PLACING HYDROXIDE IN THE THERMODYNAMIC TRANS INFLUENCE ORDER OF THE COBALT CORRINOIDS - EQUILIBRIUM-CONSTANTS FOR THE REACTION OF SOME LIGANDS WITH AQUAHYDROXOCOBINAMIDE

Diaquacobinamide (DAC) was prepared by reducing aquacyanocobinamide (Factor B) with zinc and oxidising in aqueous HCl. The macroscopic acid dissociation constants pK(Co1) and pK(Co2) for coordinated H2O in DAC were found spectrophotometrically to be 5.91 +/- 0.04 and 10.30 +/- 0.24 at 25-degrees-C; from their temperature dependence it was found that DELTAH = 45 +/- 4 and 27 +/- 2 kJ mol-1, and DELTAS = 40 +/- 14 and - 109 +/- 8 J K-1 mol-1, for pk(Co1) and pK(Co2), respectively. Equilibrium constants for the reaction of the cobinamide with cyanide were determined in the pH range 8-12 at 25-degrees-C, mu = 1.0 M (NaClO4). Two CN ligands bind to aquahydroxocobinamide and pH dependence of the observed equilibrium constants can be accounted for by assuming that both dihydroxocobinamide and hydroxocyanocobinamide are inert, while equilibrium constants for reaction of the labile species aquahydroxocobinamide and aquacyanocobinamide with HCN are insignificantly small compared to those for reaction with CN- anion. A fit of the data shows that the macroscopic equilibrium constant, log beta2, for reaction of 2CN- with the two diastereomers of aquahydroxocobinamide to produce dicyanocobinamide is 19.0 +/- 0.1. Hence the macroscopic log K for binding of CN to the two diastereomers of aquahydroxocobinamide is 11 since log K for binding of CN- to aquacyanocobinamide is 8. Equilibrium constants for binding of azide, pyridine, N-methylimidazole and 3-amino-1-propanol to aquahydroxocobinamide were determined at 25-degrees-C, mu = 1.0 M (NaClO4), pH 12. In these cases only a single ligand is bound; the macroscopic log K values are N-methylimidazole, 6.06 +/- 0.04; 3-amino-1-propanol, 4.66 +/- 0.04; pyridine, 4.19 +/- 0.01; and azide, 3.45 +/- 0.04. A comparison of the present results with others available shows that hydroxide is above water and 5,6-dimethylbenzimidazole, but below cyanide, sulfite and alkyl ligands in the trans influence order of the cobalt corrinoids.