Axial Co-N Bond Research Articles

Electrochemical and electronic properties of a series of substituted polypyridine ligands and their Co(II) complexes

DFT calculations show that, due to Jahn-Teller distortion, the d7 [Co(N,N)3]2+ complexes, with S = ½ (N,N = bipyridine or substituted bipyridine ligand) have two longer axial and four shorter equatorial Co-N bonds (elongation Jahn-Teller), while [Co(terpyridine)2]2+ with S = ½, instead has two shorter central (axial) Co-N bonds and four longer distal Co-N bonds (compression Jahn-Teller), since in the latter, the distal Co-N bonds are more flexible than the Co-N axial bonds in the rigid structure of the tridentate terpyridine ligand. The same trend is observed for the related high spin S = 3/2 Co(II) complexes, though less pronounced. The cyclic voltammograms of [Co(terpyridine)2]2+ and a series of the [Co(N,N)3]2+ complexes show at least three chemically as well as electrochemically reversible redox couples, namely CoIII/II, CoII/I and a ligand based reduction of the polypyridine-Co(I) complex. The reduction of the uncoordinated free polypyridine ligand is more than 0.5 V more negative than the reduction of the coordinated ligand in the polypyridine-Co(I) complex.

Mechanism of Co-C bond photolysis in methylcobalamin: influence of axial base.

A mechanism of Co-C bond photolysis in the base-off form of the methylcobalamin cofactor (MeCbl) and the influence of its axial base on Co-C bond photodissociation has been investigated by time-dependent density functional theory (TD-DFT). At low pH, the MeCbl cofactor adopts the base-off form in which the axial nitrogenous ligand is replaced by a water molecule. Ultrafast excited-state dynamics and photolysis studies have revealed that a new channel for rapid nonradiative decay in base-off MeCbl is opened, which competes with bond dissociation. To explain these experimental findings, the corresponding potential energy surface of the S1 state was constructed as a function of Co-C and Co-O bond distances, and the manifold of low-lying triplets was plotted as a function of Co-C bond length. In contrast to the base-on form of MeCbl in which two possible photodissociation pathways were identified on the basis of whether the Co-C bond (path A) or axial Co-N bond (path B) elongates first, only path B is active in base-off MeCbl. Specifically, path A is inactive because the energy barrier associated with direct dissociation of the methyl ligand is higher than the barrier of intersection between two different electronic states: a metal-to-ligand charge transfer state (MLCT), and a ligand field state (LF) along the Co-O coordinate of the S1 PES. Path B initially involves displacement of the water molecule, followed by the formation of an LF-type intermediate, which possesses a very shallow energy minimum with respect to the Co-C coordinate. This LF-type intermediate on path B may result in either S1/S0 internal conversion or singlet radical pair generation. In addition, intersystem crossing (ISC) resulting in generation of a triplet radical pair is also feasible.

Synthesis, characterization, electrochemical studies, and antibacterial activities of cobalt(III) complexes with Salpn-Tipe Schiff base ligands. Crystal structure of trans-[CoIII(L1)(Py)2]ClO4

The synthesis, characterization, spectroscopic and electrochemical properties of trans-[CoIII(L1)(Py)2]ClO4 (I) and trans-[CoIII(L2)(Py)2]ClO4 (II) complexes, where H2L1 = N,N′-bis(5-chloro-2-hydroxybenzylidene)-1,3-propylenediamine and H2L2 = N,N′-bis(5-bromo-2-hydroxybenzylidene)-1,3-propylenediamine, have been investigated. Both complexes have been characterized by elemental analysis, FT-IR, UV-Vis, and 1H NMR spectroscopy. The crystal structure of I has been determined by X-ray diffraction. The coordination geometry around cobalt(III) ion is best described as a distorted octahedron. The electrochemical studies of these complexes revealed that the first reduction process corresponding to Co(III/II) is electrochemically irreversible accompanied by dissociation of the axial Co-N(Py) bonds. The in vitro antimicrobial activity of the Schiff bse ligands and their corrsponding complexes have been tested against human pathogenic bacterias such as Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, and Escherichia coli. The cobalt(III) complexes showed lower antimicrobial activity than the free Schiff base ligands.

The enzymatic activation of coenzyme B12

The enzymatic "activation" of coenzyme B12 (5'-deoxyadenosylcobalamin, AdoCbl), in which homolysis of the carbon-cobalt bond of the coenzyme is catalyzed by some 10(9)- to 10(14)-fold, remains one of the outstanding problems in bioinorganic chemistry. Mechanisms which feature the enzymatic manipulation of the axial Co-N bond length have been investigated by theoretical and experimental methods. Classical mechanochemical triggering, in which steric compression of the long axial Co-N bond leads to increased upward folding of the corrin ring and stretching of the Co-C bond is found to be feasible by molecular modeling, but the strain induced in the Co-C bond seems to be too small to account for the observed catalytic power. The modeling study shows that the effect is a steric one which depends on the size of the axial nucleotide base, as substitution of imidazole (Im) for the normal 5,6-dimethylbenzimidazole (Bzm) axial base decreases the Co-C bond labilization considerably. An experimental test was thus devised using the coenzyme analog with Im in place of Bzm (Ado(Im)Cbl). Studies of the enzymatic activation of this analog by the B12-dependent ribonucleoside triphosphate reductase from Lactobacillus leichmannii coupled with studies of the non-enzymatic homolytic lability of the Co-C bond of Ado(Im)Cbl show that the enzyme is only slightly less efficient (3.8-fold, 0.8 kcal mol(-1)) at activating Ado(Im)Cbl than at activating AdoCbl itself. This suggests, in agreement with the modeling study, that mechanochemical triggering can make only a small contribution to the enzymatic activation of AdoCbl. Another possibility, electronic stabilization of the Co(II) homolysis product by compression of the axial Co-N bond, requires that enzymatic activation be sensitive to the basicity of the axial nucleotide. Preliminary studies of the enzymatic activation of a coenzyme analog with a 5-fluoroimidazole axial nucleotide suggest that the catalysis of Co-C bond homolysis may indeed be significantly slowed by the decrease in basicity.

XAS spectroscopy reveals X-ray-induced photoreduction of free and protein-bound B12cofactors

Crystal structures of several proteins with a B(12) cofactor show abnormally long axial bonds between the cofactor's Co atom and its ;lower' ligand, which is typically a protein-derived imidazole from a histidine residue. X-ray absorption spectroscopy (XAS) experiments were carried out with the following cofactor derivatives to examine the question of whether the bond elongation might be due to an X-ray-induced reduction of the cofactor's cobalt centre: aquocobalamin, cyanocobalamin, methylcobalamin, 5'-desoxyadenosylcobalamin and cob(II)alamin. Each cofactor was investigated at 100 K in a water/glycerol or water/trehalose glass, both as unbound free species and bound to the protein components of the enzyme glutamate mutase. XAS data were collected for each sample around the cobalt absorption edge before and after exhaustive (10 min) irradiation with X-rays of energy 7.76 keV. While XAS spectra for cob(II)alamin, methylcobalamin and 5'-desoxyadenosylcobalamin were the same (within experimental error) before and after irradiation, both in the free and protein-bound state, the spectra of samples with aquocobalamin and cyanocobalamin changed substantially upon irradiation. The spectra of the irradiated samples resembled each other and were similar - but not identical - to the spectrum of the reduced cob(II)alamin. The implications of these observations for the interpretation of the ;long' axial Co-N bonds observed crystallographically in B(12) proteins are discussed.

Vitamin B12 Coenzyme Models: Perspectives on Recent Developments in the Chemistry of the Cobaloximes and Related Models

Discoveries on B12 models made since 1989 are assessed in the light of the advances in structural and spectroscopic methodologies. Further studies emanating in part from these advances have confirmed, often definitively, previously identified principles describing the properties of the classic simple models (cobaloximes and iminocobaloximes) and have established some new principles defining the properties of the axial Co-C and Co-N bonds; the latter are clearly relevant to enzymatic processes. Some new simple models have been proposed and studied in relation to the classic ones and to the more complicated natural cobalamins. In several cases, the influence of the steric and electronic factors have been established, sometimes in semi-quantitative terms, also with the help of studies on the rhodium analogues of cobaloximes. New spectroscopic techniques have been introduced, which have been found useful in the study of the natural cobalamins. Recent structural analyses of the binding site in some B12-based enzymes have shown that the B12 cofactors bind in the base-off form, with displacement of the benzimidazole residue from cobalt and coordination of a histidine residue of the protein chain. Such observations have stimulated new experiments aimed at defining the mechanism of the Co-C homolytic cleavage in isomerases and mutases or at testing the recently proposed mechanism for the Co-C heterolytic cleavage in methionine synthase. As a consequence, the information now available on models and on cobalamins (e.g., on the nature of the Co-S bond) is much broader in scope, and these advances have prompted this new analysis of models. The experimental aspects have advanced much more quickly than our theoretical understanding, and hence further calculations, possibly based on more sophisticated approaches, are clearly required.

Heteronuclear NMR Studies of Cobalt Corrinoids. 18. Correlation of Structure and Magnetic Resonance Parameters in Base-On Cobalamins1

Recent X-ray crystal structure determinations (including a new X-ray determination of the structure of cyano-13-epicobalamin reported herein) create a series of seven base-on cobalamins structurally characterized by modern crystallographic techniques in which the intramolecular equilibrium constant for coordination of the axial benzimidazole ligand (Bzm) varies from 76.6 to 4.90 x 10(7). For the five normal, unepimerized cobalamins, the free energy change for this equilibrium correlates linearly with the axial Co-N bond length (r(2) = 0.99). Absolute assignment of the (1)H and (13)C NMR spectra of two of these structurally characterized cobalamins (CH(3)Cbl and CN-13-epiCbl) together with literature assignments for the other complexes now provides reliable (13)C NMR assignments and chemical shifts for all seven complexes. The magnetic anisotropies of the central cobalt atom of all seven complexes, estimated by a method described earlier, are well correlated with the axial Co-N bond distance (r(2) = 0.97) and the free energy of coordination of the Bzm ligand (r(2) = 0.95). The (31)P NMR chemical shift of the phosphodiester moiety of the nucleotide loop is excellently correlated to the axial Co-N bond length (r(2) = 0.996) of the unepimerized cobalamins and provides a reliable method of estimating this bond length. The (15)N chemical shifts of the axially coordinated Bzm nitrogen vary strongly with the axial Co-N bond distance and correlate linearly with this structural parameter (r(2) = 0.991) except for the case of H(2)OCbl(+), which deviates substantially. However, there is a good linear correlation (r(2) = 0.98) of this (15)N chemical shift with the free energy of Bzm coordination for the five unepimerized cobalamins. Attempts to correlate (13)C NMR chemical shifts with structural, thermodynamic, and corrin ring conformational parameters are discussed.

Synthesis, Characterization and Enhanced Cleavage of Diphosphate and Triphosphate Bound to Tetraaminecobalt(III) Centers. Crystal Structure of [N,N-Bis(2-aminoethyl)ethane-1,2-diamine](diphosphato)cobalt(III) Perchlorate Monohydrate

The complexes [Co( tren )(P2O7H2)] (ClO4).H2O, [Co( tren )(P3010H3)] (ClO4) (tren = N,N-bis(2- aminoethyl )ethane-1,2-diamine) and [Co( cyclen )(P2O7H2)](ClO4)(cyclen = 1,4,7,10-tetraazacy-clododecane ) have been prepared and characterized. The complex [Co( tren )(P207H2)](C104).H20 crystallized in the monoclinic space group P21/n,a 10.655(3), b 14.675(5), c 11.260(4) � , β 98.49(3) for a single-crystal structure determination R was 0.041 for 4043 'observed' reflections. Despite the presence of a primary and a tertiary nitrogen donor in the plane of the cobalt and the phosphate oxygen donors, bond distances to Co-N(primary) and Co-N(tertiary) at 1.926(3) and 1.924(3) � respectively are equivalent, as are the Co-O distances of 1.940(2) and 1 .943(3) to the N(primary) and N(tertiary) respectively. Axial Co-N(primary) bonds (av. 1.952 A) are elongated compared with the in-plane N(primary), and the axial N-Co-N angle is distorted (171.6�). The hydrolysis of the diphosphate and triphosphate complexes at pH 7.3 in the presence of an assisting complex, [Co( tn )2(OH2)(OH)]2+ ( tn = propane-1,3-diamine) or [Co( tren )(OH2)(0H)]2+, was followed by 31P n.m.r. spectroscopy, and rate enhancements of up to about 104 compared with free polyphosphate were observed, with the diphosphato complexes being least sensitive to cleavage by added complex. Intermediates in which the polyphosphate bridges two metal centres are apparently involved; the inability of [Pd(en)(0H2)2]2+(en= 1,2-ethanediamine) to enhance cleavage is consistent with this, since this ion does not form complexes with polyphosphates. [Co( tren )(ATP)](ATP=adenosine triphosphate ) was also prepared but not isolated; assisted hydrolysis with [Co( tren )(OH2)(OH)]2+ is about 104 faster than unassisted hydrolysis of free ATP under similar conditions.

Heteronuclear NMR studies of cobalamins. 31P NMR observations of cobalamins bound to a haptocorrin from chicken serum.

A vitamin B12-binding protein (haptocorrin) from chicken serum has been purified to homogeneity by photodissociative affinity chromatography and characterized by gel electrophoresis and UV-visible spectrophotometry of its aquocobalamin, hydroxocobalamin, and cyanocobalamin complexes. The haptocorrin is a glycoprotein with a molecular mass of about 70 kDa and a protein moiety of about 40 kDa. 31P NMR resonances of the haptocorrin-cobalamin complexes are relatively broad singlets (with or without proton decoupling) shifted downfield by 0.7-1.0 ppm from the position of the free cobalamin resonances. From the line width data, the relaxation of the phosphorus nucleus is found to be dominated by chemical shift anisotropy with a very minor (13%) component from dipolar interaction with the two nearest neighbor protons. The rotational correlation time of the haptocorrin at 25 degrees C is estimated to be 85 ns and the activation energy for rotational correlation 3.9 +/- 0.3 kcal mol-1. The downfield shift of the 31P resonances of cobalamins upon binding to the haptocorrin cannot be due to hydrogen bonding phosphodiester moiety or displacement of the axial base by a group on the protein. Calculations also show that the downfield shift is very unlikely to be due to dipolar deshielding of the phosphorus nucleus by the ring current of an aromatic residue of the protein. It is concluded that the downfield shift of the 31P resonance must be due to sterically induced changes in phosphodiester conformation which may, or may not, involve steric compression of the axial Co-N bond.