TY - JOUR
T1 - Probing the nature of the Co(III) ion in cobalamins
T2 - The reactions of aquacobalamin (vitamin B12a), aqua-10-chlorocobalamin and aqua-10-bromocobalamin with anionic and neutral ligands
AU - Ghadimi, Nafise
AU - Perry, Christopher B.
AU - Fernandes, Manuel A.
AU - Govender, Penny P.
AU - Marques, Helder M.
N1 - Publisher Copyright:
© 2015 Elsevier B.V. All rights reserved.
PY - 2015/8/3
Y1 - 2015/8/3
N2 - The preparation of aqua-(10-bromo)cobalamin, [H2O-(10-Br)Cbl]+, in which the C-10 H of aquacobalamin is substituted by Br, and its characterisation by spectroscopic and by single crystal XRD is described. Equilibrium constants for substitution of H2O by anionic (SO32-, NO2-, SCN-, N3-) and neutral N-donor (imidazole, 4-N,N-dimethylaminopyridine) ligands in [H2OCbl]+, [H2O-(10-Cl)Cbl]+ and [H2O-(10-Br)Cbl]+ are reported. Rate constants and ΔH‡ and ΔS‡ values for the substitution of H2O in [H2O-(10-Br)Cbl]+ by N3- and imidazole are reported, and are compared with values available for [H2OCbl]+ and [H2O-(10-Cl)Cbl]+. Br at C-10 is resonance donating to the corrin. Substitution of the C-10 H by Cl or Br favours the coordination of anionic ligands, but discriminates against the binding of neutral N-donor ligands. DFT (BP86/TZVP) calculations are used to rationalise these observations. When H is changed to Cl or Br, the metal ion becomes less positive and the corrin nucleus becomes more positive, so favouring the coordination of anionic ligands. The kinetics of the substitution of H2O by N3- and imidazole show that both react more slowly with [H2O-(10-Br)Cbl]+ than with [H2OCbl]+. Although ΔH‡ values are smaller, they do not compensate for significantly more negative values of ΔS‡, indicative of a transition state that occurs earlier along the reaction coordinate. Thus perturbation of the electronic structure of a corrin can markedly affect the chemistry of the axial coordination site of Co(III), providing evidence of the importance of cis effects in the chemistry of Co(III) macrocycles.
AB - The preparation of aqua-(10-bromo)cobalamin, [H2O-(10-Br)Cbl]+, in which the C-10 H of aquacobalamin is substituted by Br, and its characterisation by spectroscopic and by single crystal XRD is described. Equilibrium constants for substitution of H2O by anionic (SO32-, NO2-, SCN-, N3-) and neutral N-donor (imidazole, 4-N,N-dimethylaminopyridine) ligands in [H2OCbl]+, [H2O-(10-Cl)Cbl]+ and [H2O-(10-Br)Cbl]+ are reported. Rate constants and ΔH‡ and ΔS‡ values for the substitution of H2O in [H2O-(10-Br)Cbl]+ by N3- and imidazole are reported, and are compared with values available for [H2OCbl]+ and [H2O-(10-Cl)Cbl]+. Br at C-10 is resonance donating to the corrin. Substitution of the C-10 H by Cl or Br favours the coordination of anionic ligands, but discriminates against the binding of neutral N-donor ligands. DFT (BP86/TZVP) calculations are used to rationalise these observations. When H is changed to Cl or Br, the metal ion becomes less positive and the corrin nucleus becomes more positive, so favouring the coordination of anionic ligands. The kinetics of the substitution of H2O by N3- and imidazole show that both react more slowly with [H2O-(10-Br)Cbl]+ than with [H2OCbl]+. Although ΔH‡ values are smaller, they do not compensate for significantly more negative values of ΔS‡, indicative of a transition state that occurs earlier along the reaction coordinate. Thus perturbation of the electronic structure of a corrin can markedly affect the chemistry of the axial coordination site of Co(III), providing evidence of the importance of cis effects in the chemistry of Co(III) macrocycles.
KW - Aquacobalamin
KW - Co(III)
KW - Ligand substitution reactions
KW - Reaction kinetics
KW - Vitamin B
UR - http://www.scopus.com/inward/record.url?scp=84938527010&partnerID=8YFLogxK
U2 - 10.1016/j.ica.2015.07.019
DO - 10.1016/j.ica.2015.07.019
M3 - Article
AN - SCOPUS:84938527010
SN - 0020-1693
VL - 436
SP - 29
EP - 38
JO - Inorganica Chimica Acta
JF - Inorganica Chimica Acta
ER -