Effect of aglycone on the rate of cyanide cleavage of CoC-bond in nucleoside analogues of cobamide coenzyme.

Formation of an Association Complex without a Substitution Reaction Occurring between Methylcobalamin and Cyanide

“…Vitamin B 12 has two forms (adenosylcobalamin (=AdoCbl); methylcobalamin (=MeCbl)) which are essential for a number of enzyme reactions. Recently we re-investigated the reaction between adenosylcobalamin and cyanide and found, contrary to earlier literature, that the reaction proceeds in only one kinetically observable step, with rate-determining attack of the first cyanide. This reaction is of particular interest as it provides a convenient means of examining factors which influence the rate of heterolytic cobalt−carbon cleavage, an important reaction in the methylcobalamin-dependent methyltransferase reactions…”

“…We have no explanation as to why Rudakova et al observed a reaction between MeCbl and CN - , other than to suggest, as in the case of the “fast” reaction observed between AdoCbl and CN - (of which we could find no evidence for its existence 1 ), that the authors may have contaminated their reactant by exposing it to light (as for AdoCbl, MeCbl may have been converted to H 2 OCbl + by the spectrophotometer light source; for H 2 OCbl + + CN - → CNCbl + CN - ↔ (CN) 2 Cbl - , k obs(1) = 6.3 × 10 -3 s -1 and k obs(2) = 3.5 × 10 -2 s -1 for [KCN] = 0.10 M, pH 11.7, 25 °C 11 ). Alternatively, a reactant may have contained an impurity.…”

“…We first tried to repeat the experiment performed by Rudakova et al A MeCbl solution (ca. 2 × 10 -4 M MeCbl, pH 11.0 ± 0.1) was mixed with equal volumes of a NaCN solution (0.20 M NaCN, pH 11.0 ± 0.1) in a tandem cuvette to give [CN - ] f = 0.10 M (where f = final concentration of CN - after mixing) and the absorbance change of the mixture monitored spectrophotometrically (300−600 nm).…”

Equilibrium and Kinetic Studies on the Reactions of Alkylcobalamins with Cyanide

“…In earlier work, Dolphin et al reported that no reaction occurs between MeCbl and CN - , but Hayward et al reported log K of 0.1 for the binding of CN - to MeCbl. It was also reported in the literature that CN - can displace Bzm from MeCbl with k = 2.8 × 10 -2 s -1 for 0.1 M KCN, 0.1 M buffer (NaHCO 3 ), pH = 10.5, and 25 °C . In later work it was reported that MeCbl forms an association complex with CN - with no evidence for displacement of Bzm, but this study was carried out at [CN - ] between 0.1 and 1.2 M .…”