Sirovatka Jeanne M., Finke Richard G.
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523.
Inorg Chem. 1999 Apr 19;38(8):1697-1707. doi: 10.1021/ic980608x.
Adenosylcobinamide (AdoCbi(+)) plus the sterically hindered bases 1,2-dimethylimidazole, 2-methylpyridine, and 2,6-dimethylpyridine, as well as control experiments with imidazolate and 4-methylimidazolate, have been investigated to provide chemical precedent for the benzimidazole base-off, protein histidine imidazole base-on form of adenosylcobalamin (AdoCbl, also coenzyme B(12)). This imidazole base-on form of AdoCbl was observed in the recent X-ray crystallographic structural study of methylmalonyl-CoA (MMCoA) mutase; of interest to the present work is the fact that MMCoA mutase contains a long, ca. 2.5 Å, Co-N(imidazole) axial bond, at least in the enzyme's crystallographically characterized Co(II)/Co(III) state and conformation. In the present studies, upper limits for the axial-base binding K(assoc) parameters to form AdoCbi.bulky base BF(4)(-) have been obtained; these thermodynamic studies reveal that sterically hindered bases do not bind detectably to AdoCbi(+) in the ground state, which results in negligible ground-state free-energy stabilization via the formation of AdoCbi.bulky base. The sterically hindered bases do, however, bind to Co(II)Cbi(+), a good energetic model of the Ado. - - -.CoCbi homolysis transition state. Kinetic studies demonstrate that the sterically hindered bases are involved in the rate-determining step of Co-C bond homolysis, accelerating it by 200-fold; hence, Co-C cleavage does occur via the low-level and otherwise nondetectable amount of AdoCbi.bulky base formed in solution. Product studies reveal (i) that both Co-C heterolysis and homolysis occur, and (ii) that there is no simple correlation between the ratio of Co-C heterolysis to homolysis and the Co-N(axial-base) bond length. Overall, the results provide strong evidence for the dominance of axial-base transition-state effects on Co-C bond cleavage, and reveal a subtle interplay of sigma and pi effects as a function of the Co-N(axial-base) bond length.
已对腺苷钴胺酰胺（AdoCbi(+)）与空间位阻碱1,2 - 二甲基咪唑、2 - 甲基吡啶和2,6 - 二甲基吡啶进行了研究，同时也进行了咪唑盐和4 - 甲基咪唑盐的对照实验，以提供关于腺苷钴胺素（AdoCbl，也称为辅酶B₁₂）的苯并咪唑碱基脱离、蛋白质组氨酸咪唑碱基结合形式的化学先例。在最近对甲基丙二酰辅酶A（MMCoA）变位酶的X射线晶体学结构研究中观察到了这种咪唑碱基结合形式的AdoCbl；本研究感兴趣的是，MMCoA变位酶至少在该酶晶体学表征的Co(II)/Co(III)状态和构象中含有一个长约2.5 Å的Co - N(咪唑)轴向键。在本研究中，已获得了形成AdoCbi.体积庞大的碱BF₄⁻的轴向碱基结合K(assoc)参数的上限；这些热力学研究表明，空间位阻碱在基态下与AdoCbi(+)没有可检测到的结合，这导致通过形成AdoCbi.体积庞大的碱产生的基态自由能稳定作用可忽略不计。然而，空间位阻碱确实能与Co(II)Cbi(+)结合，Co(II)Cbi(+)是Ado - - -.CoCbi均裂过渡态的一个良好能量模型。动力学研究表明，空间位阻碱参与了Co - C键均裂的速率决定步骤，使其加速了200倍；因此，Co - C裂解确实是通过溶液中形成的低水平且原本无法检测到的AdoCbi.体积庞大的碱发生的。产物研究表明：（i）Co - C异裂和均裂都会发生；（ii）Co - C异裂与均裂的比例和Co - N(轴向碱基)键长之间没有简单的相关性。总体而言，这些结果为轴向碱基过渡态对Co - C键裂解的主导作用提供了有力证据，并揭示了σ和π效应作为Co - N(轴向碱基)键长函数的微妙相互作用。
