The reaction between methylcobalamin and cyanide revisited

The reaction between the vitamin B₁₂ coenzyme methylcobalamin and cyanide has been re-investigated by UV-Vis and ¹H NMR spectroscopies under the same conditions. On the basis of the results and recent results of other studies on the reaction between other alkylcobalamins and cyanide, the changes in the ¹H NMR chemical shifts and linewidths in the aromatic region observed upon addition of cyanide to methylcobalamin can be attributed to the rapid formation of (β-CH₃)(α-CN)Cbl^-, rather than the formation of MeCbl·CN^- as proposed earlier (Inorg. Chem. 36 (1997) 4891). There is excellent agreement between the equilibrium constant for formation of (β-CH₃)(α-CN)Cbl^- determined by the two methods (0.35±0.03 M^-1 and 0.31±0.01 M^-1 from the UV-Vis and ¹H NMR spectroscopic data, respectively (D₂O, pD 12.1, 25.0°C)).