Redetermination of the X-ray structure of nitroxylcobalamin: base-on nitroxylcobalamin exhibits a remarkably long Co-N(dimethylbenzimidazole) bond distance

The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

Influence of the primary and secondary coordination spheres on nitric oxide adsorption and reactivity in cobalt(ii)-triazolate frameworks

Nitric oxide (NO) is an important signaling molecule in biological systems, and as such, the ability of porous materials to reversibly adsorb NO is of interest for potential medical applications. Although certain metal-organic frameworks are known to bind NO reversibly at coordinatively unsaturated metal sites, the influence of the metal coordination environment on NO adsorption has not been studied in detail. Here, we examine NO adsorption in the frameworks Co2Cl2(bbta) (H2bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) and Co2(OH)2(bbta) using gas adsorption, infrared spectroscopy, powder X-ray diffraction, and magnetometry. At room temperature, NO adsorbs reversibly in Co2Cl2(bbta) without electron transfer, with low temperature data supporting spin-crossover of the NO-bound cobalt(ii) centers of the material. In contrast, adsorption of low pressures of NO in Co2(OH)2(bbta) is accompanied by charge transfer from the cobalt(ii) centers to form a cobalt(iii)-NO- adduct, as supported by diffraction and infrared spectroscopy data. At higher pressures of NO, characterization data indicate additional uptake of the gas and disproportionation of the bound NO to form a cobalt(iii)-nitro (NO2 -) species and N2O gas, a transformation that appears to be facilitated by secondary sphere hydrogen bonding interactions between the bound NO2 - and framework hydroxo groups. These results provide a rare example of reductive NO binding in a cobalt-based metal-organic framework, and they demonstrate that NO uptake can be tuned by changing the primary and secondary coordination environment of the framework metal centers.

The human B12 trafficking protein CblC processes nitrocobalamin

In humans, cobalamin or vitamin B₁₂ is delivered to two target enzymes via a complex intracellular trafficking pathway comprising transporters and chaperones. CblC (or MMACHC) is a processing chaperone that catalyzes an early step in this trafficking pathway. CblC removes the upper axial ligand of cobalamin derivatives, forming an intermediate in the pathway that is subsequently converted to the active cofactor derivatives. Mutations in the cblC gene lead to methylmalonic aciduria and homocystinuria. Here, we report that nitrosylcobalamin (NOCbl), which was developed as an antiproliferative reagent, and is purported to cause cell death by virtue of releasing nitric oxide, is highly unstable in air and is rapidly oxidized to nitrocobalamin (NO₂Cbl). We demonstrate that CblC catalyzes the GSH-dependent denitration of NO₂Cbl forming 5-coordinate cob(II)alamin, which had one of two fates. It could be oxidized to aquo-cob(III)alamin or enter a futile thiol oxidase cycle forming GSH disulfide. Arg-161 in the active site of CblC suppressed the NO₂Cbl-dependent thiol oxidase activity, whereas the disease-associated R161G variant stabilized cob(II)alamin and promoted futile cycling. We also report that CblC exhibits nitrite reductase activity, converting cob(I)alamin and nitrite to NOCbl. Finally, the denitration activity of CblC supported cell proliferation in the presence of NO₂Cbl, which can serve as a cobalamin source. The newly described nitrite reductase and denitration activities of CblC extend its catalytic versatility, adding to its known decyanation and dealkylation activities. In summary, upon exposure to air, NOCbl is rapidly converted to NO₂Cbl, which is a substrate for the B₁₂ trafficking enzyme CblC.

Spectral and electronic properties of nitrosylcobalamin

Nitrosylcobalamin (NOCbl) is readily formed when Co(II)balamin reacts with nitric oxide (NO) gas. NOCbl has been implicated in the inhibition of various B12-dependent enzymes, as well as in the modulation of blood pressure and of the immunological response. Previous studies revealed that among the known biologically relevant cobalamin species, NOCbl possesses the longest bond between the Co ion and the axially bound 5,6-dimethylbenzimidazole base, which was postulated to result from a strong trans influence exerted by the NO ligand. In this study, various spectroscopic (electronic absorption, circular dichroism, magnetic circular dichroism, and resonance Raman) and computational (density functional theory (DFT) and time-dependent DFT) techniques were used to generate experimentally validated electronic structure descriptions for the "base-on" and "base-off" forms of NOCbl. Further insights into the principal Co-ligand bonding interactions were obtained by carrying out natural bond orbital analyses. Collectively, our results indicate that the formally unoccupied Co 3dz(2) orbital engages in a highly covalent bonding interaction with the filled NO π* orbital and that the Co-NO bond is strengthened further by sizable π-backbonding interactions that are not present in any other Co(III)Cbl characterized to date. Because of the substantial NO(-) to Co(III) charge donation, NOCbl is best described as a hybrid of Co(III)-NO(-) and Co(II)-NO(•) resonance structures. In contrast, our analogous computational characterization of a related species, superoxocobalamin, reveals that in this case a Co(III)-O2(-) description is adequate due to the larger oxidizing power of O2 versus NO. The implications of our results with respect to the unusual structural features and thermochromism of NOCbl and the proposed inhibition mechanisms of B12-dependent enzymes by NOCbl are discussed.

Phenylvinylcobalamin: an alkenylcobalamin featuring a ligand with a large trans influence

Cob(I)alamin reacts with phenylacetylene to produce two diastereomers in which the organic ligand is coordinated to the upper (β) and lower (α) face of the corrin ring, respectively. The isomers were separated chromatographically and characterised by ESI-MS and, in the case of the β isomer, by (1)H and (13)C NMR. Only the β isomer crystallised and its molecular structure, determined by X-ray diffraction, shows that the organic ligand coordinates Co(III) through the β carbon of the phenylvinyl ligand. The Co-C bond length is 2.004(8) Å while the Co-N bond length to the trans 5,6-dimethylbenzimidazole (dmbzm) base is 2.217(8) Å, one of the longest Co-Ndmbzm bond lengths known in an organocobalamin. Unlike benzylcobalamin (BzCbl), phenylvinylcobalamin (PhVnCbl) is stable towards homolysis. DFT calculations (BP86/TZVP) on model compounds of BzCbl and PhVnCbl show that the Co-C bond dissociation energy for homolysis to Co(II) and an organic radical in the former is 8 kcal mol(-1) lower than in the latter. An analysis of the electron density at the Co-C bond critical point using Bader's QTAIM approach shows that the Co-C bond in PhVnCbl is shorter, stronger and somewhat more covalent than that in BzCbl, and has some multiple bond character. Together with calculations that show that the benzyl radical is more stable than the phenylvinyl radical, this rationalises the stability of PhVnCbl compared to BzCbl. The phenylvinyl ligand has a large trans influence. The pKa for deprotonation of dmbzm and its coordination by the metal in β-PhVnCbl is 4.60 ± 0.01, one of the highest values reported to date in cobalamin chemistry. The displacement of dmbzm ligand by CN(-) in β-PhVnCbl occurs with log K = 0.7 ± 0.1; the trans influence order of C-donor ligands is therefore CN(-) < CCH < CHCH2 = PhVn < Me < Et.

High-resolution neutron crystallographic studies of the hydration of the coenzyme cob(II)alamin

The hydration of the coenzyme cob(II)alamin has been studied using high-resolution monochromatic neutron crystallographic data collected at room temperature to a resolution of 0.92 Å on the original D19 diffractometer with a prototype 4° × 64° detector at the high-flux reactor neutron source run by the Institute Laue-Langevin. The resulting structure provides hydrogen-bonding parameters for the hydration of biomacromolecules to unprecedented accuracy. These experimental parameters will be used to define more accurate force fields for biomacromolecular structure refinement. The presence of a hydrophobic bowl motif surrounded by flexible side chains with terminal functional groups may be significant for the efficient scavenging of ligands. The feasibility of extending the resolution of this structure to ultrahigh resolution was investigated by collecting time-of-flight neutron crystallographic data during commissioning of the TOPAZ diffractometer with a prototype array of 14 modular 2° × 21° detectors at the Spallation Neutron Source run by Oak Ridge National Laboratory.