Studies on the mechanism of the reaction between 5′-deoxyadenosylcobinamide and cyanide

Abstract

The reaction between 5′-deoxyadenosylcobinamide (AdoCbi⁺OH⁻) and cyanide (NaCN or tetrabutylammonium cyanide) to form dicyanocobinamide, adenine and 1-cyano-D-erythro-2,3-dihydroxy-4-pentenol has been studied by ¹H NMR and UV-visible spectroscopies in aqueous solution at pH (pD) 11.0 and in 92% N,N-dimethylformamide (DMF)–8% D₂O. Saturation kinetics are observed by UV-visible spectroscopy at high cyanide concentrations and data were fitted to a mechanism involving rapid formation of an (β-adenosyl)(α-cyano)cobinamide intermediate, (β-Ado)(α-CN)Cbi (K_CN = 5.6 (H₂O) and 6.9 M⁻¹ (D₂O), 25.0 °C, I = 1.0 M (NaClO₄); K_CN = 4.6 (H₂O) and 3.8 M⁻¹ (D₂O), 25.0 °C, I = 2.0 M (NaClO₄), K_CN = 2.8 × 10³ M⁻¹, 25.0 °C, I = 0.50 M ((TBA)ClO₄)), followed by solvent-assisted, rate-determining cleavage of the Co–Ado bond of (β-Ado)(α-CN)Cbi (10²k₂ = 2.91 (H₂O) and 2.17 s⁻¹ (D₂O), 25.0 °C, I = 1.0 M (NaClO₄); 10²k₂ = 2.55 (H₂O) and 2.45 s⁻¹ (D₂O), 25.0 °C, I = 2.0 M (NaClO₄), 10⁵k₂ = 6.7 s⁻¹, 92% DMF–8% D₂O, 25.0 °C, I = 0.50 M ((TBA)ClO₄)). Deviations in the plots of the observed rate constant versus cyanide concentration in aqueous solution at high cyanide concentrations is attributed to medium effects, and only CN⁻, not HCN, reacts with AdoCbiOH. Evidence for rapid formation of (β-adenosyl)(α-cyano)cobinamide from AdoCbiOH plus cyanide is also obtained by investigating the ¹H NMR chemical shifts of the signals in the aromatic region immediately after mixing AdoCbiOH and cyanide as a function of cyanide concentration (D₂O, 25.0 °C; K_CN′ = 5.7 and 3.3 M⁻¹ at I = 1.0 (NaClO₄) and 2.0 M (NaClO₄), respectively). A comparison of rate and equilibria constants for the reaction between AdoCbiOH and 5′-deoxyadenosylcobalamin is made.