High nuclearity nickel compounds with three,four or five metal atoms showing antibacterial activity

The effect on DNA and the antibacterial activity of a series of high nuclearity nickel compounds with three, four and five metal atoms were examined. The compounds have a mixed ligand composition with salicylhydroxamic acid and di-2-pyridyl-ketonoxime as chelate agents. In the trinuclear compound Ni(3)(shi)(2)(Hpko)(2)(py)(2)(1), two metal ions show a square planar geometry while the third one is in an octahedral environment. The compounds with four and five nickel atoms construct metallacrown cores with two distinct connectivities. The tetranuclear vacant metallacrown [12-MC(Ni(II)N(Hshi)2(pko)2)-4](2+) shows the connectivity pattern [-O-Ni-O-N-Ni-N-](2), while the pentanuclear ([Ni(II)][12-MC(Ni(II)N(shi)2(pko)2)-4])(2+) follows the pattern [-Ni-O-N-](4). Two distinct arrangements of the chelates around the ring metal ions were observed; a 6-5-6-5-6-5-6-5 arrangement for the [12-MC(Ni(II)N(Hshi)2(pko)2)-4] core and a 6-6-5-5-6-6-5-5 arrangement for the [12-MC(Ni(II)N(shi)2(pko)2)-4] core. Magnetic variable temperature susceptibility study of the trinuclear compound revealed the presence of one paramagnetic nickel(II) ion with strong crystal field dependence, with D=5.0(4) cm(-1), g(xy)=2.7(3) and g(z)=2.3(3). The effect of the synthesized Ni(II) complexes on the integrity and electrophoretic mobility of nucleic acids was examined. Only compounds 2, 3 and 4 altered the mobility of pDNA, forming high molecular weight concatamers at low concentrations or precipitates at higher concentrations. Antibacterial activity screening of the above compounds suggests that nickel compounds 2, 3 and 4 were the most active and can act as potent antibacterial agents.     

Towards elucidating the energy of the first excited singlet state of xanthophyll cycle pigments by X-ray absorption spectroscopy

The first excited singlet state (S(1)) of carotenoids (also termed 2A(g)(-)) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S(1) (Q(y)) level of chlorophylls. The determination of carotenoid 2A(g)(-) energies by optical techniques is difficult; transitions from the ground state (S(0), 1A(g)(-)) to the 2A(g)(-) state are forbidden ("optically dark") due to parity (g <-- //--> g) as well as pseudo-parity selection rules (- <-- //--> -). Of particular interest are S(1) energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S(1) energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one pi* energy level (about 0.5 eV below the lowest 1B(u)(+) vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states.     

Synthesis, physico-chemical studies of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) complexes with some p-substituted acetophenone benzoylhydrazones and their antimicrobial activity

Complexes of the type [M(pabh)(H2O)Cl], [M(pcbh)(H2O)Cl] and [M(Hpabh)(H2O)2 (SO4)] where, M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II); Hpabh = p-amino acetophenone benzoyl hydrazone and Hpcbh = p-chloro acetophenone benzoyl hydrazone have been synthesized and characterized with the help of elemental analyses, electrical conductance, magnetic susceptibility measurements, electronic, ESR and IR spectra, thermal (TGA & DTA) and X-ray diffraction studies. Co(II), Ni(II) and Cu(II) chloride complexes are square planar, whereas their sulfate complexes have spin-free octahedral geometry. ESR spectra of Cu(II) complexes with Hpabh are axial and suggest d(x(2)-y(2) as the ground state. The ligand is bidentate bonding through > C = N--and deprotonated enolate group in all the chloro complexes, whereas, >C = N and >C = O groups in all the sulfato complexes. Thermal studies (TGA & DTA) on [Cu(Hpabh)(H2O)2(SO4)] indicate a multistep decomposition pattern, which are both exothermic and endothermic in nature. X-ray powder diffraction parameters for [Co(pabh)(H2O)Cl] and [Ni(Hpabh)(H2O)2(SO4)] correspond to tetragonal and orthorhombic crystal lattices, respectively. The ligands as well as their complexes show a significant antifungal and antibacterial activity. The metal complexes are more active than the ligand.     

Glutathionato-S-Gold(III) complexes formed as intermediates in the reduction of auricyanide by glutathione

The reduction of auricyanide ([Au(CN)(4)](-), a potential gold(III) metabolite of antiarthritic gold(I) compounds), by glutathione (G(-)SH, an anionic biological reductant) proceeds through two intermediates (I(230) and I(290)) which have previously been identified by their UV-vis spectra, but not isolated. Negative-ion electrospray ionization-mass spectroscopy (ESI-MS) has unambiguously identified them as [Au(CN)(3)(SG)](2-) and [Au(CN)(2)(SG)(2)](3-), respectively, and allowed their formation and decay to be monitored. The spectra also confirm that the products are aurocyanide ([Au(CN)(2)](-), a known metabolite of chrysotherapy agents) and oxidized glutathione (GSSG(2-)). The reactions are dependent on the presence or absence of buffering agents and the pH of the reaction media. The reaction can be driven to the first intermediate by using an excess of auricyanide or by running the reaction at low pH which prevents further reaction. At neutral pH and/or with excess of glutathione present, the reaction proceeds to the second intermediate, which is then reduced to aurocyanide. The monoanions, [Au(CN)(3)(SGH)](-) at m/z=581.2 and [Au(CN)(2)(SGH)(2)](-) at m/z=861.5 generate more intense signals than their respective dianions, [Au(CN)(3)(SG)](2-) at m/2=290.2 and [Au(CN)(2)(SG)(SGH)](2-)m/2=430.9, respectively, whereas the trianion [Au(CN)(2)(SG)(2)](3-) (m/3=281.2) was not observed. These studies demonstrate the value of ESI-MS methods for characterizing reactions of metallopharmaceuticals under biomimetic conditions and suggest that they will be useful for other systems which give strong ESI-MS signals.     

The effects of ligand substitution and deuteriation on the spectroscopic and photophysical properties of [Ru(LL)(CN)4]2- complexes.

The spectroscopic characterisation of a series of [Ru(LL)(CN)(4)](2-) complexes, where LL = 1,10-phenanthroline (phen) and its methyl- and phenyl-substituted derivatives and several deuteriated isotopologues are reported. The optical and vibrational properties of these complexes are compared with that of the series of 2,2'-bipyridine (bipy) derivatives and analogous [Ru(LL)(3)](2+) complexes. It has been demonstrated that substitution at the 4,4' positions of bipy and 4,7-positions of phen by electron donating (CH(3)) and withdrawing (C(6)H(5), COO(-)) groups induces a pronounced blue and red shift, respectively, in the lowest energy (1)MLCT absorption band of [Ru(LL)(CN)(4)](2-). The energy of the emission originating from the (3)MLCT excited state is found to be dependant on the nature of the vibrational modes of the aromatic rings and the electron donating and/or withdrawing properties of the substituents. Single-mode Franck-Condon analysis indicates that methyl substitution leads to a significant increase in the Huang-Rhys factor (S(M)), while phenyl substitution results in a decrease in S(M) for both series (bipy and phen) of complexes. The rate of non-radiative (k(nr)) and radiative decay (k(ph)) to the ground state and the parameters of thermally activated deactivation pathways (A(4th), DeltaE(4th) and A(dd), DeltaE(dd)) were estimated from the temperature dependence of luminescence quantum yields and lifetimes. It has been demonstrated that the non-radiative decay rate and the temperature dependent decay processes are more efficient for bipy complexes than for phen derivatives due to the rigidity of the latter ligand.     

The kinetics of oxidation of hemerythrin species—linear free energy relationships

The second-order rate constants (M^?1sec^?1, 25°C, pH 8.2, I = 0.15 M) for the oxidation to (semi-met)₀of deoxyhemerythrin from Phascolopsis gouldii (P.g.) and Themiste zostericola (T.z.) have been determined for Fe(CN)₅(4-NH₂py)^2? (3.6 × 10⁴ T.z.,2.8 × 10²P.g.),Fe(CN)₅NH₃^2?(2.4 × 10⁴ T.z.), Fe(CN)₆^3? (1.0 × 10⁵ T.z.,1.4 × 10²P.g.), Fe(CN)₅PPh₃^2? (7.3 × 10⁵T.z.), and Fe(CN)₄dipy- (~6 × 10⁶ T.z.,7.5 × 10⁴ P.g.). Corresponding rate constants for the oxidation of (semi-met)_R to met are: Fe(CN)₅(4-NH₂py)^2? (1.2 × 10³ P.g.), Fe(CN)₆^3? (3.4 × 10⁵ T.z., 4.5 × 10 Fe(CN)₅PPh₃^2? (4.4 × 10⁴P.g.), Fe(CN)₄dipy^? (1.7 × 10⁵P.g.), and Coterpy₂³⁺ (5.1 P.g.) The rates of oxidation of deoxy- and (semi-met)_R myohemetythrin by Fe(CN)₆^3? were too rapid for stopped-flow measurement. The Marcus relationship for cross-reactions was successfully applied to these data.     

Single molecule magnet: Heterodinuclear cyano-bridged cubic cluster [(Tp)8Fe4Ni4(CN)12] (Tp = hydrotris(1-pyrazolyl)borate)

The octanuclear cyano-bridged cluster [(Tp)₈Fe₄Ni₄(CN)₁₂] · H₂O · 24CH₃CN (1) (Tp = hydrotris(1-pyrazolyl)borate) showing magnetic properties of single-molecule magnet has been synthesized by reaction of [fac-Fe(Tp)(CN)₃]⁻ with {(Tp)Ni(NO₃)} species formed from an equimolar reaction mixture of Ni(NO₃)₂ · 6H₂O and KTp in MeCN. The X-ray analysis of 1 shows molecular cube structure in which Fe^III and Ni^II ions reside in alternate corners. The average intramolecular Fe?Ni distance is 5.124 Å. Out-of-phase ac susceptibility and reduce magnetization measurements show that 1 is a single molecule magnet with ground spin state S = 6 and spin reversal energy barrier U = 14 K. Magnetic hysteresis loops were also observed by applying fast sweeping field.     

Delayed fluorescence from Rhodopseudomonas sphaeroides reaction centers. Enthalpy and free energy changes accompanying electron transfer from P-870 to quinones

Delayed fluorescence from isolated reaction centers of Rhodopseudomonas sphaeroides was measured to study the energetics of electron transfer from the bacteriochlorophyll complex (P-870, or P) to the primary and secondary quinones (Q_A and Q_B). The analysis was based on the assumption that electron transfer between P and Q reaches equilibrium quickly after flash excitation, and stays in equilibrium during the lifetime of the P⁺Q⁻ radical pair. Delayed fluorescence of 1Q reaction centers (reaction centers that contain only Q_A) has a lifetime of about 0.1 s, which corresponds to the decay of P⁺Q⁻_A. 2Q reaction centers (which contain both Q_A and Q_B) have a much weaker delayed fluorescence, with a lifetime that corresponds to that of P⁺Q⁻_B (about 1 s). In the presence of o-phenanthroline, the delayed fluorescence of 2Q reaction centers becomes similar in intensity and decay kinetics to that of 1Q reaction centers. From comparisons of the intensities of the delayed fluorescence from P⁺Q⁻_A and P⁺Q⁻_B, the standard free energy difference between P⁺Q⁻_A and P⁺Q⁻_B is calculated to be 78 ± 8 meV. From a comparison of the intensity of the delayed fluorescence with that of prompt fluorescence, we calculate that P⁺Q⁻_A is 0.86 ± 0.02 eV below the excited singlet state of P in free energy, or about 0.52 eV above the ground state PQ_A. The temperature dependence of the delayed fluorescence indicates that P⁺Q⁻_A is about 0.75 eV below the excited singlet state in enthalpy, or about 0.63 eV above the ground state.     

Hydrogen-induced structural changes at the nickel site of the regulatory [NiFe] hydrogenase from Ralstonia eutropha detected by X-ray absorption spectroscopy

For the first time, the nickel site of the hydrogen sensor of Ralstonia eutropha, the regulatory [NiFe] hydrogenase (RH), was investigated by X-ray absorption spectroscopy (XAS) at the nickel K-edge. The oxidation state and the atomic structure of the Ni site were investigated in the RH in the absence (air-oxidized, RH(ox)) and presence of hydrogen (RH(+H2)). Incubation with hydrogen is found to cause remarkable changes in the spectroscopic properties. The Ni-C EPR signal, indicative of Ni(III), is detectable only in the RH(+H2) state. XANES and EXAFS spectra indicate a coordination of the Ni in the RH(ox) and RH(+H2) that pronouncedly differs from the one in standard [NiFe] hydrogenases. Also, the changes induced by exposure to H(2) are unique. A drastic modification in the XANES spectra and an upshift of the K-edge energy from 8339.8 (RH(ox)) to 8341.1 eV (RH(+H2)) is observed. The EXAFS spectra indicate a change in the Ni coordination in the RH upon exposure to H(2). One likely interpretation of the data is the detachment of one sulfur ligand in RH(+H2) and the binding of additional (O,N) or H ligands. The following Ni oxidation states and coordinations are proposed: five-coordinated Ni(II)(O,N)(2)S(3) for RH(ox) and six-coordinated Ni((III))(O,N)(3)X(1)S(2) [X being either an (O,N) or H ligand] for RH(+H2). Implications of the structural features of the Ni site of the RH in relation to its function, hydrogen sensing, are discussed.     

The structure of the Ni-Fe site in the isolated HoxC subunit of the hydrogen-sensing hydrogenase from Ralstonia eutropha

The regulatory Ni-Fe hydrogenase (RH) from Ralstonia eutropha which forms a [HoxBC]2 complex functions as a hydrogen sensor under aerobic conditions. We have studied a novel Strep-tag isolate of the RH large subunit, HoxC(ST), which lacks the Fe-S clusters of HoxB, allowing for structure determination of the catalytic site by X-ray absorption spectroscopy both at the Ni and, for the first time, also at the Fe K-edge. This technique, together with Fourier-transform infrared spectroscopy, revealed a Ni-Fe site with [O1(CysS)2Ni(II)(mu-SCys)2Fe(II)(CN)2(CO)] structure in about 50% of HoxC(ST) and a [(CysS)2Fe(II)(CN)2(CO)] site lacking Ni in the remainder protein. Possibly both sites may be intermediates in the maturation process of the RH.     

Bias from H2 cleavage to production and coordination changes at the Ni-Fe active site in the NAD+-reducing hydrogenase from Ralstonia eutropha

The soluble NAD+-reducing Ni-Fe hydrogenase (SH) from Ralstonia eutropha H16 is remarkable because it cleaves hydrogen in the presence of dioxygen at a unique Ni-Fe active site (Burgdorf et al. (2005) J. Am. Chem. Soc. 127, 576). By X-ray absorption (XAS), FTIR, and EPR spectroscopy, we monitored the structure and oxidation state of its metal centers during H2 turnover. In NADH-activated protein, a change occurred from the (CN)O2Ni(II)(mu-S)2Fe(II)(CN)3(CO) site dominant in the wild-type SH to a standard-like S2Ni(II)(mu-S)2Fe(II)(CN)2(CO) site as the prevailing species in a specific mutant protein, HoxH-H16L. The wild-type SH primarily was active in H2 cleavage. The nonstandard reaction mechanism does not involve stable EPR-detectable trivalent Ni oxidation states, namely, the Ni-A,B,C states as observed in standard hydrogenases. In the HoxH-mutant protein H16L, H2 oxidation was impaired, but H2 production occurred via a stable Ni-C state (Ni(III)-H(-)-Fe(II)), suggesting a reaction sequence similar to that of standard hydrogenases. It is proposed that reductive activation by NADH of both wild-type and H16L proteins causes the release of an oxygen species from Ni and is initiated by electron transfer from a [2Fe-2S] cluster in the HoxU subunit that at first becomes reduced by electrons from NADH. Electrons derived from H2 cleavage, on the other hand, are transferred to NAD+ via a different pathway involving a [4Fe-4S] cluster in HoxY, which is reducible only in wild-type SH but not in the H16L variant.     

Ligand-field excited states of hexacyanochromate and hexacyanocobaltate as sensitisers for near-infrared luminescence from Nd(iii) and Yb(iii) in cyanide-bridged d-f assemblies.

Crystallisation of [Co(CN)(6)](3-) or [Cr(CN)(6)](3-) with Ln(iii) salts (Ln = Nd, Gd, Yb) from aqueous dmf afforded the cyanide-bridged d/f systems [Ln(dmf)(4)(H(2)O)(3)(micro-CN)Co(CN)(5)] (-, discrete dinuclear species) and {[Cr(CN)(4)(micro-CN)(2)Ln(H(2)O)(2)(dmf)(4)]}(infinity) (-, infinite cyanide-bridged chains with alternating Cr and Ln centres). With Ln = Gd the characteristic long-lived phosphorescence from d-d excited states of the [M(CN)(6)](3-) units was apparent in the red region of the spectrum, with lifetimes of the order of 1 micros, since the heavy atom effect of the Gd(iii) promotes inter-system crossing at the [M(CN)(6)](3-) units to generate the phosphorescent spin-forbidden excited states. With Ln = Yb or Nd however, the d-block luminescence was completely quenched due to fast (>10(8) s(-1)) energy-transfer to the Ln(iii) centre, resulting in the characteristic sensitised emission from Yb(iii) and Nd(iii) in the near-IR region. For both - and -, calculations based on spectroscopic overlap between emission of the donor (Co) and absorption of the acceptor (Ln) suggest that the Dexter energy-transfer mechanism is responsible for the complete quenching that we observe.     

The photorelease of nitrogen monoxide (NO) from pentacyanonitrosyl coordination compounds of group 8 metals.

The photodetachment of NO from [M(II)(CN)5NO]2- with M = Fe, Ru, and Os, upon laser excitation at various wavelengths (355, 420, and 480 nm) was followed by various techniques. The three complexes showed a wavelength-dependent quantum yield of NO production Phi(NO), as measured with an NO-sensitive electrode, the highest values corresponding to the larger photon energies. For the same excitation wavelength the decrease of Phi(NO) at 20 degrees C in the order Fe > Ru > Os, is explained by the increasing M-N bond strength and inertness of the heavier metals. Transient absorption data at 420 nm indicate the formation of the [M(III)(CN)5H2O]2- species in less than ca. 1 micros for M = Fe and Ru. The enthalpy content of [Fe(III)(CN)5H2O]2- with respect to the parent [Fe(II)(CN)5NO]2- state is (190 +/- 20) kJ mol(-1), as measured by laser-induced optoacoustic spectroscopy (LIOAS) upon excitation at 480 nm. The production of [Fe(III)(CN)5H2O]2- is concomitant with an expansion of (8 +/- 3) ml mol(-1) consistent with an expansion of the water bound through hydrogen bonds to the CN ligands plus the difference between NO release into the bulk and water entrance into the first coordination sphere. The activated process, as indicated by the relatively strong temperature dependence of the Phi(NO) values and by the temperature dependence of the appearance of the [Fe(III)(CN)5H2O]2- species, as determined by LIOAS, is attributed to NO detachment in less than ca. 100 ns from the isonitrosyl (ON) ligand (MS1 state).     

Synthesis, structures and magnetic properties of nickel(II), manganic(II) and zinc(II) complexes containing pyridyl-substituted nitronyl nitroxide and tris(2-benzimidazolymethyl)amine

Three new magnetic compounds were synthesized by using 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NIT2Py) and tris(2-benzimidazolymethyl)amine (NTB) as ligands. The structures and magnetic properties of the complexes with formula [Ni(NIT2Py)(NTB)](ClO₄)₂(CH₃OH) 1, [Mn(NIT2Py)(NTB)](ClO₄)₂2 and {[Zn(NIT2Py)₂(CH₃OH)₂](ClO₄)₂}{[Zn(NTB)(H₂O)](ClO₄)₂} 3 were characterized. Compounds 1 and 2 both have [M(NIT2Py)(NTB)] structural units, where the metal ion is in an octahedral environment bound to one NIT2Py through one pyridyl nitrogen atom and one nitroxide oxygen atom. However, compound 3, the chelating zinc ion has two crystallographically independent molecules in the asymmetric unit: one is six coordinated octahedral structure [Zn(NIT2Py)₂(CH₃OH)₂](ClO₄)₂, and the other one is five coordinated pyramidal structure [Zn(NTB)(H₂O)](ClO₄)₂. The magnetic behaviors of these compounds indicate that both the nickel ion and the manganese ion are antiferromagnetically coupled with the NIT2Py ligand with a coupling constant of −19.44 and −0.37 cm⁻¹, respectively, whereas two NIT2Py ligands in compound 3 are ferromagnetically coupled with a coupling constant of 19.1 cm⁻¹.     

A quantum chemical investigation of the electronic structure of thionine

We have examined the electronic and molecular structure of 3,7-diaminophenothiazin-5-ium dye (thionine) in the electronic ground state and in the lowest excited states. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Equilibrium geometries were optimized employing (time-dependent) density functional theory (B3LYP functional) combined with the TZVP basis set. Solvent effects were estimated using the COSMO model and micro-hydration with up to five explicit water molecules. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet and triplet states at the ground state geometry to be of π→π* (S(1), S(2), T(1), T(2)) and n→π* (S(3), T(3)) character. This order changes when the molecular structure in the electronically excited states is relaxed. Geometry relaxation has almost no effect on the energy of the S(1) and T(1) states (~0.02 eV). The relaxation effects on the energies of S(2) and T(2) are moderate (0.14-0.20 eV). The very small emission energy results in a very low fluorescence rate. While we were not able to locate the energetic minimum of the S(3) state, we found a non-planar minimum for the T(3) state with an energy which is very close to the energy of the S(1) minimum in the gas phase (0.04 eV above). When hydration effects are taken into account, the n→π* states S(3) and T(3) are strongly blueshifted (0.33 and 0.46 eV), while the π→π* states are only slightly affected (<0.06 eV).     

Iron-oxidation-state-dependent O-O bond cleavage of meta-chloroperbenzoic acid to form an iron(IV)-oxo complex

The mechanism of formation of [Fe^IV(O)(N4Py)]²⁺ (2, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) from the reaction of [Fe^II(N4Py)(CH₃CN)]²⁺ (1) with m-chloroperbenzoic acid (mCPBA) in CH₂Cl₂ at −30 °C has been studied on the basis of the visible spectral changes observed and the reaction stoichiometry. It is shown that the conversion of 1 to 2 in 90% yield requires 1.5 equiv. peracid and takes place in two successive one-electron steps via an [Fe^III(N4Py)OH]²⁺(3) intermediate. The first oxidation step uses 0.5 equiv. peracid and produces 0.5 equiv. 3-chlorobenzoic acid, while the second step uses 1 equiv. peracid and affords byproducts derived from chlorophenyl radical. We conclude that the Fe^II(N4Py) center promotes O-O bond heterolysis, while the Fe^III(N4Py) center favors O-O bond homolysis, so the nature of O-O bond cleavage is dependent on the iron oxidation state.     

When being straight bends rules: a rationale for the linear FeNO unit in the low-spin square-pyramidal {FeNO}7 tetracyanonitrosylferrate(2-) anion

All low-spin S=1/2 heme-NO complexes feature FeNO angles of about 140 degrees . In contrast, the square-pyramidal [Fe(CN)(4)(NO)](2-) complex features an exactly linear {FeNO}(7) unit. We have sought here to determine a possible, simple molecular orbital (MO) rationale for these structural variations. A DFT-based (DFT=density functional theory) MO analysis shows that the linearity of the latter stems from the greater pyramidalization of the Fe center, relative to nitrosylheme, which results in significant differences in d orbital hybridization. Thus, the singly occupied molecular orbital (SOMO) of [Fe(CN)(4)(NO)](2-) , while primarily Fe dz2-based, also has a significant amount of 4p(z) character, which makes it less stereochemically active, accounting for the linearity of the FeNO unit.     

Spectroscopic characterization of the [Fe(His)<Subscript>4</Subscript>(Cys)] site in 2Fe-superoxide reductase from<Emphasis Type="Italic"> Desulfovibrio vulgaris</Emphasis>

The electronic and vibrational properties of the [Fe(His)(4)(Cys)] site (Center II) responsible for catalysis of superoxide reduction in the two-iron superoxide reductase (2Fe-SOR) from Desulfovibrio vulgaris have been investigated using the combination of EPR, resonance Raman, UV/visible/near-IR absorption, CD, and VTMCD spectroscopies. Deconvolution of the spectral contributions of Center II from those of the [Fe(Cys)(4)] site (Center I) has been achieved by parallel investigations of the C13S variant, which does not contain Center I. The resonance Raman spectrum of ferric Center II has been assigned based on isotope shifts for (34)S and (15)N globally labeled proteins. As for the [Fe(His)(4)(Cys)] active site in 1Fe-SOR from Pyrococcus furiosus, the spectroscopic properties of ferric and ferrous Center II in D. vulgaris 2Fe-SOR are indicative of distorted octahedral and square-pyramidal coordination geometries, respectively. Differences in the properties of the ferric [Fe(His)(4)(Cys)] sites in 1Fe- and 2Fe-SORs are apparent in the rhombicity of the S=5/2 ground state ( E/ D=0.06 and 0.28 in 1Fe- and 2Fe-SORs, respectively), the energy of the CysS(-)(p(pi))-->Fe(3+)(d(pi)) CT transition (15150+/-150 cm(-1) and 15600+/-150 cm(-1) in 1Fe- and 2Fe-SORs, respectively) and in changes in the Fe-S stretching region of the resonance Raman spectrum indicative of a weaker Fe-S(Cys) bond in 2Fe-SORs. These differences are interpreted in terms of small structural perturbations in the Fe coordination sphere with changes in the Fe-S(Cys) bond strength resulting from differences in the peptide N-H.S(Cys) hydrogen bonding within a tetrapeptide bidentate "chelate". Observation of the characteristic intervalence charge transfer transition of a cyano-bridged [Fe(III)-NC-Fe(II)(CN)(5)] unit in the near-IR VTMCD spectra of ferricyanide-oxidized samples of both P. furiosus 1Fe-SOR and D. vulgaris 2Fe-SOR has confirmed the existence of novel ferrocyanide adducts of the ferric [Fe(His)(4)(Cys)] sites in both 1Fe- and 2Fe-SORs.     

Preparation, structures and electrochemical property of phosphine substituted diiron azadithiolates relevant to the active site of Fe-only hydrogenases

Mono- and di-phosphine diiron azadithiolate complexes [{(mu-SCH(2))(2)N(4-NO(2)C(6)H(4))}Fe(2)(CO)(5)(PMe(3))] (2), [{(mu-SCH(2))(2)N(4-NO(2)C(6)H(4))}{Fe(CO)(2)L}(2)] (3, L=PMe(3); 4, PMe(2)Ph) and the mu-hydride diiron complex [3(FeHFe)](+)[PF(6)](-) were prepared as biomimetic models of the active site of Fe-only hydrogenases. The complexes 2-4 and [3(FeHFe)](+)[PF(6)](-) were characterized by IR, (31)P, (1)H and (13)C NMR spectra and their molecular structures were determined by single crystal X-ray analyses. The PMe(3) ligand in complex 2 lies on the basal position. The PMe(3)-disubstituted complex 3 exists as two configuration isomers, transoid basal/basal and apical/basal, in the crystalline state, while two PMe(2)Ph ligands of 4 are in an apical/basal orientation. The variable temperature (31)P NMR spectra of 2 and 3 were made to have an insight into the existence of the possible conformation isomers of 2 and 3 in solution. The [3(FeHFe)](+) cation possesses the sole transoid ba/ba geometry as other reported mu-hydride diiron analogues. The electrocatalytic property of {(mu-SCH(2))(2)NC(6)H(5)}[Fe(CO)(2)PMe(3)](2) (5) was studied for proton reduction in the presence of HOAc.     

Synthesis, structural characterization and antimicrobial activities of 4- and 6-coordinate nickel(II) complexes with three thiosemicarbazones and semicarbazone ligands

To investigate the relationship between antimicrobial activities and the molecular structures of nickel(II) complexes with thiosemicarbazone and semicarbazone ligands, nickel(II) complexes with ligands Hmtsc, Hatsc, Hasc and H2dmtsc, were prepared and characterized by elemental analysis, FT-IR, 1H and 13C NMR spectroscopies, magnetic susceptibility measurements, UV-Vis absorption spectra, TG/DTA and single-crystal X-ray analysis. Their antimicrobial activities were evaluated by the MIC against four bacteria (B. subtilis, S. aureus, E. coli and P. aeruginosa), two yeasts (C. albicans and S. cerevisiae) and two molds (A. niger and P. citrinum). The 4-coordinate, diamagnetic nickel(II) complexes showed antimicrobial activities which were different from those of free ligands or the starting nickel(II) compounds; [Ni(mtsc)(OAc)] 1 showed selective and effective antimicrobial activities against two Gram-positive bacteria (B. subtilis and S. aureus) and modest activities against a yeast (S. cerevisiae), [Ni(mtsc)Cl] 3 exhibited moderate activities against a Gram-positive bacterium (S. aureus), and [Ni(atsc)(OAc)] 5 showed modest activities against two Gram-positive bacteria (B. subtilis and S. aureus). On the other hand, the 6-coordinate, paramagnetic nickel(II) complexes with two protonated or deprotonated ligands ([Ni(mtsc)2] 2, [Ni(atsc)(mtsc)] 4, [Ni(atsc)2] 6, [Ni(Hatsc)2](NO3)(2)7, [Ni(Hatsc)2]Cl(2)8 and [Ni(Hasc)2](OAc)(2)9) and the sterically crowded 4-coordinate, diamagnetic nickel(II) complex ([Ni(dmtsc)] 10) did not inhibit the growth of the test organisms. The structure-activity correlation in this series of nickel(II) complexes was discussed based on their ligand-replacement abilities.