Cobalt-bleomycin-deoxyribonucleic acid system. Evidence of deoxyribonucleic acid bound superoxo and mu-peroxo cobalt-bleomycin complexes

Co(II) interacts with bleomycin in aqueous solution, in the presence of air, to give a short-lived mononuclear superoxo Co(III) complex (I). Then, two molecules of complex I react together, with the loss of oxygen, to yield the dinuclear mu-peroxo Co(III) complex (II); the dimerization follows a second-order rate law with k2 = 200 +/- 50 M-1 s-1 at 25 degrees C. The rate of dimerization is lowered by a factor of 2000 when DNA is present at a molar ratio of [nucleotide]/[Co] higher than 16. These results and studies of circular dichroism and electron paramagnetic resonance spectra of complexes strongly suggest the binding of the superoxo complex to DNA (I') as well as that of the mu-peroxo complex (II'); the binding of 1 molecule of complex II for every 2.9 base pairs in DNA has been determined with an apparent equilibrium constant of 8.4 x 10(4) M-1.     

Structural characterization of adenine nucleotides bound to Escherichia coli adenylate kinase. 2. 31P and 13C relaxation measurements in the presence of cobalt(II) and manganese(II)

13C spin-lattice relaxation rates have been measured for two complexes of Escherichia coli adenylate kinase (AKe), viz., AKe. [U-(13)C]ATP and AKe.[U-(13)C]AMP.GDP in the presence of the substituent activating paramagnetic cation Mn(II) for the purpose of determination of the enzyme-bound conformations of ATP and AMP. (GDP has been added to the AMP complex with the enzyme in order to hold the cation in the bound complex.) Measurements of relaxation times at three different (13)C frequencies, 181.0, 125.7, and 75.4 MHz, indicate that the relaxation times in the enzyme-nucleotide complexes with the paramagnetic cation are not exchange-limited; i.e. , they are larger than the effective lifetimes of cation binding to these complexes and are, therefore, dependent on the cation-(13)C distances. An analysis of the frequency-dependent relaxation data allowed all of the ten Mn(II)-(13)C distances to be determined in each of the complexes. Similar measurements of the (31)P relaxation rate made on AKe.ATP and AKe.AMP.GDP complexes in the presence of Co(II) as the activating cation yielded Co(II)-(31)P distances for each adenine nucleotide. These distances, together with the interproton distances determined previously from TRNOESY experiments [Lin, Y., and Nageswara Rao, B. D. (2000) Biochemistry 39, 3636-3646], led to a complete characterization of both ATP and AMP conformations in AKe-bound complexes. These conformations differ significantly from the nucleotide conformations in crystals of AKe. AP(5)A and AKe.AMP.AMPPNP as determined by X-ray crystallography.     

Spectroscopic properties of the cobalt(II)-substituted alpha-fragment of rabbit liver metallothionein

The C-terminal segment of rabbit liver metallothionein 1 (alpha-fragment) containing four paramagnetic Co(II) ions was obtained by stoichiometric replacement of the originally bound diamagnetic Cd(II) ions. The latter form was prepared by limited proteolysis with subtilisin as described previously [Winge, D. R., & Miklossy, K. A. (1982) J. Biol. Chem. 257, 3471-3476]. Electronic absorption, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) measurements were employed to monitor the stepwise incorporation of Co(II) ions into the metal-free fragment. Absorption and MCD spectra of the apofragment containing the first 3 Co(II) equiv show the typical features of tetrahedral tetrathiolate Co(II) coordination. However, in the d-d region only small changes in the visible and no apparent change in the near-infrared region are discernible when the fourth Co(II) is bound. This unusual spectral behavior was not seen in Co(II) substitution of native metallothionein [Vasák, M., & K?gi, J. H. R. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6709-6713] and may indicate a different cluster geometry. In the charge-transfer region, the binding of all 4 Co(II) equiv is accompanied by characteristic increments of the thiolate S----Co(II) bands. As in the formation of Co(II)7-metallothionein, the development of the charge-transfer and EPR spectral properties upon binding of the first 2 Co(II) equiv to the apofragment is indicative of isolated, noninteracting tetrahedral tetrathiolate Co(II) complexes. The binding of the additional Co(II) ion is accompanied by a red shift in the charge-transfer region and by the dramatic loss of paramagnetism in the EPR spectra, both diagnostic of the formation of metal-thiolate cluster structures.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synthesis, spectroscopic, and antitumor activity of metal chelates of S-methyl-N-(l-isoquinolyl)-methylendithiocarbazate

Complexes of Mn(III), Fe(III), Fe(II), Co(III), Ni(II), Cu(II), Zn(II), and Pt(II) with S-methyl-N-(l-isoquinolyl) methylendithiocarbazate (N-N-SH) were isolated and characterized by elemental analysis, conductance measurement, magnetic susceptibilities, and spectroscopic studies. On the basis of these studies, a highly distorted, high-spin, chloro-bridged, polymeric octahedral structure for [Mn(N-N-S)Cl2]; a distorted, low-spin, monomeric octahedral structure for [Fe(N-N-S)2]; a distorted, high-spin, octahedral structure for [Ni(N-N-S)2]; and a square-planar structure for [M(N-N-S)X] (M = Ni, Cu, Pt or Zn and X = Cl- or -OAc) are suggested. With Fe(III), the complex [Fe(N-N-S)2][FeCl4] was isolated while the Co(II) was oxidized to yield the Co(III) ion as [Co(N-N-S)2]2[CoCl4]. All these complexes were screened for their antitumor activity against P 388 lymphocytic leukemia test system in mice. Except for Mn(III), Fe(III), and Co(III) complexes, all were found to possess significant activity; the Cu(II) and Zn(II) complexes showed a T/C% value of 160 and 195, respectively, at their optimum dosages.     

Synthesis,characterization of some transition metal(II) complexes of acetone <Emphasis Type="Italic">p-</Emphasis>amino acetophenone salicyloyl hydrazone and their anti microbial activity

Complexes of the type [M(apash)Cl] and [M(Hapash)(H2O)SO4], where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II); Hapash = acetone p-amino acetophenone salicyloyl hydrazone have been synthesized and characterized by elemental analyses, molar conductance, magnetic moments, electronic, ESR and IR spectra, thermal studies (TGA & DTA) and X-ray diffraction studies. The ligand coordinates through two >C=N and a deprotonated enolate group in all the chloro complexes, whereas through two >C=N- and a >C=O group in all the sulfato complexes. The electronic spectra suggest a square planar geometry for Co(II), Ni(II) and Cu(II) chloride complexes and an octahedral geometry for the sulfate complexes. ESR data show an isotropic symmetry for [Cu(apash)Cl] and [Cu(Hapash)(H2O)SO4] in solid state. However, ESR spectra of both Cu(II) complexes indicate the presence of unpaired electron in d x2-y2. The X-ray diffraction parameters for [Co(apash)Cl] and [Cu(Hapash)(H2O)SO4] complexes correspond to a tetragonal and an orthorhombic crystal lattices, respectively. Thermal studies of [Co(apash)Cl] complex shows a multi-step decomposition pattern. Most of the complexes show better antifungal activity than the standard miconazole against a number of pathogenic fungi. The antibacterial activity of these complexes has been evaluated against E. coli and Clostridium sp. which shows a moderate activity.     

Cobalt(II) and zinc(II) binding to a ferredoxin maquette

We have examined the Co(II) and Zn(II) affinity of the prototype ferredoxin maquette ligand, NH(2)-KLCEGG.CIACGAC.GGW-CONH2 (IAA), which was originally designed to bind a [4Fe-4S] cluster. UV-Vis spectroscopy demonstrates tight 1:1 complex formation between Co(II) and IAA. The intensity of the S-->Co(II) charge transfer bands at 304 and 340 nm and the ligand field bands between 630 and 728 nm indicate Co(II) coordination by the four cysteine thiolates of IAA in a pseudo-tetrahedral geometry. A dissociation constant value of 5.3 microM was determined for the Co(II)-IAA complex at pH 6.5. Zn(II) readily displaces Co(II) from IAA as evinced by loss of the Co(II) spectral features. The dissociation constant for Zn(II), 20 pM at pH 6.5, was determined be competition experiments with Co(II)-IAA. These results demonstrate that the ferredoxin maquette ligand is an excellent ligand for Zn(II).     

Transition metal complexes of buparvaquone as potent new antimalarial agents. 1. Synthesis,X-ray crystal-structures,electrochemistry and antimalarial activity against Plasmodium falciparum

New Cu(II), Ni(II), Co(II), Fe(II), and Mn(II) metal complexes of buparvaquone [3-trans(4-tert.-butylcyclohexyl)methyl-2-hydroxy-1,4-naphthoquione] (L1H) have been synthesized and characterized using IR, electron paramagnetic resonance (EPR) spectroscopy, microanalytical methods and single crystal X-ray diffraction methods. The single crystal structures were determined for ligand L1H [space group P-1 with a=6.2072(14) A, b=10.379 (2) A, c=13.840 (3) A, V=878.7(3) A(3), Z=2, D(calcd.)=1.234 mg/m(3)] and copper complex [Cu(L1)(2)(C(2)H(5)OH)(2)] C1 [space group I2/a with a=17.149(14) A, b=9.4492(8) A, c=26.946(3) A, V=4335.3(7)A(3), Z=4, D(calcd.)=1.233 mg/m(3)]. All the metal complexes along with the parent ligand have been studied for their electrochemical properties using cyclic voltammetric techniques. The compounds were tested for their in vitro antimalarial activity against Plasmodium falciparum strains. A correlation between the antimalarial activity and the redox property of these complexes is presented. The copper complex C1 exhibits significantly higher growth inhibitory activity both in vitro and in vivo than the parent ligand.     

Nuclear magnetic resonance studies of substrate interaction with cobalt substituted alcohol dehydrogenase from liver.

The role of zinc in liver alcohol dehydrogenase has been studied by replacement of 1.3 and 3.5 of the four Zn(II) ions with Co(II) and measuring the effects of the paramagnetic Co(II) on the relaxation rates of the protons of water, ethanol, and isobutyramide. Water relaxation studies at 8, 24, 100, and 220 MHz indicate two classes of bound Co(II). The similar to 2 readily replaced Co(II) ions retain one fast exchanging water proton in their inner coordination spheres, while the similar to 2 slowly exchanging Co(II) ions coordinate no detectable water protons, indicating that the former replaced Zn(II) at the "catalytic sites" and the latter replaced Zn(II) at the "structural sites" detected crystallographically. Ethanol, acetaldehyde, and isobutyramide bind with appropriate affinities to the Co(II) substituted alcohol dehydrogenases decreasing the number of fast exchanging protons at the catalytic Co(II) site by greater than or equal to 54 percent. Coenzyme binding causes smaller changes in the water relaxation rate which may be due to local conformation changes. The paramagnetic effects of Co(II) at the catalytic site on the relaxation rates of the methyl protons of isobutyramide at 100 and 220 MHz indicate that this analog binds at a site 9.1 A from the catalytic Co(II). This distance decreases to 6.9 A when NADH is bound, and a Co(II) to methyne proton distance of 6.6 A is determined indicating a conformation change leading to the formation of a second sphere enzyme-Co(II)-isobutyramide complex in which a hydroxyl or water ligand intervenes between the metal and the substrate analog. Similar behavior is observed in the enzyme-ethanol complexes. The paramagnetic effects of Co(II), at the catalytic site, on the relaxation rates of the protons of ethanol at 100 and 220 MHz, indicate that this substrate bind at a site 12-14 A distant from the catalytic Co(II) but that this distancedecreases to 6.3 A in the abortive enzyme-NADH-ethanol complex. The role of the catalytic Co(II) thus appears to be the activation of a hydroxyl or water ligand which polarizes the aldehyde carbonyl group by hydrogen bonding. The role of the structural Co(II), which is more distant from isobutyramide (9-11 A), may be that of a template for protein conformation changes. By combining the present distances with those from previous magnetic resonance studies on the liver enzyme, the arrangement of coenzyme, metal, and substrate at the active site in solution can be constructed. This arrangement is consistent with that of ADP-ribose and zinc in the crystalline complex of liver alcohol dehydrogenase as determined by X-ray crystallography (Branden et al., (1973), Proc. Natl. Acad. Sci. U.S.A.70, 2439).     

Co-ordination of transition metal ions by galactaric acid: a potentiometric and spectroscopic study

A solution study on the ability of galactaric acid [GalaH(2), HOOC(CH)(4)COOH] in the complexation of biological metal ions such as Co(II) and Ni(II) and toxic metal ions such as Cd(II), Pb(II) and Hg(II), is reported. The stability constants of the complex species are determined by means of potentiometric measurements. Galactaric acid behaves as chelate ligand through carboxylic oxygen and alpha-hydroxy group towards Co(II) and Ni(II), while in the Pb(II) and Cd(II) containing system it co-ordinates the metal ion with carboxylic oxygen and two alcoholic hydroxy groups. The prevailing species at acidic or neutral pH is [MGala] which is also isolated in the solid state and characterized by means of IR spectroscopy. On increasing pH, the [MGalaH(-1)](-) species is also formed where the co-ordinated OH group undergoes deprotonation in all metal ion complexes except those with Hg(II), where the co-ordination of hydroxide ion is suggested as the precipitation of the metal hydroxide occurs at pH 7.     

Electron-paramagnetic-resonance studies on cobalt(II) carbonic anhydrase. Low-spin cyanide complexes

The low-spin cyanide complexes of three Co(II) carbonic anhydrases were investigated by electron paramagnetic resonance (e.p.r.) at 9 and 35GHz. Well-defined and closely axial spectra were obtained only in the absence of oxygen. Several mole equivalents of cyanide were required for complete formation of the complexes in frozen solution, although large excesses caused abstraction of the cobalt. Experiments with [(13)C]cyanide showed that the low-spin complexes contained two CN(-) groups in an environment similar to that of the in-plane ligands in [Co(CN)(5)](3-). A combined e.p.r. and spectrophotometric titration confirmed the presence of two CN(-) ligands. A 5-co-ordinate square pyramidal structure involving three protein ligands was proposed. The dicyanide complex could be oxygenated reversibly, producing a characteristic new e.p.r. spectrum. The O(2) molecule was thought to occupy the remaining octahedral metal site in a formally Co(III) species. The optical spectrum of the dicyanide lacked the prominent d-d bands of the high-spin monocyanide. Both e.p.r. and optical data indicated that the low-spin complex was formed much more fully in frozen solution than at room temperature. Differences in behaviour between the high- and low-activity enzymes suggested some variation in conformational flexibility at the metal binding site.     

Mononuclear and binuclear Co(III)-dioxygen adducts of bleomycin. Circular dichroism and electron paramagnetic resonance studies

Co(II) interacts with bleomycin in aqueous solution, in the presence of air, to give a short lived mononuclear superoxo-Co(III) complex (I) identified previously, by Sugiura, by electron paramagnetic resonance measurements. This complex rapidly releases O₂ to yield the dinuclear μ-peroxo-Co(III) complex (II), but is stabilized by the presence of DNA yielding a new superoxo long lived species (I′). The absorption and circular dichroism spectra of the three species (I,I′,II) have been characterized.     

Metal(II) complexes of 1-formylisoquinoline thiosemicarbazone: their preparation,characterization and antitumour activity

Complexes of Co(II), Ni(lI), Cu(II), Zn(II) and Pt(II) with 1-formylisoquinoline thiosemicarbazone (1-iqtsc-H) were prepared and characterized by elemental analyses, conductance measurement and spectral studies. On the basis of these studies a distorted octahedral structure for [Co(1-iqtsc)₂]·2H₂O, a distorted trigonal-bipyramidal structure for [Ni- (1-iqtsc-H)Cl₂], [Cu(1-iqtsc-H)Cl₂] and [Zn(1-iqtsc- H)(OAc)₂]·H₂O and a square-planar structure for [Pt(1-iqtsc)Cl] are suggested. All these metal(II) complexes were screened for their antitumour activity in the P388 lymphocytic leukaemia test system in mice. Except for Pt(Il), the complexes were found to possess significant activity; the Ni(II) complex showed a T/C value of 161 at the optimum dosage.     

ESR evidence of the formation of a new superoxide complex of tetra-p-tolyporphyrinatocobalt(II) in aprotic solvents

The reactions of the superoxide ion (O₂⁻) with tetra-p-tolyporphyrinatocobalt(II) [Co(II)TTP] in dimethyl sulfoxide(DMSO) have been investigated by use of electron spin resonance (ESR) spectroscopy. In the absence of oxygen, Co(II)TTP in DMSO gives the DMSO adduct, Co(II)(TPP)(DMSO). When this DMSO adduct is exposed to air, an oxygen complex, Co(II)(TTP)(DMSO)(O₂), is formed in which the binding state between Co(II) and O₂ has been considered formally as Co(III)O₂⁻. When the superoxide ion (O₂⁻ is added to this oxygen complex, a new superoxide complex, Co(II)(TTP)(O₂⁻)₂, is formed. The same superoxide adduct is formed by the reaction of O₂⁻ with Co(II)TTP in the absence of oxygen.     

The interaction of metal ions with nucleic acids. NMR, EPR, CD and spectrophotometric study of the copper (II) interaction with 6-ketopurine ribosides

The copper (II)-inosine system in water-DMSO solutions was investigated as a function of pH and the molar ratio between the ligand and copper(II) ion by the EPR, NMR, CD and visible absorption spectrometric methods. It was concluded that a simple M.[-N]L copper(II)-inosine 1:1 complex is formed over the pH range 1.4-5.0, while M.[-N]L2 complexes are present in the solutions of pH 5.0-6.2. From pH 6.2 to 7.8 a diamagnetic, hydroxybridged complex M2.(OH)2.[-N]L4 dominates. At pH values of 7.8-9.2 an insoluble, oxybridged species (M.O.[-N]L)n is formed in addition to the soluble paramagnetic M.[-N-1)L4 complex. Above pH 9.1 the nitrogenbridged polymeric complex (M.[-N-1].M[-N-7] )n is formed which is stable up to pH 12.5, and above pH 12.5 the only species found is the M.[-OH]L2 chelate complex in which inosine is coordinated to copper through the two ionized hydroxyl groups.     

Stable [57Co]-bleomycin complex with a very high specific radioactivity for use at very low concentrations.

A method for the preparation of 57Co-labelled bleomycin (BLM) possessing very high specific radioactivity and suitable for use at the nanomolar concentration range is described, and validated using a biological assay. Chelation of BLM with Co(II) results in a very stable complex. However, association does not occur below the micromolar concentration range. A nanomolar [57Co]BLM solution with maximal specific radioactivity can be easily prepared, without handling unreasonable amounts of radioactivity, provided that: equimolar solutions of BLM and [57Co]Cl2 are first mixed at the micromolar concentration range and that the mixture is then diluted a thousand times to reach the nanomolar concentration range.     

Hydrophobic interactions of transcobalamin II (TC II) from mammalian sera

The hydrophobic properties of mammalian transcobalamin IIs (TC II) were studied by chromatography of radioactive cyanocobalamin (CN[57Co]Cbl)-labeled serum on phenyl-Sepharose CL-4B. Mammalian holo TC IIs (CN[57Co]Cbl-TC II) exhibited species variability in their affinity for the hydrophobic matrix in the order: dog greater than mouse greater than human greater than rat greater than rabbit. Phenyl-Sepharose chromatography of the isolated CN[57Co]Cbl-TC II peaks from gel filtration of dog and rat serum showed no hydrophobic change in dog TC II, but an increase in hydrophobicity of rat TC II. Phenyl-Sepharose chromatography of CN[57Co]Cbl-labeled rabbit serum (holo TC II) and the unlabeled serum (apo TC II) showed apo TC II to be more hydrophobic than holo TC II as has been shown for human TC II (Begley et al., Biochem Biophys Res Commun 103:434-441, 1981). Thus mammalian holo TC IIs differ in their hydrophobic properties and apo TC II, in man and rabbit, is more hydrophobic than holo TC II. In addition, isolation of the TC II in some animal sera by gel filtration may result in a TC II that is more hydrophobic than the native molecule.     

Interaction between transcobalamin II and immobilized Cibacron Blue F3GA

Human serum transcobalamin II (TC II), a vitamin B₁₂ (Cbl) transport protein, complexes with Cibacron Blue F3GA, a reactive blue dye which can bind to proteins that require nucleotides as cofactors. Apo-TC II and holo-TC II both bind, but intrinsic factor (IF) and R-type binders of Cbl do not. Other mammalian species TC II also complex with the dye. Greater than 87% of the applied TC II-CN-[⁵⁷Co]Cbl remains bound to the dye even at pH 4.0. At pH values below this, the CN-[⁵⁷Co]Cbl dissociates off TC II which remains bound to the dye. High salt concentrations will break the TC II-dye complex. Ionic forces were considered not to be involved since complexing also occurred at pH 9.0, 2.5 pH units above the isoelectric point of TC II. Failure to dissociate the TC II-dye complex with 50% glycerol makes hydrophobic interactions unlikely. In addition to the potential uses of TC II-Cibacron Blue F3GA complexes in a total scheme for protein purification, the possibility that TC II is a nucleotide-requiring protein should be explored.     

Coordination scheme and stereochemical configuration of manganese(II) adenosine 5'-diphosphate at the active site of 3-phosphoglycerate kinase

The structure of the MnIIADP complex at the active site of 3-phosphoglycerate kinase from yeast has been investigated by electron paramagnetic resonance (EPR) spectroscopy. Inhomogeneous broadening in the EPR signals for Mn(II) resulting from unresolved superhyperfine coupling to 17O regiospecifically incorporated into ADP shows that Mn(II) is coordinated to the alpha- and beta-phosphate groups of ADP at the active site of the enzyme. The EPR pattern for the enzyme-MnIIADP complex is characteristic of a predominantly axially symmetric zero-field splitting tensor. The symmetry and magnitude of the zero-field splitting interaction suggest that there is an additional negatively charged oxygen ligand in the coordination sphere of Mn(II). EPR measurements for solutions of the enzyme-MnIIADP complex in 17O-enriched water indicate that there are also two or three water molecules in the coordination sphere of the metal ion. EPR data for complexes with the two epimers of [alpha-17O]ADP have been used to determine the stereochemical configuration of the MnIIADP complex at the active site. EPR spectra for Mn(II) in the enzymic complex with (Rp)-[alpha-17O]ADP show an inhomogeneous broadening due to superhyperfine coupling with 17O whereas spectra for (Sp)-[alpha-17O]ADP complexes are indistinguishable from those for matched samples with unlabeled ADP. These results show that 3-phosphoglycerate kinase selectivity binds the alpha configuration of the alpha, beta chelate of MnIIADP. Addition of 3-phosphoglycerate to form the dead-end complex (enzyme-MnIIADP-3-phosphoglycerate) does not alter the EPR spectrum, but addition of vanadate to this complex causes marked changes in the spectral parameters.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of the substrate radical intermediate derived from ethanolamine during catalysis by ethanolamine ammonia-lyase

Rapid-mix freeze-quench (RMFQ) methods and electron paramagnetic resonance (EPR) spectroscopy have been used to characterize the steady-state radical in the deamination of ethanolamine catalyzed by adenosylcobalamin (AdoCbl)-dependent ethanolamine ammonia-lyase (EAL). EPR spectra of the radical intermediates formed with the substrates, [1-13C]ethanolamine, [2-13C]ethanolamine, and unlabeled ethanolamine were acquired using RMFQ trapping methods from 10 ms to completion of the reaction. Resolved 13C hyperfine splitting in EPR spectra of samples prepared with [1-13C]ethanolamine and the absence of such splitting in spectra of samples prepared with [2-13C]ethanolamine show that the unpaired electron is localized on C1 (the carbinol carbon) of the substrate. The 13C splitting from C1 persists from 10 ms throughout the time course of substrate turnover, and there was no evidence of a detectable amount of a product like radical having unpaired spin on C2. These results correct an earlier assignment for this radical intermediate [Warncke, K., et al. (1999) J. Am. Chem. Soc. 121, 10522-10528]. The EPR signals of the substrate radical intermediate are altered by electron spin coupling to the other paramagnetic species, cob(II)alamin, in the active site. The dipole-dipole and exchange interactions as well as the 1-13C hyperfine splitting tensor were analyzed via spectral simulations. The sign of the isotropic exchange interaction indicates a weak ferromagnetic coupling of the two unpaired electrons. A Co2+-radical distance of 8.7 A was obtained from the magnitude of the dipole-dipole interaction. The orientation of the principal axes of the 13C hyperfine splitting tensor shows that the long axis of the spin-bearing p orbital on C1 of the substrate radical makes an angle of approximately 98 degrees with the unique axis of the d(z2) orbital of Co2+.