Equilibrium and kinetic studies on the reactions of alkylcobalamins with cyanide.
Equilibrium and kinetic studies on the reactions of alkylcobalamins with cyanide.
复制标题
烷基钴胺素与氰化物反应的平衡和动力学研究。
DOI：
10.1021/ic010558n
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发表时间：
2001
影响因子：
4.6
通讯作者：
vanEldik,R
中科院分区：
文献类型：
--
作者：
Hamza,MS;Zou,X;Brown,KL;vanEldik,R
Ligand substitution equilibria of different alkylcobalamins (RCbl, R = Me, CH2Br, CH2CF3, CHF2, CF3) with cyanide have been studied. It was found that CN-first substitutes the 5,6-dimethylbenzimidazole (Bzm) moiety in the α-position, followed by substitution of the alkyl group in the β-position trans to Bzm. The formation constantsKCNfor the 1:1 cyanide adducts (R(CN)Cbl) were found to be 0.38 ± 0.03, 0.43 ± 0.03, and 123 ± 9 M-1for R = Me, CH2Br, and CF3, respectively. In the case of R = CH2CF3, the 1:1 adduct decomposes in the dark with CN-to give (CN)2Cbl. The unfavorable formation constants for R = Me and CH2Br indicate the requirement of very high cyanide concentrations to produce the 1:1 complex, which cause the kinetics of the displacement of Bzm to be too fast to follow kinetically. The kinetics of the displacement of Bzm by CN-could be followed for R = CH2CF3and CF3to form CF3CH2(CN)Cbl and CF3(CN)Cbl, respectively, in the rate-determining step. Both reactions show saturation kinetics at high cyanide concentration, and the limiting rate constants are characterized by the activation parameters: R = CH2CF3, ΔH⧧= 71 ± 1 kJ mol-1, ΔS⧧= −25 ± 4 J K-1mol-1, and ΔV⧧= +8.9 ± 1.0 cm3mol-1; R = CF3, ΔH⧧= 77 ± 3 kJ mol-1, ΔS⧧= +44 ± 11 J K-1mol-1, and ΔV⧧= +14.8 ± 0.8 cm3mol-1, respectively. These parameters are interpreted in terms of an Idand D mechanism for R = CH2CF3and CF3, respectively. The results of the study enable the formulation of a general mechanism that can account for the substitution behavior of all investigated alkylcobalamins including coenzyme B12.
