Thermal decomposition and cobalt-carbon bond dissociation energies of organocobalamins:  neopentyl-, (cyclopentylmethyl)-, (cyclohexylmethyl)-, (tetrahydrofurfuryl)- and ((tetrahydro-2H-pyryl)methyl)cobalamin

Kim, Sook Hui; Chen, Hui Lan.; Feilchenfeld, Natalie B.; Halpern, Jack

doi:10.1021/ja00218a021

Cited by 69 publications

(41 citation statements)

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“…3 ) . The rate of Cbl' + RX k, RCbl"' + X - (3) this reaction depends on the structure of R X , but it is generally high enough for the reaction to be completed within a second under pseudo-first-order conditions, even at RX concentrations in the mmol range [30]. Generally, the RCb1""s are irreversibly reduced at a potential E, (Co-R) that is more negative than Eo2 (Eqn.…”

mentioning

confidence: 99%

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Zhou

Tinembart

Scheffold

et al. 1990

Helvetica Chimica Acta

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The irreversible-reduction potentials of 26 alkylcob(II1)alamins (RCbl"'; la-z) and 26 alkylcob(I1I)yrinates (RCby'"'; 2a-2) (E, la-z and Ep 2a-2, resp.) have been measured in situ by single-scan voltammetry of hydroxocob(II1)alamin hydrochloride (vitamin BIZb. HCl; 1) or heptamethyl cob(1I)yrinate perchlorate (ClO.,'Cby'"; 2) in presence of the corresponding alkyl halides (RX; 3-2) in DMF. The reduction potentials of alkylcobalt complexes exhibiting half-life times as short as a few seconds become measurable by this technique. Thermodynamic cycles prove that the observed reduction potentials are closely related to the standard reduction potentials E"(R-Co"' + eC%R' + Co'). Electron-withdrawing groups and/or an increased degree of substitution at the Co-bound C-atom in RCbl"' and RCby'"' shift E,(la-z) and E (2a-2) towards positive potentials. Linear correlations have been found between Ep(la-z) (EP(2a-z)) of RCbl" (R'Cby'"') and the pK, of RH (or the Tuft v*-or the Hummeft o--values of R) within each class of R, i.e. MeCbl"' (Me'Cby'"'), primary RCbl"' (RCby'"') and secondary RCbll'P(R'Cby'"L). The correlations allow to distinguish between electronic effects of the Co-bound alkyl residues and their steric interactions with the corrin side chains. The correlations have further been used to visualize the light-induced formal insertion of an olefin into the Co,C-bond of an alkylcobalamin (Scheme 2, la-lu), a key step in the vitamin-B,,-catalized C,C-bond formation.

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confidence: 99%

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Zhou

Tinembart

Scheffold

et al. 1990

Helvetica Chimica Acta

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“…Cage effects by proteins have been previously observed in the AdoCbl-dependent glutamate mutase 45,46 and in the MeCblbinding domain of methionine synthase 47 . In addition to thermolysis of AdoCbl and adenosylcobinamide [39][40][41] , homolytic fission followed by radical-mediated β-elimination is well documented in alkylcobalamins and alkylcobinamides [48][49][50] . The generation of 4',5'-anhydroadenosine (rather than more reactive species as in photolysis in vitro) as the organic partner of CarH photolysis may represent a safety mechanism in ensuring that the reactive 5'-deoxyadenosyl radical is not released.…”

Section: Carh-bound Adocbl: Products Of Photolysis and The Co-c Bondmentioning

confidence: 99%

New light on vitamin B12: The adenosylcobalamin-dependent photoreceptor protein CarH

Chemaly¹

2016

S. Afr. J. Sci.

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Adenosylcobalamin (AdoCbl), or coenzyme B12, is a cofactor for enzymes important in metabolism in humans (and other mammals) and bacteria. AdoCbl contains a Co-C bond and is extremely light sensitive, but, until recently, this light sensitivity appeared to have no physiological function. Recently, AdoCbl has been found to act as cofactor for a photoreceptor protein (CarH) that controls the expression of DNA coding for transcription of the proteins needed for synthesis of carotenes in certain non-photosynthetic bacteria. In 2015, the X-ray crystal structures of two dark states of the photoreceptor protein from the bacterium Thermus thermophilus were determined: CarH bound to AdoCbl and CarH bound to a large portion of the cognate DNA operator (and AdoCbl); a light state was also determined in which CarH was bound to cobalamin in which the Co-C bond had been broken. The breaking of the Co-C bond of Ado-Cbl acts as a trigger for the regulatory switch that allows the transcription of DNA. In the two dark states AdoCbl is bound to a conserved histidine from CarH, which displaces the lower 5,6-dimethylbenzimidazole ligand of AdoCbl. In the light state the 5’-deoxyadenosyl group of AdoCbl is replaced by a second histidine from CarH, giving a bis-histidine cobalamin and 4’,5’-anhydroadenosine. Genes for B12-dependent photoreceptors are widespread in bacteria. Control of DNA transcription may represent an evolutionarily ancient function of AdoCbl, possibly pre-dating its function as a protein cofactor.

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“…The weak cobalt-carbon bond in adenosylcobalamin [Co(III)-CH2--] readily undergoes homolytic scission to Co(I1) and the 5'-deoxyadenosyl radical (32,33). This process constitutes a chemical precedent for the capacity of metallic cofactors, especially species of adenosyl-Co(III), to potentiate the reversible formation of the 5'-deoxyadenosyl radical.…”

Section: The Importance Of Iron-sulfur Clusters and Cobaltmentioning

confidence: 99%

Lysine 2,3‐Aminomutase and the Mechanism of the Interconversion of Lysine and β‐Lysine

Frey

Reed

1993

Advances in Enzymology - And Related Areas of Molecular Biology

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Thermal decomposition and cobalt-carbon bond dissociation energies of organocobalamins: neopentyl-, (cyclopentylmethyl)-, (cyclohexylmethyl)-, (tetrahydrofurfuryl)- and ((tetrahydro-2H-pyryl)methyl)cobalamin

Cited by 69 publications

References 0 publications

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

New light on vitamin B12: The adenosylcobalamin-dependent photoreceptor protein CarH

Lysine 2,3‐Aminomutase and the Mechanism of the Interconversion of Lysine and β‐Lysine

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