- Sterically Induced, Spontaneous Co-C Bond Homolysis and β-Elimination Reactions of Primary and Secondary Organocobalamins
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Sterically hindered secondary alkylcobalamins carrying hydrogen in the β-position decompose in neutral aqueous solutions spontaneously by way of β-elimination.The cleavage of the Co-C bond in these compounds is caused by "upward" distortions of the corrin
- Schrauzer, Gerhard N.,Grate, John H.
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- Vitamin B12 and redox homeostasis: Cob(II)alamin reacts with superoxide at rates approaching superoxide dismutase (SOD)
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(Graph Presented) We report a kinetic study of the reaction between superoxide and an important intracellular form of vitamin B12, cob(II)alamin. Superoxide is implicated in the pathophysiology of many inflammatory diseases, whereas vitamin Bs
- Suarez-Moreira, Edward,Yun, June,Birch, Catherine S.,Williams, John H. H.,McCaddon, Andrew,Brasch, Nicola E.
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- Cobalamins and the spectrochemical series
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UV-visible-NIR spectra of a variety of cobalamins were run in water and methanol. A broad absorption band (band A) with extinction coefficients of about an order of magnitude less than those of the αβ bands was found in the red and NIR regions for Cl-cobalamin (Cl-cbl), Br-cbl, I-cbl, SC(NH 2)2-cbl+ and SeCN-cbl. OCrO3- cbl-, which also has a broad absorption band in the NIR was prepared for the first time. After deconvolution, similar broad bands were seen in the visible region for many other cobalamins. The wavelengths for band A placed the cobalamins in an order similar to the spectrochemical series but different from that of the αβ and γ bands (π-π* transitions), which follow the nephelauxetic series. Band A was ascribed to a ligand-to-metal charge transfer (LMCT) transition from a π orbital in the corrin ring to Co(iii). This is the first systematic study of LMCT bands in cobalamins.
- Chemaly, Susan M.
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- Synthesis, spectroscopic characterization, axial base coordination equilibrium and photolytic kinetics studies of a new coenzyme B12 analogue-3′-deoxy-2′,3′-anhydrothymidylcobalamin
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A new coenzyme B12 (AdoCbl) analogue, 3′-deoxy-2′, 3′-didehydrothymidylcobalamin (2′,3′-anThyCbl) was prepared by the reaction of 5′-iodo-3′-deoxy-2′,3′-dihydrothymidine with reduced B12a, and characterized by UV-Vis, CD, ESI-MS and NMR spectroscopies. Its axial base (dbzm) coordination equilibria with pH's and temperatures were investigated and showed similar features to those of coenzyme B12. Photolytic dynamics studies under homolytic and heterolytic conditions demonstrated that the Co-C bond of the analogue is slightly more photolabile relative to coenzyme B12. The Royal Society of Chemistry.
- Zhang, Xin,Shen, Xujie,Yan, Hong,Chen, Huilan
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p. 2336 - 2342
(2007/10/03)
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- Thermal Decomposition and Cobalt-Carbon Bond Dissociation Energies of Organocobalamins: Neopentyl-, (Cyclopentylmethyl)-, (Cyclohexylmethyl)-, (Tetrahydrofurfuryl)- and ((Tetrahydro-2H-pyryl)methyl)cobalamin
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The title compounds were prepared and characterized and their thermal decomposition reactions were studied in aqueous solutions of varying pH and containing varying concentrations of cob(II)alamin (B12) and of bis(dimethylglyoximato)cobalt(II), .
- Kim, Sook-Hui,Chen, Hui Lan,Feilchenfeld, Natalie,Halpern, Jack
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p. 3120 - 3126
(2007/10/02)
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- Electron transfer. 90. Further oxidations of vitamin B12r (Cob(II)alamin)
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Vitamin B12r (cob(II)alamin), the Co(II) derivative of vitamin B12, reduces FeIII (to FeII), VV (to VIV), and BrO2- (to Br-) in aqueous solution. The rate law for reaction with FeIII(aq) features an inverse-[H+] term and, in the presence of added Cl- or Br-, a sizable halide-proportional term as well. These kinetic dependencies point to specific redox bridging roles for Fe(III)-bound hydroxide and halide. The very high specific rates for oxidations by Fe(NCS)2+ (1.1 × 105) and Fe(N3)2+ (>1 × 107 M-1 s-1) are in accord with this assignment. Oxidation by FeIII(aq) is inhibited by fluoride, reflecting the conversion of Fe(III) to inactive FeF2+. The specific rate pertaining to the [H+]-independent component in the oxidation by FeIII(aq) corresponds to a self-exchange rate for the cob(II,III)alamin couple of 10-4.5 M-1 s-1, or about 10-8 times the corresponding value for cob(I,II)alamin. The acidity pattern for oxidation by vanadium(V) is consistent with partition of V(V) into a reactive protonated form (pKA = 3.5) and an unreactive deprotonated species. The limiting specific rate (4.4 × 105 M-1 s-1) points to a predominant inner-sphere path for this reaction. Oxidations by bromite (BrO2-) exhibit a prominent [H+]-proportional term, attributed to reaction of B12r with HBrO2, proceeding at a specific rate of 4 × 106 M-1 s-1, i.e. about as rapidly as the analogous reaction with HClO2. Oxidations of B12r by IrCl62-, Fe(CN)63-, and Fe(bpy)33+ are too rapid to measure (k > 7 × 106 M-1 s-1 at 25°C and μ = 1.0 M) by our methods.
- Balasubramanian,Chithambarathanu Pillai,Carlson,Linn Jr.,Gould
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p. 780 - 783
(2008/10/08)
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- Electron transfer. 92. Reductions of vitamin B12a (hydroxocobalamin) with formate and related formyl species
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Vitamin B12a (hydroxocobalamin) is reduced to B12r (cob(II)alamin) with formate in aqueous media. One unit of formate consumes nearly two molecules of B12a. At formate concentrations below 0.1 M, reactions are first order in both reagents. Rates vary with pH, approach a maximum in the range pH 5-7, and conform to eq 4 in the text, indicating that the active species are the formate anion and the protonated form of B12a. At formate concentrations exceeding 0.1 M, the formate dependence exhibits kinetic saturation, pointing to the formation of a B12a-formate complex having Kassn = 4.6 M-1. The reaction is inhibited moderately by acetate and thiocyanate but severely by imidazole. The observed deuterium isotope effect, kHCOO-/kDCOO- = 1.8, is very close to that reported for the Cannizzaro reaction of benzaldehyde and is thus consistent with a path entailing migration of hydride from a formyl carbon to cobalt. The proposed mechanism for this reaction (sequence (6) - (9)) then features an internal hydride shift (k = 0.016 s-1) within a B12a-formate complex to yield a protonated CoI (B12s-like) intermediate, which very rapidly undergoes a comproportionation reaction with unreacted B12a. The reduction proceeds inconveniently slowly, or not at all, with a number of formyl-substituted carboxylic acids in which the aldehydo group is not properly positioned for hydride migration to carboxyl-bound CoIII or, in the case of glyoxylic acid, is nearly completely converted by hydration to its less reactive gem-diol form.
- Linn Jr.,Gould
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p. 1625 - 1628
(2008/10/08)
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- Electron transfer. 93. Further reactions of transition-metal-center oxidants with vitamin B12s (Cob(I)alamin)
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Vitamin B12, (cob(I)alamin) reduces europium(III), titanium(IV) (TiO(C2O4)22-), and uranium(VI) in aqueous solution. These oxidants undergo one-electron changes, leading in each case to the cobalt product cob(II)alamin (B12r). The reduction of Eu3+, which is inhibited by TES buffer, but not by glycine, is outer sphere. Its limiting specific rate (1 × 102 M-1 s-1), incorporated in the Marcus treatment, yields a B12s,B12r self-exchange rate of 104.8±0.5 M-1 s-1. Reductions of TiO(C2O4)22- are accelerated by H+ and by acetic acid. Kinetic patterns suggest three competing reaction paths involving varying degrees of protonation of the Ti(IV) center or its association with acetic acid. The very rapid reduction of U(VI) (k = 4 × 106 M-1 s-1) yields U(V) in several buffering media, even when B12s is taken in excess. The much slower conversion of U(V) to U(IV), although thermodynamically favored, appears to be retarded by the extensive reorganization of the coordination sphere of oxo-bound U(V) that must accompany its acceptance of an additional electron. The observed specific rate for the B12s-U(VI) reaction is in reasonable agreement, in the framework of the Marcus formalism, with reported values of the formal potential and the self-exchange rate for U(V,VI).
- Pillai, G. Chithambarathanu,Ghosh,Gould
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p. 1868 - 1871
(2008/10/08)
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- Acid-base properties of α-ribazole and the thermodynamics of dimethylbenzimidazole association in alkylcobalamins
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1-α-D-Ribofuranosyl-5,6-dimethylbenzimidazole (α-ribazole) has been prepared by cerous hydroxide catalyzed hydrolysis of cyanocobalamin, purified, and characterized by elemental analysis and 1H and 13C NMR and UV-visible spectroscopy. Values of the pKa of N-3-protonated α-ribazole have been determined at several temperatures (ionic strength 1.0 M) and the pKa of N-1-protonated α-ribazole has been estimated to be -7.2 from UV-visible spectral changes in sulfuric acid-water mixtures. Seven alkylcobalamins have been synthesized by standard reductive alkylation procedures and purified chromatographically. It has been found that reductive alkylation with CF3Br produces mixtures of (trifluoromethyl)cobalamin and (difluoromethyl)cobalamin because the former is reductively converted to the latter by reducing agents commonly employed for reduction of cobalt(III) cobalamins to cob(I)alamin. The pKa's for the base-on-base-off transition of these seven alkylcobalamins and methylcobalamin have been determined at the same temperatures as the α-ribazole pKa's. From these values the apparent binding constants for ligation of the free-base benzimidazole nucleotide and the enthalpy and entropy changes for this ligand substitution have been calculated. The enthalpy change has been found to be quite insensitive to the nature of the organic ligand while the entropy change is quite sensitive. These results are discussed in terms of the probable importance of steric effects of the organic ligands on the base-on-base-off pKa's of alkylcobalamins.
- Brown, Kenneth L.,Hakimi, Janette M.,Nuss, Debra M.,Montejano, Yolanda D.,Jacobsen, Donald W.
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p. 1463 - 1471
(2008/10/08)
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