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 (KCN = 5.6 (H2O) and 6.9 M−1 (D2O), 25.0 °C, I = 1.0 M (NaClO4); KCN = 4.6 (H2O) and 3.8 M−1 (D2O), 25.0 °C, I = 2.0 M (NaClO4), KCN = 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; KCN′ = 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.