Synthesis, characterization and bioactivity of two novel trinuclear Cu(II)/Ni(II) complexes with pentadentate ligand N-2-methyl-acryloyl-salicylhydrazide

Two new trinuclear complexes, Cu₃L₂(py)₂ (1) and Ni₃L₂(py)₄ (2), have been synthesized and characterized, where L³⁻ is N-2-methyl-acryloyl-salicylhydrazidate. Central metal ion and two terminal metal ions in the two complexes are combined by two bridging deprotonated L³⁻ ligands, forming a bent trinuclear structure unit with an M-N-N-M-N-N-M core. The bent angles in complexes 1 and 2 are 167.6(1)° and 75.4(1)°, respectively. Three nickel ions in compound 2 exhibit alternating square-planar and octahedral geometries, while three copper ions in compound 1 follow square-planar mode. The studies in solution integrity and stability of compounds 1 and 2 show they are soluble and stable in DMF. UV-Vis titrations demonstrate compound 1 is stable in DMF even in the presence of excess metal ions. Antibacterial screening data indicate the two compounds all have stronger antimicrobial activities against the tested microorganisms than ligand. The trinuclear copper compound 1 is more active than monocopper compounds in the previous study, and the trinuclear nickel compound 2 is less active than tetranuclear nickel compound in the previous study.     

Urease-null and hydrogenase-null phenotypes of a phylloplane bacterium reveal altered nickel metabolism in two soybean mutants

Mutation at either of two genetic loci (Eu2 or Eu3) in soybean (Glycine max [L.] Merr.) results in a pleiotropic elimination of the activity of both major urease isozymes. Surprisingly, the phenotype of a phylloplane bacterium, Methylobacterium mesophilicum, living on the leaves of eu2/eu2 or eu3-e1/eu3-e1 mutants is also affected by these plant mutations. The bacteria isolated from leaves of these soybean mutants have transient urease- and hydrogenase-deficient phenotypes that can be corrected by the addition of nickel to free-living cultures. The same bacterium growing on wild-type soybeans or on urease mutants eu1-sun/eu1-sun or eu4/eu4, each deficient in only one urease isozyme, are urease-positive. These results suggest that the bacterium living on the eu2/eu2 or eu3-e1/eu3-e1 mutant is unable to produce an active urease or hydrogenase because it is effectively starved for nickel. We infer that mutations at Eu2 or Eu3 result in defects in nickel metabolism but not in Ni²⁺ uptake or transport, because eu2/eu2 and eu3-e1/eu3-e1 mutants exhibit normal uptake of ⁶³NiCl₂. Moreover, wild-type plants grafted on mutant rootstocks produce seeds with fully active urease, indicating unimpeded transport of nickel through mutant roots and stems.     

Metal complexes of 1-methyl-3,4-diphenylpyrazole,a ligand formed by the reaction between benzil and N,N,-dimethylhydrazine

New complexes of the general formulae MnX₂L₂ (X = Cl,Br), MnBr₂L₃, CoX₂L₂ (X = Cl, Br, I, NCS, NO₃), NiX₂L₂ (X = Cl, NO₃), NiBr₂L₃·H₂O, NiL₂L₄·H₂O, CuCl₂L, CuBr₂L₂·H₂O, Cu(NO₃)₂L₂, ZnX₂L₂ (X = Cl, Br, NO₃, Zn(NCS)₂L₂·H₂O, CdX₂L₂ (X = I, NO₃) and HgCl₂L, where L is 1-methyl-3,4-diphenylpyrazole, have been prepared and characterized by elemental analysis, conductivity measurements, magnetic moments and spectral (¹H-NMR, IR and electronic) studies. The ligands is formed by the reaction between benzil and N,N-dimethylhydrazine. The nitrogen of the >CN bond is the donor atom to the metal ions. The bis-ligand halide complexes are pseudotetrahedral, while the nitrate complexes contain octahedrally coordinated metal ions. The IR spectra of MCl₂L (M = Cu, Hg) are indicative of the presence of both terminal and bridging metal-halogen bonds supporting polymeric structures. The stereochemistry and the nature of the nickel(II) complexes are markedly dependent upon the anions; the chloride complex is pseudotetrahedral, the iodide square planar, the nitrate polymeric octahedral, while the proposed structural formula for NiBr₂L₃·H₂O comprises Nickel(II) atoms present in both square planar and octahedral coordination environments.     

Studies of copper(II) and nickel(II) complexation by bis(1,4,7-triazacyclononane) ligands containing alkene and alkyne bridging groups

The copper(II) and nickel(II) complexes of three new 1,2-bis(1,4,7-triazacyclononane) ligands containing unsaturated four carbon bridging groups is studied by continuous variation UV-Vis spectroscopic and pH potentiometric equilibrium experiments. The cis-butene-2 (L_C) linked ligand may form monomeric MN₆-type complexes while the trans-butene-2 (L_T) and butyne-2 (L_Y) ligands are prevented by their stereochemistry from forming monomeric complexes and form oligomeric complexes. It is determined that the stability of the CuL_C²⁺ complex is not appreciably different from the oligomeric complexes of L_T and L_Y. Single-crystal X-ray structure determinations are made on three square pyramidal Cu₂L⁴⁺ complexes: [Cu₂L_CCl₄] (1), [Cu₂L_YCl₄] (2), and [Cu₂L_T(NO₃)₂(H₂O)₂](NO₃)₂ (3). The structure of [Ni₂(L_C)₂](ClO₄)₄ · 2H₂O (4) is a binuclear dimer that contains two nickel(II) ions sandwiched between two ligands, indicating that bis([9]aneN₃) ligands with four linker atom chains may form either monomeric or oligomeric structures.     

Synthesis, structures and properties of macrocyclic nickel(II) supramolecules with imidazole pendants

Two new nickel(II) complexes with the composition [Ni(L+H)(CH₃CN)₂](ClO₄)₃ (1) and [Ni(L)(tp)]·6H₂O (2), (L = 3,10-bis{3-(1-imidazolyl)propyl}-1,3,5,8,10,12-hexaazacyclotetradecane, tp = terephthalate) have been synthesized and structurally characterized by a combination of analytical, spectroscopic and X-ray diffraction methods. The structure of 1 consists of monomeric cations of the formula [Ni(L+H)(CH₃CN)₂]³⁺ and perchlorate ions. The nickel(II) ion is six-coordinate with bonds to the four nitrogen atoms of the macrocycle and two nitrogen atoms of the axial acetonitrile ligands. One of the protonated imidazole pendants of the macrocycle is hydrogen bonded to the imidazole group of the neighboring nickel(II) macrocycle, forming an undulated 1D supramolecule. Then, the two 1D supramolecular chains are further interconnected by C-H···π interactions between the methyl group of the acetonitrile ligand and one of the imidazole groups to form a 2D double stranded supramolecular polymer. In the structure of 2, the 1D coordination polymer is formed with nickel(II) macrocycles and bridging terephthalate ions, where each 1D chain is interconnected with π-π interactions of pendant imidazole moieties of the macrocycles, resulting in the formation of a 2D supramolecule.     

Complexation of a four-strand tetraalcohol with labile metal ions probed by electrospray mass spectrometry

The saturated, stereodefined tetraalcohol 2,3,5,6-endo,endo,endo,endo-tetrakis(hydroxymethyl)bicyclo[2.2.1]heptane (tetol, L¹) and the simple alcohol butane-1,3-diol (L²) form complexes with alkali metal ions (lithium, sodium, potassium, rubidium and caesium), alkali earth cations (magnesium, calcium, strontium and barium) and Ga(III) and Ce(IV) in aqueous solution, characterised by electrospray ionisation mass spectrometry (ESMS). Metal ion exchange between the Li⁺ complex of L¹ and the other metal ions is rapid, with a range of M(L¹)_n ^m+ species detected, in addition to solvated species. With the alkal metal ions, M(L¹)⁺ and M(L¹)₂ ⁺ are dominant, although speciation varies with metal ion size. For the alkaline earth ions, a range of complex ions up to n=8 are observed, although n=1-3 dominate. A preference for M(L¹)₂ ²⁺ with Mg²⁺ versus M(L¹)₃ ²⁺ with Ca²⁺ may again relate to a larger ion size. For the higher-charged Ga(III) and Ce(IV), hydroxo species M(OH)(L¹)_n ^(m−1)+ are dominant reflecting bulk solution behaviour, which the ESMS studies appear to map generally.     

Synthesis and solvatochromism studies of new mixed-chelate dinuclear copper(II) complexes with different counter ions

Four new symmetric mixed-chelate dinuclear complexes type [Cu₂(L)₂(TAE)]X₂, where TAE = tetraacetylethane; L = N,N-dimethyl-N′-benzylethylenediamine (L¹) or N,N′-dibenylethylenediamine (L²); X = ClO₄ or BPh₄ have been synthesized and characterized on the bases of elemental analysis, spectroscopic and conductance measurements. The X-ray crystal analysis of [Cu₂(L¹)₂(TAE)](ClO₄)₂ demonstrated that the two copper(II) ions are not equivalent. The axial position of the first copper is occupied by a ClO₄⁻ ion with a square pyramidal geometry whereas; the second copper ion resides in an octahedral environment determined by two perchlorate anions. However, in solution, the perchlorate ions are driven out by solvent molecules leading to their solvatochromism. The solvatochromism of the complexes were investigated in various organic solvents and also were compared with those of the corresponding mononuclear complexes [Cu(L)(acac)]ClO₄. Their solvatochromism were also investigated with different solvent parameters models using stepwise multiple linear regression (SMLR) method. The results suggested that the DN parameter of the solvent has the dominate contribution to the shift of the d-d absorption band of the complexes. The results demonstrated that the complexes with counter ions of BPh₄ are more solvatochromic in very weak donor solvents owing to their disinclination in ion-pair formation.     

Dinuclear Ni(II) and Cu(II) complexes with indolecarboxamide ligand: Synthesis, structure and properties

A potential tetradentate indolecarboxamide ligand, H₄L³ is synthesized and investigated for its coordination abilities towards Ni(II) and Cu(II) ions. Two H₄L³ ligands in their tetra-deprotonated form [L³]⁴⁻, were found to coordinate two metal centers resulting in the formation of [Ni₂(L³)₂]⁴⁻ (5) and [Cu₂(L³)₂]⁴⁻ (6) complexes. The crystal structure of 6 displays the formation of a dinuclear structure where two fully deprotonated ligands, [L³]⁴⁻ hold two copper(II) ions together. Even more interesting is the fact that both deprotonated ligands, [L³]⁴⁻ coordinate the copper ions in an identical and symmetrical fashion. The Na⁺ cations present in the complex 6 stitch together the dinuclear units resulting in the formation of a coordination chain polymer. Four sodium ions connect two dinuclear units via interacting with the O_amide groups. Further, Na⁺ cations were found to coordinate several DMF molecules; some of them are terminal and a few are bridging in nature. The solution state structure (determined by the NMR spectral analysis) of the diamagnetic complex 5 also supported the fact that two deprotonated ligands, coordinate two nickel ions in an identical and symmetrical fashion. Absorption spectral studies reveal that the solid-state square-planar geometry is retained in solution and both complexes do not show any tendency to coordinate potential axial ligands. The variable-temperature magnetic measurements and EPR spectra indicate spin-spin exchange between two copper centers in complex 6. The electrochemical results for both complexes show three irreversible oxidative responses that correspond to the oxidation of first and second metal ion followed by the ligand oxidation, respectively.     

Synthesis and structures of morpholine substituted new vic-dioxime ligand and its Ni(II) complexes

N,N^′-bis[4-(2-aminoethyl)morpholino]glyoxime (H₂L) (Fig. 1), has been prepared in various yields using three different methods. The most efficient of these methods is the technique of microwave irradiation. The crystal structures of H₂L, and of two nickel(II) complexes 1 and 2 have been determined by single crystal X-ray diffraction. Both nickel(II) complexes have a metal-ligand ratio of 1:2 in which the ligand coordinates through the two nitrogen atoms as do most vic-dioximes. The nickel(II) complexes are either hydrogen (1) or boron diphenyl bridged (2). Complex 1 was synthesized by reacting H₂L with nickel(II) chloride in refluxing ethanol. Complex 2 was prepared at room temperature in an ethanol solution containing excess NaBPh₄. Elemental analyses, NMR(¹H, ¹³C), IR and mass data are also presented.     

Complexes of 2-acetyl-gamma-butyrolactone and 2-furancarbaldehyde thiosemicarbazones: antibacterial and antifungal activity

Cobalt, nickel, copper and zinc coordination compounds of two thiosemicarbazones with general composition ML₂ (L: monodeprotonated ligand corresponding to 2-acetyl-γ-butyrolactone thiosemicarbazone, HL¹, and 2-furancarbaldehyde thiosemicarbazone, HL²) and also complexes with general composition MCl₂(HL²) were synthesized (except [NiCl₂(HL²)] and [Co(L²)₂]). The interaction of CuCl₂ with HL² gave [CuCl(HL²)], a copper(I) complex. The ligands and metal complexes were characterized by IR, ¹H and ¹³C NMR spectroscopy, and magnetic susceptibility measurements. The crystal structure of [Ni(L²)₂] · 2dmso was determined and a trans-square planar coordination of the two κ²-N,S chelate rings forming polymeric strips through H-bonds with dmso was observed. Actually, in all the reported complexes both ligands behaved as κ²-N,S chelates, except in the case of [Co(L¹)₂] in which HL¹ is tridentate κ³-N,S,O. The antimicrobial properties of all compounds were studied using a wide spectrum of bacterial and fungal strains. The copper complexes of HL² were the most active against all strains, including dermatophytes and phytopathogenic fungi. Most of the studied compounds, especially [Cu(L¹)₂], presented good activity against Haemophilus influenzae, a very harmful bacterium to humans.     

Synthesis and characterization of new adjacent-bridged tetraaza macrocyclic compounds with C-alkyl groups: crystal structure and spectral properties of a copper(II) complex

New structurally constrained tetraaza macrocycles 2,3-dimethyl-1,5,12,16-tetraazapentacyclo[14.6.3.3^5.12.0^6.11.0^17.22]octacosane (L⁶) and 2,2,4,10,12,18,20-octamethyl-1,5,9,13-tetraazatricyclo[11.3.2. 2^5.9]eicosane (L⁷) have been prepared in high yield by direct reaction of 1,3-dibromopropane with 3,14-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.0^7.12]docosane (L³) or 2,5,5,7,9,12,12,14-octamethyl-1,4,8,11-tetraazacyclodecane (L⁵). The macrocycle L⁶ readily reacts with anhydrous copper(II) ion to yield [CuL⁶]²⁺ in dry methanol but does not react with nickel(II) ion, showing a high copper(II) ion selectivity. On the other hand, neither of the metal ions is inserted into the cavity of L⁷ in similar conditions. The copper(II) complex [CuL⁶](ClO₄)₂ has a severely distorted square-planar coordination polyhedron with a rarely observed trans-IV type N-conformation. The visible absorption spectrum of [CuL⁶]²⁺ shows the d-d transition band at 490 nm (ε=690 M⁻¹ cm⁻¹); the wavelength is quite similar to that of [CuL³]²⁺, but the molar absorption coefficient is extraordinarily larger than those of [CuL³]²⁺ and other related tetraaza macrocyclic copper(II) complexes.     

Synthesis, structure and Mössbauer characterization of polymeric iron(II) complexes with bidentate thiourea ligands

Complexes of FeCl₂ with the known bis(3-methyl-2-thione-imidazolyl)methane (L¹) and the new bis(3-tert-butyl-2-thione-imidazolyl)methane (L²) are reported. For both [L¹FeCl₂]_n (3) and [L²FeCl₂]_n (4) X-ray crystallography reveals that 1D-polymeric chain structures are present in the solid state, with the two mercaptoimidazolyl units of L¹ and L² coordinating to different metal ions. Complexes 3 and 4 are further characterized by Mössbauer spectroscopy and SQUID magnetometry. NMR spectroscopy suggests that the complexes largely dissociate in polar solvents. X-ray structures of L² and its precursor bis(imidazolium) salt are also reported.     

Urease from Arthrobacter oxydans,a nickel-containing enzyme

In Arthrobacter oxydans, Klebsiella aerogenes and Sporosarcina ureae, growth with urea as a nitrogen source turned out to be more sensitive to inhibition by EDTA than that with ammonia. The inhibition was overcome by added nickel chloride, but not by other divalent metal ions tested. In A. oxydans the uptake of ⁶³Ni was paralleled by an increase in urease (urea amidohydrolase, EC 3.5.1.5) activity under certain conditions. Following growth with radioactive nickel, urease from this strain was enriched by heat treatment and acetone fractionation. Copurification of ⁶³Ni and urease was observed during subsequent Sephadex gel chromatography. Almost the entire labelling was detected together with the purified enzyme after focusing on polyacrylamide gel. The relative molecular mass of the purified urease was estimated to be 242,000. The pH optimum was 7.6, the K _m-value 12.5 mmol/l and the temperature optimum 40°C; heat stability was observed up to 65°C. In presence of 10 mmol/l EDTA the protein-nickel binding remained intact at pH 7; at pH 5 and below, nickel was irreversibly removed with concommitant loss of enzyme activity. The results demonstrated that nickel ions are required for active urease formation in the bacterial strains studied, and that urease from A. oxydans is a nickel-containing enzyme.Dedicated to Professor Dr. H.-G. Schlegel on the occasion of his 60th birthday     

Synthesis, characterization and X-ray crystal structures of copper(II) and nickel(II) complexes with potentially hexadentate Schiff base ligands

Copper(II) and nickel(II) complexes of potentially N₂O₄ Schiff base ligands 2-({[2-(2-{2-[(1-{2-hydroxy-5-[2-phenyl-1-diazenyl]phenyl}methylidene)amino] phenoxy}ethoxy) phenyl]imino}methyl)4-[2-phenyl-1-diazenyl]phenol (H₂L¹) and 2-({[2-(4-{2-[(1-{2-hydroxy-5-[2-phenyl-1-diazenyl]phenyl}methylidene)amino] phenoxy}butoxy) phenyl]imino}methyl)4-[2-phenyl-1-diazenyl]phenol (H₂L²) prepared of 5-phenylazo salicylaldehyde (1) and two various diamines 2-[2-(2-aminophenoxy)ethoxy]aniline (2) and 2-[4-(2-aminophenoxy)butoxy]aniline (3) were synthesized and characterized by a variety of physico-chemical techniques. The single-crystal X-ray diffractions are reported for CuL¹ and NiL². The CuL¹ complex contains copper(II) in a near square-planar environment of N₂O₂ donors. The NiL² complex contains nickel(II) in a distorted octahedral geometry coordination of N₂O₄ donors. In all complexes, H₂L¹ behaves as a tetradentate and H₂L² acts as a hexadentate ligand. Cyclic voltammetry of copper(II) complexes indicate a quasi-reversible redox wave in the negative potential range.     

Metal complexes with two tri-aza,tri-oxa pendant-armed macrocyclic ligands: Synthesis,characterization, crystal structures and fluorescence studies

Metal complexes of two new tri-aza, tri-oxa macrocycles containing ethyl acetate (L¹) or carboxymethyl (H₂L²) pendant arms with hydrated nitrate or perchlorate salts of alkaline earth, post-transition and lanthanide metal ions have been synthesized and characterized by microanalysis FAB MS, conductivity measurements, IR, UV–Vis spectroscopy and fluorescence emission studies. The synthesis and characterization of the Pb(II) complexes with the armless macrocyclic precursors L (Schiff base macrocycle) and L′ (diaminic reduced macrocycle) are also reported. The crystal structures of complexes [PbL(ClO₄)(H₂O)](ClO₄), [PbL′(ClO₄)](ClO₄) and ([Zn₂L²(Cl)(H₂O)](ClO₄))_∞ have been determined. In both lead(II) complexes, the metal ion is located inside the macrocyclic cavity and is coordinated by all N₃O₃ donor atoms in the complex with L′ but only by the nitrogen atoms present in the ligand in the complex with L. In both cases, the coordination sphere of the metal atom is completed with a perchlorate anion or a water molecule in the iminic complex of L. X-ray studies on the Zn(II) complex show the presence of a supramolecular structure that is consistent with a linear polymer formed alternately by an endomacrocyclic metal atom coordinated to a macrocyclic ligand and an exomacrocyclic metal ion in distorted octahedral and tetrahedral environments, respectively. UV–Vis and fluorescent emission studies were carried out on the ligands L¹ and H₂L² and their metal complexes, but only the luminescence spectra of the Eu(III) and Tb(III) complexes with L¹ in aqueous solution at ca. pH 7 show the characteristic visible emission of the metal. The value of the quantum yield determined for the Eu(III) complex is similar to that reported in the literature for other Eu(III) complexes.     

Synthesis,DNA Binding,and Oxidative Cleavage Studies of Fe(II) and Co(III) Complexes Containing Bioactive Ligands

The two complexes containing bioactive ligands of the type and [Fe(L)] (PF₆)₂ (1) (where L = [1-{[2-{[2-hydroxynaphthalen-1-yl)methylidine]amino}phenyl)imino] methyl}naphthalene-2-ol]) and [Co(L₁L₂)] (PF₆)₃ (2) (where L₁L₂ = mixed ligand of 2-seleno-4-methylquinoline and 1,10-phenanthroline in the ratio 1:2, respectively) were synthesized and structurally characterized. The DNA binding property of the complexes with calf thymus DNA has been investigated using absorption spectra, viscosity measurements, and thermal denaturation experiments. Intrinsic binding constant K_b has been estimated at room temperature. The absorption spectral studies indicate that the complexes intercalate between the base pairs of the CT-DNA tightly with intrinsic DNA binding constant of 2.8 × 10⁵ M^?1 for (1) and 4.8 × 10⁵ M^?1 for (2) in 5 mM Tris-HCl/50 mM NaCl buffer at pH 7.2, respectively. The oxidative cleavage activity of (1) and (2) were studied by using gel electrophoresis and the results show that complexes have potent nuclease activity.     

Alkali and alkaline-earth metal complexes of different nuclearities with azamacrocyclic pendant-arms ligands

A new series of alkali and alkaline-earth complexes of Na(I), Ca(II), Sr(II) and Ba(II) with two pyridyl pendant-arms azamacrocyclic ligands, L¹ and L², were synthesized by reaction of the ligand and the appropriate perchlorate salt in refluxing acetonitrile. The reactions gave analytically pure products that were characterized by elemental analysis, FAB mass spectrometry, IR, conductivity measurements and ¹H NMR spectroscopy. The crystal structure of [NaL¹](ClO₄), [BaL¹](ClO₄)₂ · 2CH₃CN and [Ca₂L²(μ-ClO₄)(H₂O)₂](ClO₄)₂ · Cl · (H₂O)_0.5 could be also determined. The crystal structure of the Na(I) and Ba(II) complexes with L¹ show endomacrocyclic mononuclear species, where the metal ions are in N8 and N10 coordination environment, respectively. The geometry around the metal ions can be described as cube and bicapped cube geometry for Na(I) and Ba(II), respectively. Instead, the crystal structure of the Ca(II) compound with L² shows an exomacrocyclic dinuclear complex where both metal ions are in a similar N5O2 environment joined through a bidentate perchlorate anion. The coordination geometry for both Ca(II) ions can be described as distorted pentagonal bipyramid.     

A study on mono-, bis- and tris-N-functionalized 1,4,7-triazacyclononane with benzimidazolyl groups and their nickel(II) complexes

Three new ligands, 1-(benzimidazolyl-2-methyl)-1,4,7-triazacyclononane L¹, 1,4-bis(benzimidazolyl-2-methyl)-1,4,7-triazacyclonone L², and 1,4,7-tris-(benzimidazolyl-2-methyl)-1,4,7-triazacyclonane L³, were synthesized by a straightforward one-pot method. Their nickel(II) complexes , [NiL²CH₃CN](ClO₄)₂ · 2CH₃OH · H₂O (or [NiL²Cl] · ClO₄) and [Ni(H₋₂L³)] · H₂O were obtained and characterized by electrospray mass spectrum, ¹H NMR, CV and other physical methods. Their crystal structures were determined by X-ray analyses. The crystal structure of the nickel(II) complex of L¹ shows that two Ni(II) atoms are bridged by two Cl⁻ anions. A ferromagnetic exchange coupling and zero-field splitting effect exist in complex 1.     

Characterization of urease from Sporosarcina ureae

Alkaline stable (pH 7.75–12.5) urease from Sporosarcina ureae was purified over 400-fold by ion exchange and hydrophobic interaction chromatography. The cytoplasmic enzyme was remarkably active with a specific activity of greater than 9300 μmol urea degraded min^-1 mg protein^-1 at pH 7.5, where it has optimal activity. Although S. ureae is closely related to Bacillus pasteurii, known to posses a homopolymeric urease containing 1 nickel per subunit [M _r=65000], the S. ureae enzyme is comprised of three subunits [apparent M _r=63100 (α), 14500 (β), and 8500 (γ)] in an estimated ∝βγ stoichiometry and contains 2.1±0.6 nickel ions per ∝βγ unit as measured by atomic absorption spectrometry. Stationary phase cultures sometimes possessed low levels of urease activity, but the specific activity of cell extracts of partially purified urease preparations from such cultures could be elevated by heat treatment, dilution, or dialysis to values comparable to those observed in samples from exponentially grown cells.     

Substrate-induced Assembly of Methanococcoides burtonii d-Ribulose-1,5-bisphosphate Carboxylase/Oxygenase Dimers into Decamers

Like many enzymes, the biogenesis of the multi-subunit CO₂-fixing enzyme ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) in different organisms requires molecular chaperones. When expressed in Escherichia coli, the large (L) subunits of the Rubisco from the archaeabacterium Methanococcoides burtonii assemble into functional dimers (L₂). However, further assembly into pentamers of L₂ (L₁₀) occurs when expressed in tobacco chloroplasts or E. coli producing RuBP. In vitro analyses indicate that the sequential assembly of L₂ into L₁₀ (via detectable L₄ and L₆ intermediates) occurs without chaperone involvement and is stimulated by protein rearrangements associated with either the binding of substrate RuBP, the tight binding transition state analog carboxyarabinitol-1,5-bisphosphate, or inhibitory divalent metal ions within the active site. The catalytic properties of L₂ and L₁₀ M. burtonii Rubisco (MbR) were indistinguishable. At 25 °C they both shared a low specificity for CO₂ over O₂ (1.1 mol·mol⁻¹) and RuBP carboxylation rates that were distinctively enhanced at low pH (∼4 s⁻¹ at pH 6, relative to 0.8 s⁻¹ at pH 8) with a temperature optimum of 55 °C. Like other archaeal Rubiscos, MbR also has a high O₂ affinity (K_m(O₂) = ∼2.5 μm). The catalytic and structural similarities of MbR to other archaeal Rubiscos contrast with its closer sequence homology to bacterial L₂ Rubisco, complicating its classification within the Rubisco superfamily.