Dinuclear Center Research Articles

Resonance Raman of the nitrite reductase action of heme‐copper oxidoreductases

Several themes have emerged in the studies of biological control of nitrite and nitric‐oxide chemistry.[1‐2] Apparently, evolution has clearly selected means by which to take advantage of the unique NO2 and NO chemistry, and a number of enzymes have evolved that are able to activate NO2/NO kinetically, and control their redox chemistry. In bacterial respiration the reduction of NO2/NO to N2O by terminal oxidoreductases supports the hypothesis of a common evolutionary origin of bacterial denitrification and aerobic respiration. Comparison of the enzymes responsible for the activation of NO in denitrification and bacterial respiration may provide the means to identify conserved structural features, which can be assumed to be involved in basic functions common to both classes of enzymes. The most commonly encountered metals in the biological reduction of NO2 to NO and subsequently to N2O are iron and copper in mononuclear, binuclear, and dinuclear centers. In P450Nor the electrons needed for the reduction of NO are directly transferred from NADH. This way, a single molecule of NO binds at the heme Fe, and addition of two electrons to heme Fe3+‐NO yields the two electron reduced species Fe2+‐N=O‐. A second NO moleculeattacks the N atom of the ferrous‐NO species to transiently yield hyponitrite (HONNO–), and thus the N‐N bond formation. Cleavage of the N‐O bond produces the ferric enzyme, N2O and H2O. Resonance Raman studies of the NO2‐ligand binding properties of heme‐copper oxidases will be presented.References. Discrete Ligand Binding and Electron Transfer Properties of ba3‐Cytochrome c Oxidase from Thermus thermophilus: Evolutionary Adaption to Low Oxygen and …C Koutsoupakis, T Soulimane, C. Varotsis Accounts of chemical research 52 (5), 1380‐139, 2019. The structure of a ferrous heme‐nitro species in the binuclear heme a3/CuBcenter of ba3‐cytochrome c oxidase as determined by resonance Raman spectroscopy A. Loullis, MR Noor, T Soulimane, E Pinakoulaki Chemical Communications 51 (2), 286‐289, 2015.

Cu(I) Binding to Designed Proteins Reveals a Putative Copper Binding Site of the Human Line1 Retrotransposon Protein ORF1p.

Long interspersed nuclear elements-1 (L1) are autonomous retrotransposons that encode two proteins in different open reading frames (ORF1 and ORF2). The ORF1p, which may be an RNA binding and chaperone protein, contains a three-stranded coiled coil (3SCC) domain that facilitates the formation of the biologically active homotrimer. This 3SCC domain is composed of seven amino acid (heptad) repeats as found in native and designed peptides and a stammer that modifies the helical structure. Cysteine residues occur at three hydrophobic positions (2 a and 1 d sites) within this domain. We recently showed that the cysteine layers in ORF1p and model de novo designed peptides bind the toxic metalloid lead(II) with high affinities, a feature that had not been previously recognized. However, there is little understanding of how essential metal ions might interact with this metal binding domain. We have, therefore, investigated the copper(I) binding properties of analogous de novo designed 3SCCs that contain cysteine layers within the hydrophobic core. The results from UV-visible and X-ray absorption spectroscopy show that these designed peptides bind Cu(I) with high affinity in a pH-dependent manner. At pH 9, monomeric trigonal planar Cu(I)S3 centers are formed with 1 equiv of metal, while dinuclear centers form with a second equivalent of metal. At physiologic pH conditions, the dinuclear center forms cooperatively. These data suggest that ORF1p is capable of binding two copper ions to its tris(cysteine) layers. This has major implications for ORF1p coiled coil domain stability and dynamics, ultimately potentially impacting the resulting biological activity.

DFT Calculations for Mössbauer Properties on Dinuclear Center Models of the Resting Oxidized Cytochrome c Oxidase.

Mössbauer isomer shift and quadrupole splitting properties have been calculated using the OLYP‐D3(BJ) density functional method on previously obtained (W.‐G. Han Du, et al., Inorg Chem. 2020, 59, 8906–8915) geometry optimized Fea3 3+−H2O−CuB 2+ dinuclear center (DNC) clusters of the resting oxidized (O state) “as‐isolated” cytochrome c oxidase (CcO). The calculated results are highly consistent with the available experimental observations. The calculations have also shown that the structural heterogeneities of the O state DNCs implicated by the Mössbauer experiments are likely consequences of various factors, particularly the variable positions of the central H2O molecule between the Fea3 3+ and CuB 2+ sites in different DNCs, whether or not this central H2O molecule has H‐bonding interaction with another H2O molecule, the different spin states having similar energies for the Fea3 3+ sites, and whether the Fea3 3+ and CuB 2+ sites are ferromagnetically or antiferromagnetically spin‐coupled.

Hydrothermal syntheses, crystal structures and properties of two H2Tiron-bridged rare earth metal dinuclear complexes with 1,10-phenanthroline ligands

Two isostructural rare earth metal compounds, [RE2(H2Tiron)3(phen)2] (RE = Ho (1), Lu (2)) (Na2H2Tiron = 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt, phen = 1,10-phenanthroline), have been synthesized by hydrothermal methods and characterized by single-crystal X-ray diffraction, elemental and thermogravimetric analyses. Compounds 1 and 2 occur in the same triclinic crystal system and P-1 space group, where three H2Tiron2- ligands show the same bridged coordination mode. The dinuclear centers show double-capped triangular prismatic and dodecahedral geometries, respectively. The dinuclear compounds are stitched together via π-π stacking interactions to form 1 D chain structures. Compounds 1 and 2 have good thermal stability, which may be attributed to the introduction of phen auxiliary ligands. The emission spectrum of 1 exhibits four obvious peaks centered at 355 nm, 458 nm, 625 nm and 733 nm. The magnetic coupling interactions between Ho ions are ferromagnetic in 1.

4-Amino-1,2,4-triazole-3-thione-derived Schiff bases as metallo-β-lactamase inhibitors.

Resistance to β-lactam antibiotics in Gram-negatives producing metallo-β-lactamases (MBLs) represents a major medical threat and there is an extremely urgent need to develop clinically useful inhibitors. We previously reported the original binding mode of 5-substituted-4-amino/H-1,2,4-triazole-3-thione compounds in the catalytic site of an MBL. Moreover, we showed that, although moderately potent, they represented a promising basis for the development of broad-spectrum MBL inhibitors. Here, we synthesized and characterized a large number of 4-amino-1,2,4-triazole-3-thione-derived Schiff bases. Compared to the previous series, the presence of an aryl moiety at position 4 afforded an average 10-fold increase in potency. Among 90 synthetic compounds, more than half inhibited at least one of the six tested MBLs (L1, VIM-4, VIM-2, NDM-1, IMP-1, CphA) with Ki values in the μM to sub-μM range. Several were broad-spectrum inhibitors, also inhibiting the most clinically relevant VIM-2 and NDM-1. Active compounds generally contained halogenated, bicyclic aryl or phenolic moieties at position 5, and one substituent among o-benzoic, 2,4-dihydroxyphenyl, p-benzyloxyphenyl or 3-(m-benzoyl)-phenyl at position 4. The crystallographic structure of VIM-2 in complex with an inhibitor showed the expected binding between the triazole-thione moiety and the dinuclear centre and also revealed a network of interactions involving Phe61, Tyr67, Trp87 and the conserved Asn233. Microbiological analysis suggested that the potentiation activity of the compounds was limited by poor outer membrane penetration or efflux. This was supported by the ability of one compound to restore the susceptibility of an NDM-1-producing E.coli clinical strain toward several β-lactams in the presence only of a sub-inhibitory concentration of colistin, a permeabilizing agent. Finally, some compounds were tested against the structurally similar di-zinc human glyoxalase II and found weaker inhibitors of the latter enzyme, thus showing a promising selectivity towards MBLs.

A Water Molecule Residing in the Fea33+···CuB2+ Dinuclear Center of the Resting Oxidized as-Isolated Cytochrome c Oxidase: A Density Functional Study.

Although the dinuclear center (DNC) of the resting oxidized “as-isolated” cytochrome c oxidase (CcO) is not a catalytically active state, its detailed structure, especially the nature of the bridging species between the Fea33+ and CuB2+ metal sites, is still both relevant and unsolved. Recent crystallographic work has shown an extended electron density for a peroxide type dioxygen species (O1–O2) bridging the Fea3 and CuB centers. In this paper, our density functional theory (DFT) calculations show that the observed peroxide type electron density between the two metal centers is most likely a mistaken analysis due to overlap of the electron density of a water molecule located at different positions between apparent O1 and O2 sites in DNCs of different CcO molecules with almost the same energy. Because the diffraction pattern and the resulting electron density map represent the effective long-range order averaged over many molecules and unit cells in the X-ray structure, this averaging can lead to an apparent observed superposition of different water positions between the Fea33+ and CuB2+ metal sites.

DFT Fea3-O/O-O Vibrational Frequency Calculations over Catalytic Reaction Cycle States in the Dinuclear Center of Cytochrome c Oxidase.

Density functional vibrational frequency calculations have been performed on eight geometry optimized cytochrome c oxidase (CcO) dinuclear center (DNC) reaction cycle intermediates and on the oxymyoglobin (oxyMb) active site. The calculated Fe-O and O-O stretching modes and their frequency shifts along the reaction cycle have been compared with the available resonance Raman (rR) measurements. The calculations support the proposal that in state A[Fea33+-O2-•···CuB+] of CcO, O2 binds with Fea32+ in a similar bent end-on geometry to that in oxyMb. The calculations show that the observed 20 cm-1 shift of the Fea3-O stretching mode from the PR to F state is caused by the protonation of the OH- ligand on CuB2+ (PR[Fea34+═O2-···HO--CuB2+] → F[Fea34+═O2-···H2O-CuB2+]), and that the H2O ligand is still on the CuB2+ site in the rR identified F[Fea34+═O2-···H2O-CuB2+] state. Further, the observed rR band at 356 cm-1 between states PR and F is likely an O-Fea3-porphyrin bending mode. The observed 450 cm-1 low Fea3-O frequency mode for the OH active oxidized state has been reproduced by our calculations on a nearly symmetrically bridged Fea33+-OH-CuB2+ structure with a relatively long Fea3-O distance near 2 Å. Based on Badger's rule, the calculated Fea3-O distances correlate well with the calculated νFe-O-2/3 (νFe-O is the Fea3-O stretching frequency) with correlation coefficient R = 0.973.

Artificial bioinorganic clusters of dinuclear 3d-transition metal ions coordinated by an inorganic coordination ligand

A new biologically relevant cubane-type artificial inorganic ligand was developed to support 3d-transition metal ions of Mn(II), Co(II), Ni(II), Zn(II), Fe(III), and also In(III). All of the reactions produce dinuclear cationic clusters of [({Co(tacn)}2HMo3O13∙H2O){M(H2O)2}]2n+ (M = Mn2+ (4Mn), Co2+ (4Co), Ni2+ (4Ni), Zn2+ (4Zn), n = 2; M = Fe3+ (5), In3+ (6), n = 4), where tacn is the abbreviation of 1,4,7-triazacyclononane ligand. Clusters 4 do not dissolve in any solvents at all to prevent the recrystallization or the further characterization whereas the high solubility of clusters 5 and 6 allowed us to characterize the solution state properties. The dinuclear cores are separated at a fixed-distance in the range of 5.66–5.77 Å for M(II) complexes and 5.45–5.65 Å for M(III) complexes. Each 3d-transition metal ion is supported by two pairs of cis-oxido groups from a cubane unit formed by CoMo3O3(OH) group, and remaining two sites on the 3d-transition metal centers are occupied by two water molecules. The hydroxido group on the cubane unit provides an interaction route through bifurcated hydrogen bondings between dinuclear centers. Three consecutive stepwise reduction waves were observed at −0.73, −0.91, and −1.16 mV vs Fc+/Fc for 5 in cyclic voltammetry and the comparison with the redox inactive In3+ complex revealed that two iron centers are interacted each other through the inorganic ligand which gave a reduction wave observed at −0.91 mv vs Fc+/Fc.

A Stereoselective Tyrosinase Model Compound Derived from an m-Xylyl-l-histidine Ligand.

The aim of mimicking enzyme activity represents an important motivation for the development of new catalysts. A challenging objective is the development of chiral complexes for bioinspired enantioselective oxidation reactions. Herein, we report a new chiral dinuclear copper(II) complex based on a m-xylyl-bis(histidine) ligand (mXHI) as a biomimetic catalyst for tyrosinase and catechol oxidase. The new ligand improves a previous system also containing two tridentate N3 units derived from l-histidine that were connected by a short, rigid ethanediamine bridge. In mXHI the bridge is provided by the more extended m-xylyl moiety. The dicopper(II) complex [Cu2(mXHI)]4+ was studied as a catalyst for stereoselective oxidations of enantiomeric couples of chiral catechols of biological interest (L/D-dopa, L/D-dopa methyl ester, and ( R/ S)-norepinephrine), showing excellent discrimination capability, particularly for the methyl esters of dopa enantiomers. The catechol oxidation was studied in acetate buffer as slightly acidic medium, and a role of acetate as bridging ligand between the two coppers, preorganizing the dinuclear center in a more catalytic efficient structure, could be established. The oxidation of β-naphthol and 3,5-ditertbutylphenol was studied as a model monophenolase reaction. The oxidation proceeds stoichiometrically, and the partial incorporation of 18O into β-naphthol when the reaction was performed using 18O2 suggests the existence of two competitive reaction pathways, a genuine monooxygenase mechanism and a radical pathway. However, the more challenging reaction on derivatives of l-/d-tyrosine did not lead to the desired monooxygenase product but only to products of radical oxidation. Complex [Cu2(mXHI)]4+ was also used for the catalytic sulfoxidation of thioanisole in the presence of hydroxylamine as cosubstrate, in a preliminary attempt to model the reaction of external monooxygenases. The reaction proceeds with 25 turnovers, but the enantiomeric excess of sulfoxide was modest.

Serendipitous crystallization and structure determination of bacterioferritin from Achromobacter.

Bacterioferritins (Bfrs) are ferritin-like molecules with a hollow spherical 24-mer complex design that are unique to bacterial and archaeal species. They play a critical role in storing iron(III) within the complex at concentrations much higher than the feasible solubility limits of iron(III), thus maintaining iron homeostasis within cells. Here, the crystal structure of bacterioferritin from Achromobacter (Ach Bfr) that crystallized serendipitously during a crystallization attempt of an unrelated mycobacterial protein is reported at 1.95 Å resolution. Notably, Fe atoms were bound to the structure along with a porphyrin ring sandwiched between the subunits of a dimer. Furthermore, the dinuclear ferroxidase center of Ach Bfr has only a single iron bound, in contrast to the two Fe atoms in other Bfrs. The structure of Ach Bfr clearly demonstrates the substitution of a glutamate residue, which is involved in the interaction with the second Fe atom, by a threonine and the consequent absence of another Fe atom there. The iron at the dinuclear center has a tetravalent coordination, while a second iron with a hexavalent coordination was found within the porphyrin ring, generating a heme moiety. Achromobacter spp. are known opportunistic pathogens; this structure enhances the current understanding of their iron metabolism and regulation, and importantly will be useful in the design of small-molecule inhibitors against this protein through a structure-guided approach.

A Water Dimer Shift Activates a Proton Pumping Pathway in the PR → F Transition of ba3 Cytochrome c Oxidase.

Broken-symmetry density functional calculations have been performed on the [Fea34+,CuB2+] state of the dinuclear center (DNC) for the PR → F part of the catalytic cycle of ba3 cytochrome c oxidase (CcO) from Thermus thermophilus (Tt), using the OLYP-D3-BJ functional. The calculations show that the movement of the H2O molecules in the DNC affects the pKa values of the residue side chains of Tyr237 and His376+, which are crucial for proton transfer/pumping in ba3 CcO from Tt. The calculated lowest energy structure of the DNC in the [Fea34+,CuB2+] state (state F) is of the form Fea34+═O2-···CuB2+, in which the H2O ligand that resulted from protonation of the OH- ligand in the PR state is dissociated from the CuB2+ site. The calculated Fea34+═O2- distance in F (1.68 Å) is 0.03 Å longer than that in PR (1.65 Å), which can explain the different Fea34+═O2- stretching modes in P (804 cm-1) and F (785 cm-1) identified by resonance Raman experiments. In this F state, the CuB2+···O2- (ferryl-oxygen) distance is only around 2.4 Å. Hence, the subsequent OH state [Fea33+-OH--CuB2+] with a μ-hydroxo bridge can be easily formed, as shown by our calculations.

Kinetics of α-MnOOH Nanoparticle Formation through Enzymatically Catalyzed Biomineralization inside Apoferritin

While biomineralization in apoferritin has effectively synthesized highly monodispersed nanoparticles of various metal oxides and hydroxides, the detailed kinetics and mechanisms of Mn(III) (hydr)oxide formation inside apoferritin cavities have not been reported. To address this knowledge gap, we first identified the phase of solid Mn(III) formed inside apoferritin cavities as α-MnOOH. To analyze the oxidation and nucleation mechanism of α-MnOOH inside apoferritin by quantifying oxidized Mn, we used a colorimetric method with leucoberbelin blue (LBB) solution. In this method, LBB disassembled apoferritin by inducing an acidic pH environment, and reduced α-MnOOH nanoparticles. The LBB-enabled kinetic analyses of α-MnOOH nanoparticle formation suggested that the orders of reaction with respect to Mn2+ and OH– are 2 and 4, respectively, and α-MnOOH formation follows two-step pathways: First, soluble Mn2+ undergoes apoferritin-catalyzed oxidation at the ferroxidase dinuclear center, forming a Mn(III)-protein ...

Effect of chaotropes on the kinetics of iron release from ferritin by flavin nucleotides

BackgroundFerritins are ubiquitous multi-subunit iron storage and detoxification proteins that play a critical role in iron homeostasis. Ferrous ions that enter the protein's shell through hydrophilic channels are rapidly oxidized at dinuclear centers on the H-subunit before transfer to the protein's cavity for storage. The mechanisms of iron loading have been extensively studied, but little is known about iron mobilization. Fe(III) reduction can occur via rapid reduction by suitable reducing agents followed by chelation of Fe(II) ions or via direct and slow Fe(III) chelation. Here, the iron release kinetics from ferritin by FMNH2 in the presence of various chaotropic agents are studied and their in-vivo physiological significance discussed. MethodsThe iron release kinetics from horse and human ferritins by FMNH2 were monitored at 522nm where the Fe(II)–bipyridine complex absorbs. The experiments were performed in the presence of different concentrations of three chaotropic agents, urea, guanidine HCl, and triton. Results and conclusionsUnder our experimental conditions, iron reductive mobilization by the non-enzymatic FMN/NAD(P)H system is limited by the concentration of FMNH2 and is independent on the type or amount of chaotropes present. Diffusion of FMNH2 through the ferritin pores is an unlikely mechanism for ferritin iron reduction. An iron mobilization mechanism involving rapid electron transfer through the protein shell is discussed. General significanceCaution must be exercised when interpreting the kinetics of iron mobilization from ferritin using the FMN/NAD(P)H system. The kinetics are highly dependent on the amount of dissolved oxygen and the concentration of reagents used.

Water exit pathways and proton pumping mechanism in B-type cytochrome c oxidase from molecular dynamics simulations

Cytochrome c oxidase (CcO) is a vital enzyme that catalyzes the reduction of molecular oxygen to water and pumps protons across mitochondrial and bacterial membranes. While proton uptake channels as well as water exit channels have been identified for A-type CcOs, the means by which water and protons exit B-type CcOs remain unclear. In this work, we investigate potential mechanisms for proton transport above the dinuclear center (DNC) in ba3-type CcO of Thermus thermophilus. Using long-time scale, all-atom molecular dynamics (MD) simulations for several relevant protonation states, we identify a potential mechanism for proton transport that involves propionate A of the active site heme a3 and residues Asp372, His376 and Glu126II, with residue His376 acting as the proton-loading site. The proposed proton transport process involves a rotation of residue His376 and is in line with experimental findings. We also demonstrate how the strength of the salt bridge between residues Arg225 and Asp287 depends on the protonation state and that this salt bridge is unlikely to act as a simple electrostatic gate that prevents proton backflow. We identify two water exit pathways that connect the water pool above the DNC to the outer P-side of the membrane, which can potentially also act as proton exit transport pathways. Importantly, these water exit pathways can be blocked by narrowing the entrance channel between residues Gln151II and Arg449/Arg450 or by obstructing the entrance through a conformational change of residue Tyr136, respectively, both of which seem to be affected by protonation of residue His376.

Hydrostable coordination polymers of a methyl-functionalized 4,4′-bipyridine ligand: structure, stability, magnetic and luminescent properties

The reactions of aromatic dicarboxylic acids and methyl-functionalized 4,4′-bipyridine ligands with metal salts under hydrothermal conditions generated four structurally diverse cobalt(II), zinc(II) and cadmium(II) coordination polymers, [Co(CH3-BDC)(dmbpy)0.5] n (1), [Cd(OH-HBDC)2(dmbpy)] n (2), [Zn(NDC)(dmbpy)] n , (3) and {[Cd(DBA)(dmbpy)0.5]·2H2O} n (4) (CH3–H2BDC = 5-methylisophthalic acid, OH–H2BDC = 5-hydroxyisophthalic acid, H2NDC = 1,4-naphthalenedicarboxylic acid, H2DBA = 4,4′-methylenedibenzoic acid, dmbpy = 2,2′-dimethyl-4,4′-bipyridine). All four complexes have been structurally characterized by X-ray crystallography. Complex 1 shows a 3D jsm topology structure with two 1D channels parallel to the a and b axes. Complex 2 has a zigzag chain in which the OH-HBDC ligands point alternately up and down. Complexes 3 and 4 show 2D (4,4) networks when the dinuclear metal centers and their ligands are regarded as nodes and linkers, respectively. Complex 3 also shows twofold interpenetration with 1D channels along the b axis. Two nets of complex 4 interlock in parallel, giving rise to a polycatenated layer (2D → 2D). Thermogravimetric and chemical stabilities, magnetic and luminescent properties of these complexes were investigated.

A broken-symmetry density functional study of structures, energies, and protonation states along the catalytic O-O bond cleavage pathway in ba3 cytochrome c oxidase from Thermus thermophilus.

Broken-symmetry density functional calculations have been performed on the [Fea3, CuB] dinuclear center (DNC) of ba3 cytochrome c oxidase from Thermus thermophilus in the states of [Fea3(3+)-(HO2)(-)-CuB(2+), Tyr237(-)] and [Fea3(4+)[double bond, length as m-dash]O(2-), OH(-)-CuB(2+), Tyr237˙], using both PW91-D3 and OLYP-D3 functionals. Tyr237 is a special tyrosine cross-linked to His233, a ligand of CuB. The calculations have shown that the DNC in these states strongly favors the protonation of His376, which is above propionate-A, but not of the carboxylate group of propionate-A. The energies of the structures obtained by constrained geometry optimizations along the O-O bond cleavage pathway between [Fea3(3+)-(O-OH)(-)-CuB(2+), Tyr237(-)] and [Fea3(4+)[double bond, length as m-dash]O(2-)HO(-)-CuB(2+), Tyr237˙] have also been calculated. The transition of [Fea3(3+)-(O-OH)(-)-CuB(2+), Tyr237(-)] → [Fea3(4+)[double bond, length as m-dash]O(2-)HO(-)-CuB(2+), Tyr237˙] shows a very small barrier, which is less than 3.0/2.0 kcal mol(-1) in PW91-D3/OLYP-D3 calculations. The protonation state of His376 does not affect this O-O cleavage barrier. The rate limiting step of the transition from state A (in which O2 binds to Fea3(2+)) to state PM ([Fea3(4+)[double bond, length as m-dash]O(2-), OH(-)-CuB(2+), Tyr237˙], where the O-O bond is cleaved) in the catalytic cycle is, therefore, the proton transfer originating from Tyr237 to O-O to form the hydroperoxo [Fea3(3+)-(O-OH)(-)-CuB(2+), Tyr237(-)] state. The importance of His376 in proton uptake and the function of propionate-A/neutral-Asp372 as a gate to prevent the proton from back-flowing to the DNC are also shown.

Influence of Steric Hindrance of N‐Heterocyclic Ancillary Ligands on the Structure and Magnetic Properties of Manganese(II) Coordination Polymers

AbstractThree biphenyl‐3,5‐dicarboxylic acid (H2L) based coordination polymers, namely, [Mn3(L)3(2,2′‐bpy)2]n (1), {[Mn(L)(phen)]·(MeOH)}n (2), and [Mn(L)(dipt)]n (3), (2,2′‐bpy = 2,2′‐bipyridine, phen = 1,10‐phenanthroline, and dipt = 2,9‐dimethyl‐1,10‐phenanthroline) were synthesized and characterized by single‐crystal X‐ray diffraction and analyses of their magnetic properties. 1 is a trinuclear manganese structure with a 2D motifs, which can join by hydrogen bond bridges to give 3D supramolecular architectures. 2 has a dinuclear center forming a 1D supramolecular ladder chain. The mononuclear complex 3 displays 1D metal‐organic chains driven by μ2‐L linkers. Their structural differences were investigated, revealing that the influence of steric hindrance on the structures of acid‐based coordination polymers is realized through changing the N‐heterocyclic ancillaries of diverse steric hindrance. Obviously, with decreasing of the steric hindrance of the N‐donor ligand, complexes 1–3 show structures from 1D to 2D and mononuclear to multinuclear. Magnetic susceptibility measurements indicate that 1 and 2 have dominating antiferromagnetic couplings between metal ions, whereas compound 3 is paramagnetic.

Functionalized Paddle Wheel Complexes from BODIPY Carboxylic Acids

AbstractA series of BODIPY carboxylic acids carrying aromatic linking units between the luminophor and the carboxylate functional group was prepared using Sonogashira and Stille type coupling protocols. Ferrocene and hydroquinone units could be introduced by either of these methods. Metal complex formation of the BODIPY carboxylic acid ligands was investigated with the divalent metal ions of copper and rhodium. Copper forms insoluble material, which crystallized in one case. The X‐ray crystallographic analysis shows the presence of a BODIPY‐appended paddle wheel complex with typical Cu···Cu and Cu–O distances and four BODIPY units in a distance of 9.681 Å and 9.747 Å from the dinuclear center. Treatment with donor solvents results in the decomposition/monomerization of the compound, which could be shown crystallographically for pyridine. Rhodium(II) ions form soluble paddle wheel complexes with four different BODIPY carboxylates. The crystallographic analysis of one example shows the isostructural nature of the dirhodium and the dicopper derivatives. Optical spectroscopy and cyclic voltammetry provide first insights into efficient intramolecular energy‐ and electron transfer pathways for the rhodium complexes.

Broken Symmetry DFT Calculations/Analysis for Oxidized and Reduced Dinuclear Center in Cytochrome c Oxidase: Relating Structures, Protonation States, Energies, and Mössbauer Properties in ba3 Thermus thermophilus.

The Fea33+···CuB2+ dinuclear center (DNC) structure of the as-isolated oxidized ba3 cytochrome c oxidase (CcO) from Thermus thermophilus (Tt) is still not fully understood. When the proteins are initially crystallized in the oxidized state, they typically become radiolyticly reduced through X-ray irradiation. Several X-ray crystal structures of reduced ba3 CcO from Tt are available. However, depending on whether the crystals were prepared in a lipidic cubic phase environment or in detergent micelles, and whether the CcO’s were chemically or radiolyticly reduced, the X-ray diffraction analysis of the crystals showed different Fea32+···CuB+ DNC structures. On the other hand, Mössbauer spectroscopic experiments on reduced and oxidized ba3 CcOs from Tt (Zimmermann et al., Proc. Natl. Acad. Sci. USA 1988, 85, 5779–5783) revealed multiple 57Fea32+ and 57Fea33+ components. Moreover, one of the 57Fea33+ components observed at 4.2 K transformed from a proposed “low-spin” state to a different high-spin species when the temperature was increased above 190 K, whereas the other high-spin 57Fea33+ component remained unchanged. In the current Article, in order to understand the heterogeneities of the DNC in both Mössbauer spectra and X-ray crystal structures, the spin crossover of one of the 57Fea33+ components, and how the coordination and spin states of the Fea33+/2+ and Cu2+/1+ sites relate to the heterogeneity of the DNC structures, we have applied density functional OLYP calculations to the DNC clusters established based on the different X-ray crystal structures of ba3 CcO from Tt. As a result, specific oxidized and reduced DNC structures related to the observed Mössbauer spectra and to spectral changes with temperature have been proposed. Our calculations also show that, in certain intermediate states, the His233 and His283 ligand side chains may dissociate from the CuB+ site, and they may become potential proton loading sites during the catalytic cycle.

Structure and properties of the catalytic site of nitric oxide reductase at ambient temperature

Nitric oxide reductase (Nor) is the third of the four enzymes of bacterial denitrification responsible for the catalytic formation of laughing gas (N2O). Here we report the detection of the hyponitrite (HO–N=N–O−) species (νN–N=1332cm−1) in the heme b3 Fe–FeB dinuclear center of Nor from Paracoccus denitrificans. We have also applied density functional theory (DFT) to characterize the bimetallic-bridging hyponitrite species in the reduction of NO to N2O by Nor and compare the present results with those recently reported for the N–N bond formation in the ba3 and caa3 oxidoreductases from Thermus thermophilus.