual member of the SOD family due to the presence of oxygen-sensitive Ni-SCys bonds at its active site. Reported in this account is the synthesis and properties of the Ni^II complex of the N₃S₂ ligand [N₃S₂^Me2]^3- ([N₃S₂^Me2]^3- = deprotonated form of 2-((2-mercapto-2-methylpropyl)(pyridin-2-ylmethyl)amino)-N-(2-mercaptoethyl)acetamide), namely Na[Ni(N₃S₂^Me2)] (2), as a NiSOD model that features sterically robust gem-(CH₃)₂ groups on the thiolate α-C positioned trans to the carboxamide. The crystal structure of 2, coupled with spectroscopic measurements from ¹H NMR, X-ray absorption, IR, UV-vis, and mass spectrometry (MS), reveal a planar Ni^II (S = 0) ion coordinated by only the N₂S₂ basal donors of the N₃S₂ ligand. While the structure and spectroscopic properties of 2 resemble those of NiSOD_red and other models, the asymmetric S ligands open up new reaction paths upon chemical oxidation. One unusual oxidation product is the planar Ni^II-N₃S complex [Ni(L^ox)] (5; L^ox = 2-(5,5-dimethyl-2-(pyridin-2-yl)thiazolidin-3-yl)-N-(2-mercaptoethyl)acetamide), where two-electron oxidation takes place at the substituted thiolate and py-CH₂ carbon to generate a thiazolidine heterocycle. Electrochemical measurements of 2 reveal irreversible events wholly consistent with thiolate redox, which were identified by comparison to the Zn^II complex Na[Zn(N₃S₂^Me2)] (3). Although no reaction is observed between 2 and azide, reaction of 2 with superoxide produces multiple products on the basis of UV-vis and MS data, one of which is 5. Density functional theory (DFT) computations suggest that the HOMO in 2 is π* with primary contributions from Ni-dπ/S-pπ orbitals. These contributions can be modulated and biased toward Ni when electron-withdrawing groups are placed on the thiolate α-C. Analysis of the oxidized five-coordinate species 2^ox* by DFT reveal a singly occupied spin-up (α) MO that is largely thiolate based, which supports the proposed Ni^III-thiolate/Ni^II-thiyl radical intermediates that ultimately yield 5 and other products.

Design and reactivity of Ni-complexes using pentadentate neutral-polypyridyl ligands: Possible mimics of NiSOD

Superoxide plays a key role in cell signaling, but can be cytotoxic within cells unless well regulated by enzymes known as superoxide dismutases (SOD). Nickel superoxide dismutase (NiSOD) catalyzes the disproportion of the harmful superoxide radical into hydrogen peroxide and dioxygen. NiSOD has a unique active site structure that plays an important role in tuning the potential of the nickel center to function as an effective catalyst for superoxide dismutation with diffusion controlled rates. The synthesis of structural and functional analogues of NiSOD provides a route to better understand the role of the nickel active site in superoxide dismutation. In this work, the synthesis of a series of nickel complexes supported by nitrogen rich pentadentate ligands is reported. The complexes have been characterized through absorption spectroscopy, mass spectrometry, and elemental analysis. X-ray absorption spectroscopy was employed to establish the oxidation state and the coordination geometry around the metal center. The reactivity of these complexes toward KO₂ was evaluated to elucidate the role of the coordination sphere in controlling superoxide dismutation reactivity.

Synthesis and Speciation-Dependent Properties of a Multimetallic Model Complex of NiSOD That Exhibits Unique Hydrogen-Bonding

The complex Na₃[{Ni^II(nmp)}₃S₃BTA^alk)] (1) (nmp^2- = deprotonated form of N-(2-mercaptoethyl)picolinamide; H₃S₃BTA^alk = N¹,N³,N⁵-tris(2-mercaptoethyl)benzene-1,3,5-tricarboxamide, where H = dissociable protons), supported by the thiolate-benzenetricarboxamide scaffold (S₃BTA^alk), has been synthesized as a trimetallic model of nickel-containing superoxide dismutase (NiSOD). X-ray absorption spectroscopy (XAS) and ¹H NMR measurements on 1 indicate that the Ni^II centers are square-planar with N₂S₂ coordination, and Ni-N and Ni-S distances of 1.95 and 2.16 Å, respectively. Additional evidence from IR indicates the presence of H-bonds in 1 from the approximately -200 cm^-1 shift in ν_NH from free ligand. The presence of H-bonds allows for speciation that is temperature-, concentration-, and solvent-dependent. In unbuffered water and at low temperature, a dimeric complex (1^A; λ = 410 nm) that aggregates through intermolecular NH···O═C bonds of BTA units is observed. Dissolution of 1 in pH 7.4 buffer or in unbuffered water at temperatures above 50 °C results in monomeric complex (1^M; λ = 367 nm) linked through intramolecular NH···S bonds. DFT computations indicate a low energy barrier between 1^A and 1^M with nearly identical frontier MOs and Ni-ligand metrics. Notably, 1^A and 1^M exhibit remarkable stability in protic solvents such as MeOH and H₂O, in stark contrast to monometallic [Ni^II(nmp)(SR)]^- complexes. The reactivity of 1 with excess O₂, H₂O₂, and O₂^•- is species-dependent. IR and UV-vis reveal that 1^A in MeOH reacts with excess O₂ to yield an S-bound sulfinate, but does not react with O₂^•-. In contrast, 1^M is stable to O₂ in pH 7.4 buffer, but reacts with O₂^•- to yield a putative [Ni^II(nmp)(O₂)]^- complex from release of the BTA-thiolate based on EPR.

Novel nickel(ii) complexes of sterically modified linear N4 ligands: effect of ligand stereoelectronic factors and solvent of coordination on nickel(ii) spin-state and catalytic alkane hydroxylation

The donor atom type and diazacyclo backbone of the ligands and solvent of coordination dictate the Ni(ii) spin state (4, LS; 1–3, 5, HS) and catalytic activity of complexes.

Mechanism of Selective Nickel Transfer from HypB to HypA, Escherichia coli [NiFe]-Hydrogenase Accessory Proteins

[NiFe]-hydrogenase enzymes catalyze the reversible reduction of protons to molecular hydrogen and serve as a vital component of the metabolism of many pathogens. The synthesis of the bimetallic catalytic center requires a suite of accessory proteins, and the penultimate step, nickel insertion, is facilitated by the metallochaperones HypA and HypB. In Escherichia coli, nickel moves from a site in the GTPase domain of HypB to HypA in a process accelerated by GDP. To determine how the transfer of nickel is controlled, the impacts of HypA and nucleotides on the properties of HypB were examined. Integral to this work was His2Gln HypA, a mutant with attenuated nickel affinity that does not support hydrogenase production in E. coli. This mutation inhibits the translocation of nickel from HypB. H2Q-HypA does not modulate the apparent metal affinity of HypB, but the stoichiometry and stability of the HypB-nickel complex are modulated by the nucleotide. Furthermore, the HypA-HypB interaction was detected by gel filtration chromatography if HypB was loaded with GDP, but not a GTP analogue, and the protein complex dissociated upon binding of nickel to His2 of HypA. In contrast, a nucleotide does not modulate the binding of zinc to HypB, and loading zinc into the GTPase domain of HypB inhibits formation of the complex with HypA. These results demonstrate that GTP hydrolysis controls both metal binding and protein-protein interactions, conferring selective and directional nickel transfer during [NiFe]-hydrogenase biosynthesis.

Synthesis and Spectral Studies of Ni(II) Dithiocarbamate Complexes and Their Use as Precursors for Nickel Sulphides Nanocrystals

Ni(II) dithiocarbamate complexes have been synthesized and characterized by UV-Vis, FTIR, and NMR spectroscopic techniques. Electronic spectra measurements indicate that the complexes are four-coordinate square planar geometry while the FTIR confirmed that the dithiocarbamates act as bidentate chelating ligands. The compounds were used as single source precursors and thermolysed at 220°C to prepare HDA-capped NiS nanocrystals which were characterized by absorption and photoluminescence (PL) spectra measurements, powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Absorption spectra studies showed that the synthesized NiS nanoparticles are blue-shifted relative to the bulk material and PL studies showed emission maxima that are red-shifted compared to the absorption band edges. The XRD patterns of the as-prepared NiS nanoparticles revealed cubic crystalline phases. TEM images showed spherical and close-to-spherical nanocrystals with the size in the range 12–38 nm for NiS1, 8–11 nm for NiS2, and 9–16 nm for NiS3. SEM images showed homogeneous surface morphology and EDS confirmed the presence of Ni and S and the formation of NiS nanoparticles.

Coordination properties of dithiobutylamine (DTBA), a newly introduced protein disulfide reducing agent

The acid-base properties and metal-binding abilities of (2S)-2-amino-1,4-dimercaptobutane, otherwise termed dithiobutylamine (DTBA), which is a newly introduced reagent useful for reducing protein and peptide disulfides, were studied in solution using potentiometry, (1)H NMR spectroscopy, spectropolarimetry, and UV-vis spectroscopy. The list of metal ions studied here includes Zn(II), Cd(II), Ni(II), Co(II), and Cu(I). We found that DTBA forms specific and very stable polynuclear and mononuclear complexes with all of these metal ions using both of its sulfur donors. DTBA forms complexes more stable than those of the commonly used disulfide reducing agent DTT, giving it more interference capacity in studies of metal binding in thiol-containing biomolecules. The ability of DTBA to strongly bind metal ions is reflected in its limited properties as a thiol protectant in their presence, which is manifested through slower disulfide reduction kinetics. We found that this effect correlated with the stabilities of the complexes. Additionally, the reducing properties of DTBA toward MMTS-modified papain (MMTS = S-methylmethanethiosulfonate) were also significantly affected by the investigated metal ions. In this case, however, electrostatic interactions and stereospecific effects, rather than metal-binding abilities, were found to be responsible for the reduced protective properties of DTBA. Despite its limitations, a high affinity toward metal ions makes DTBA an attractive agent in competition studies with metalloproteins.

Steric and electronic effects on arylthiolate coordination in the pseudotetrahedral complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate)

Synthesis and characterization of several new pseudotetrahedral arylthiolate complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate; Ar = Ph, 2,4,6-(i)Pr3C6H2, C6H4-4-Cl, C6H4-4-Me, C6H4-4-OMe) are reported, including X-ray crystal structures of the first two complexes. With prior results, two series of complexes are spanned, [(Tp(Ph,Me))Ni-S-2,4,6-RC6H2] (R'' = H, Me, (i)Pr) plus the xylyl analogue [(Tp(Ph,Me))Ni-S-2,6-Me2C6H3], as well as [(Tp(Ph,Me))Ni-S-C6H4-4-Y] (Y = Cl, H, Me, OMe), intended to elucidate steric and/or electronic effects on arylthiolate coordination. In contrast to [(Tp(Me,Me))Ni-SAr] analogues that adopt a sawhorse conformation, the ortho-disubstituted complexes show enhanced trigonal and Ni-S-Ar bending, reflecting the size of the 3-pyrazole substituents. Moreover, weakened scorpionate ligation is implied by spectroscopic data. Little spectroscopic effect is observed in the series of para-substituted complexes, suggesting the observed effects are primarily steric in origin. The relatively electron-rich and encumbered complex [(Tp(Ph,Me))Ni-S-2,4,6-(i)Pr3C6H2] behaves uniquely when dissolved in CH3CN, forming a square planar solvent adduct with a bidentate scorpionate ligand, [(κ(2)-Tp(Ph,Me))Ni(NCMe)(S-2,4,6-(i)Pr3C6H2)]. This adduct was isolated and characterized by X-ray crystallography. Single-point DFT and TD-DFT calculations on a simplified [(κ(2)-Tp)Ni(NCMe)(SPh)] model were used to clarify the electronic spectrum of the adduct, and to elucidate differences between Ni-SAr bonding and spectroscopy between pseudotetrahedral and square planar geometries.

Hexacoordinate nickel(II)/(III) complexes that mimic the catalytic cycle of nickel superoxide dismutase

A functional model complex of nickel superoxide dismutase (NiSOD) with a non-peptide ligand which mimics the full catalytic cycle of NiSOD is unknown. Similarly, it has not been fully elucidated whether NiSOD activity is a result of an outer- or inner-sphere electron-transfer mechanism. With this in mind, two octahedral nickel(II)/(III) complexes of a bis-tridentate N2 S donor carboxamide ligand, N-2-phenylthiophenyl-2'-pyridinecarboxamide (HL(Ph)), have been synthesized, structurally characterized, and their SOD activities examined. These complexes mimic the full catalytic cycle of NiSOD. Electrochemical experiments support an outer-sphere electron-transfer mechanism for their SOD activity.

Metal-binding and redox properties of substituted linear and cyclic ATCUN motifs

The amino-terminal copper and nickel binding (ATCUN) motif is a short peptide sequence found in human serum albumin and other proteins. Synthetic ATCUN-metal complexes have been used to oxidatively cleave proteins and DNA, cross-link proteins, and damage cancer cells. The ATCUN motif consists of a tripeptide that coordinates Cu(II) and Ni(II) ions in a square planar geometry, anchored by chelation sites at the N-terminal amine, histidine imidazole and two backbone amides. Many studies have shown that the histidine is required for tight binding and square planar geometry. Previously, we showed that macrocyclization of the ATCUN motif can lead to high-affinity binding with altered metal ion selectivity and enhanced Cu(II)/Cu(III) redox cycling (Inorg. Chem. 2013, 52, 2729-2735). In this work, we synthesize and characterize several linear and cyclic ATCUN variants to explore how substitutions at the histidine alter the metal-binding and catalytic properties. UV-visible spectroscopy, EPR spectroscopy and mass spectrometry indicate that cyclization can promote the formation of ATCUN-like complexes even in the absence of imidazole. We also report several novel ATCUN-like complexes and quantify their redox properties. These findings further demonstrate the effects of conformational constraints on short, metal-binding peptides, and also provide novel redox-active metallopeptides suitable for testing as catalysts for stereoselective or regioselective oxidation reactions.

Metal binding properties of Escherichia coli YjiA, a member of the metal homeostasis-associated COG0523 family of GTPases

GTPases are critical molecular switches involved in a wide range of biological functions. Recent phylogenetic and genomic analyses of the large, mostly uncharacterized COG0523 subfamily of GTPases revealed a link between some COG0523 proteins and metal homeostasis pathways. In this report, we detail the bioinorganic characterization of YjiA, a representative member of COG0523 subgroup 9 and the only COG0523 protein to date with high-resolution structural information. We find that YjiA is capable of binding several types of transition metals with dissociation constants in the low micromolar range and that metal binding affects both the oligomeric structure and GTPase activity of the enzyme. Using a combination of X-ray crystallography and site-directed mutagenesis, we identify, among others, a metal-binding site adjacent to the nucleotide-binding site in the GTPase domain that involves a conserved cysteine and several glutamate residues. Mutations of the coordinating residues decrease the impact of metal, suggesting that metal binding to this site is responsible for modulating the GTPase activity of the protein. These findings point toward a regulatory function for these COG0523 GTPases that is responsive to their metal-bound state.

The oxidation of phenylhydrazine by tyrosinase

Tyrosinase was found to catalyze the oxidation of phenylhydrazine to phenol in a reaction that did not resemble those typically performed by tyrosinase. The kinetics of this reaction was investigated by measuring the initial velocity of the formation of phenol (25 °C). The values of k cat and K M for the oxidation of phenylhydrazine were obtained as 11.0 s(-1) and 0.30 mM, respectively. The generation of superoxides during the oxidation of phenylhydrazine by tyrosinase was monitored by nitroblue tetrazolium (NBT) assay. In the phenylhydrazine-tyrosinase reaction, 1 mol O2 was required for the production of 1 mol phenol and 1/6 mol superoxide. The decomposition of superoxide by superoxide dismutase enhanced the rate constant of the oxidation of phenylhydrazine. Phenol formed in the oxidation of phenylhydrazine by tyrosinase was further oxidized by tyrosinase to an o-quinone, after the oxidation of phenylhydrazine by tyrosinase was almost completed.

Effects of select histidine to cysteine mutations on transcriptional regulation by Escherichia coli RcnR

The RcnR metalloregulator represses the transcription of the Co(II) and Ni(II) exporter, RcnAB. Previous studies have shown that Co(II) and Ni(II) bind to RcnR in six-coordinate sites, resulting in derepression. Here, the roles of His60, His64, and His67 in specific metal recognition are examined. His60 and His64 correspond to ligands that are important for Cu(I) binding in the homologous Cu(I)-responsive metalloregulator, CsoR. These residues are known to be functionally important in RcnR transcriptional regulation. X-ray absorption spectroscopy (XAS) was used to examine the structure of bound cognate and noncognate metal ions, and lacZ reporter assays were used to assess the transcription of rcnA in response to metal binding in the three His → Cys mutations, H60C, H64C, and H67C. These studies confirm that both Ni(II) and Co(II) use His64 as a ligand. H64C-RcnR is also the only known mutant that retains a Co(II) response while eliminating the response to Ni(II) binding. XAS data indicate that His60 and His67 are potential Co(II) ligands. The effects of the mutations of His60, His64, and His67 on the structures of the noncognate metal ions [Zn(II) and Cu(I)] reveal that these residues have distinctive roles in binding noncognate metals. None of the His → Cys mutants in RcnR confer any response to Cu(I) binding, including H64C-RcnR, where the ligands involved in Cu(I) binding in CsoR are present. These data indicate that while the secondary, tertiary, and quaternary structures of CsoR and RcnR are quite similar, small changes in primary sequence reveal that the specific mechanisms involved in metal recognition are quite different.

Cytosolic Ni(II) sensor in cyanobacterium: nickel detection follows nickel affinity across four families of metal sensors

Efflux of surplus Ni(II) across the outer and inner membranes of Synechocystis PCC 6803 is mediated by the Nrs system under the control of a sensor of periplasmic Ni(II), NrsS. Here, we show that the product of ORF sll0176, which encodes a CsoR/RcnR-like protein now designated InrS (for internal nickel-responsive sensor), represses nrsD (NrsD is deduced to efflux Ni(II) across the inner membrane) from a cryptic promoter between the final two ORFs in the nrs operon. Transcripts initiated from the newly identified nrsD promoter accumulate in response to nickel or cobalt but not copper, and recombinant InrS forms specific, Ni(II)-inhibited complexes with the nrsD promoter region. Metal-dependent difference spectra of Ni(II)- and Cu(I)-InrS are similar to Cu(I)-sensing CsoR and dissimilar to Ni(II)/Co(II)-sensing RcnR, consistent with factors beyond the primary coordination sphere switching metal selectivity. Competition with chelators mag-fura-2, nitrilotriacetic acid, EDTA, and EGTA estimate K_D Ni(II) for the tightest site of InrS as 2.05 (±1.5) × 10⁻¹⁴m, and weaker K_D Ni(II) for the cells' metal sensors of other types: Zn(II) co-repressor Zur, Co(II) activator CoaR, and Zn(II) derepressor ZiaR. Ni(II) transfer to InrS occurs upon addition to Ni(II) forms of each other sensor. InrS binds Ni(II) sufficiently tightly to derepress Ni(II) export at concentrations below K_D Ni(II) of the other sensors.

Dipeptide-based models of nickel superoxide dismutase: solvent effects highlight a critical role to Ni-S bonding and active site stabilization

Nickel superoxide dismutase (Ni-SOD) catalyzes the disproportionation of the superoxide radical to O(2) and H(2)O(2) utilizing the Ni(III/II) redox couple. The Ni center in Ni-SOD resides in an unusual coordination environment that is distinct from other SODs. In the reduced state (Ni-SOD(red)), Ni(II) is ligated to a primary amine-N from His1, anionic carboxamido-N/thiolato-S from Cys2, and a second thiolato-S from Cys6 to complete a NiN(2)S(2) square-planar coordination motif. Utilizing the dipeptide N(2)S(2-) ligand, H(2)N-Gly-l-Cys-OMe (GC-OMeH(2)), an accurate model of the structural and electronic contributions provided by His1 and Cys2 in Ni-SOD(red), we constructed the dinuclear sulfur-bridged metallosynthon, [Ni(2)(GC-OMe)(2)] (1). From 1 we prepared the following monomeric Ni(II)-N(2)S(2) complexes: K[Ni(GC-OMe)(SC(6)H(4)-p-Cl)] (2), K[Ni(GC-OMe)(S(t)Bu)] (3), K[Ni(GC-OMe)(SC(6)H(4)-p-OMe)] (4), and K[Ni(GC-OMe)(SNAc)] (5). The design strategy in utilizing GC-OMe(2-) is analogous to one which we reported before (see Inorg. Chem. 2009, 48, 5620 and Inorg. Chem. 2010, 49, 7080) where Ni-SOD(red) active site mimics can be assembled at will with electronically variant RS(-) ligands. Discussed herein is our initial account pertaining to the aqueous behavior of isolable, small-molecule Ni-SOD model complexes (non-maquette based). Spectroscopic (FTIR, UV-vis, ESI-MS, XAS) and electrochemical (CV) measurements suggest that 2-5 successfully simulate many of the electronic features of Ni-SOD(red). Furthermore, the aqueous studies reveal a dynamic behavior with regard to RS(-) lability and bridging interactions, suggesting a stabilizing role brought about by the protein architecture.

Effects of metal on the biochemical properties of Helicobacter pylori HypB, a maturation factor of [NiFe]-hydrogenase and urease

The biosyntheses of the [NiFe]-hydrogenase and urease enzymes in Helicobacter pylori require several accessory proteins for proper construction of the nickel-containing metallocenters. The hydrogenase accessory proteins HypA and HypB, a GTPase, have been implicated in the nickel delivery steps of both enzymes. In this study, the metal-binding properties of H. pylori HypB were characterized, and the effects of metal binding on the biochemical behavior of the protein were examined. The protein can bind stoichiometric amounts of Zn(II) or Ni(II), each with nanomolar affinity. Mutation of Cys106 and His107, which are located between two major GTPase motifs, results in undetectable Ni(II) binding, and the Zn(II) affinity is weakened by 2 orders of magnitude. These two residues are also required for the metal-dependent dimerization observed in the presence of Ni(II) but not Zn(II). The addition of metals to the protein has distinct impacts on GTPase activity, with zinc significantly reducing GTP hydrolysis to below detectable levels and nickel only slightly altering the k(cat) and K(m) of the reaction. The regulation of HypB activities by metal binding may contribute to the maturation of the nickel-containing enzymes.

Orientation and stereodynamic paths of planar monodentate ligands in square planar nickel N2S complexes

The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N(2)SNi moieties. Square planar [N(2)SNiL](n+) complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N(2)SNi](2) complexes. The solid state molecular structures of three mononuclear products, and three monothiolate bridged dinickel complexes, reveal that the plane of the added monodentate ligand orients largely orthogonal to the N(2)SNiL square plane. Variable temperature (1)H NMR characterization of dynamic processes and ground state isomer ratios of imidazole complexes in their stopped exchange limiting spectra, readily correlate with density functional theory (DFT)-guided interpretation of Ni-L rotational activation barriers. Full DFT characterization finds Ni-L bond lengthening as well as a tetrahedral twist distortion in the transition state, reaching a maximum in the NHC complex, and relating mainly to the steric hindrance derived both from the ligand and the binding pocket. In the case of the imidazole ligands a minor electronic contribution derives from intramolecular electrostatic interactions (imidazole C-2 C-H(delta+)- - S(delta-) interaction). Computational studies find this donor-acceptor interaction is magnified in O-analogues, predicting coplanar arrangements in the ground state of N(2)ON(imid)Ni complexes.

Bisamidate and mixed amine/amidate NiN2S2 complexes as models for nickel-containing acetyl coenzyme A synthase and superoxide dismutase: an experimental and computational study

The distal nickel site of acetyl-CoA synthase (Ni(d)-ACS) and reduced nickel superoxide dismutase (Ni-SOD) display similar square-planar Ni(II)N(2)S(2) coordination environments. One difference between these two sites, however, is that the nickel ion in Ni-SOD contains a mixed amine/amidate coordination motif while the Ni(d) site in Ni-ACS contains a bisamidate coordination motif. To provide insight into the consequences of the different coordination environments on the properties of the Ni ions, we systematically examined two square-planar Ni(II)N(2)S(2) complexes, one with bisthiolate-bisamidate ligation (Et(4)N)(2)(Ni(L1)).2H(2)O (2) [H(4)L1 = N-(2-mercaptoacetyl)-N'-(2-mercaptoethyl)glycinamide] and another with bisthiolate-amine/amidate ligation K(Ni(HL2)) (3) [H(4)L2 = N-(2''-mercaptoethyl)-2-((2'-mercaptoethyl)amino)acetamide]. Although these two complexes differ only by a single amine versus amidate ligand, their chemical properties are quite different. The stronger in-plane ligand field in the bisamidate complex (Ni(II)(L1))(2-) (2) results in an increase in the energies of the d --> d transitions and a considerably more negative oxidation potential. Furthermore, while the bisamidate complex (Ni(II)(L1))(2-) (2) readily forms a trinuclear species (Et(4)N)(2)({Ni(L1)}(2)Ni).H(2)O (1) and reacts rapidly with O(2), presumably via sulfoxidation, the mixed amine/amidate complex (Ni(II)(HL2))(-) (3) remains monomeric and is stable for days in air. Interestingly, the Ni(III) species of the bisamidate complex formed by chemical oxidation with I(2) can be detected by electron paramagnetic resonance (EPR) spectroscopy while the mixed amine/amidate complex immediately decomposes upon oxidation. To explain these experimentally observed properties, we performed S K-edge X-ray absorption spectroscopy and low-temperature (77 K) electronic absorption measurements as well as both hybrid density functional theory (hybrid-DFT) and spectroscopy oriented configuration interaction (SORCI) calculations. These studies demonstrate that the highest occupied molecular orbital (HOMO) of the bisamidate complex (Ni(II)(L1))(2-) (2) has more Ni character and is significantly destabilized relative to the mixed amine/amidate complex (Ni(II)(HL2))(-) (3) by approximately 6.2 kcal mol(-1). The consequence of this destabilization is manifested in the nucleophilic activation of the doubly filled HOMO, which makes (Ni(II)(L1))(2-) (2) significantly more reactive toward electrophiles such as O(2).