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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Thiamin deprotonation mechanism. Carbanion development stabilized by the LUMOs of thiazolium and pyrimidylimine working in tandem and release governed by a H-bond switch
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Thiamin deprotonation mechanism. Carbanion development stabilized by the LUMOs of thiazolium and pyrimidylimine working in tandem and release governed by a H-bond switch

机译：硫胺素的质子化机理。噻唑鎓和嘧啶嘧啶的LUMO协同作用并通过H键开关控制释放，从而实现了碳的发展

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Our previous paper (J. Phys. Chem. A 2005, 109, 7606) using computed atomic charges, based on the quantum theory of atoms in molecules (QTAIM), on azolium models of thiamin diphosphate has shown that only sulfur acts as an effective electron sink in the formation of the thiamin carbanion intermediate. Herein we apply natural bond orbital (NBO) theory to the analysis of orbital contributions to canonical molecular orbitals (CMOs) of six abbreviated azolium analogs of the carbanion to better understand the unique function of sulfur. The NBO/CMO data provide a description of the origin of the first thiamin electron sink: sulfur performing in the sigma- and pi-orbitals of the transition state as well as in the carbanion, and its advantages due to low electronegativity and moderate size. At the next level of thiamin modeling, we include the six-membered pyrimidine ring to represent the prerequisite V-structure in the iminopyrimidine tautomeric form. This model is subjected to incremental deprotonation and MO decomposition. The 4'-pyrimidylimine moiety, in addition to being an internal base to abstract the C2 proton, also performs as the second electron sink. Thus, the LUMOs of the thiazolium and pyrimidylimine systems working in tandem stabilize the developing charges in these transient structures, with facilitation from their HOMOs. Further, the absence of detectable amounts of the C2 carbanion in (13)C2-labeled thiamin-enzyme complex by NMR is explained. Both NBO analysis and the QTAIM topological electronic properties suggest the operation of a H-bonding scheme that leads to the formation of a cryptic C2 carbanion that is not accumulated. The shielding of the carbanion by the N4'-H hydrogen bond is weakened by N1'-H deprotonation. Consequently, prior return of the N1' proton to the nearby glutamate may be the switch for streaming a timed-release of the unstable C2 carbanion to the incoming substrate.
机译：我们先前的论文（J. Phys。Chem。A 2005，109，7606）基于分子中原子的量子理论（QTAIM），在硫胺二磷酸盐的偶氮模型上使用计算的原子电荷表明，只有硫才是有效的电子沉没在硫胺素碳负离子的中间体中。在本文中，我们将自然键轨道（NBO）理论用于分析对碳六碳氮原子的简写类似物对规范分子轨道（CMO）的轨道贡献，以更好地理解硫的独特功能。 NBO / CMO数据描述了第一个硫胺素电子吸收体的起源：硫在过渡态的σ和pi轨道以及碳负离子中表现，并且由于低电负性和中等尺寸而具有优势。在下一阶段的硫胺素建模中，我们包括六元嘧啶环，以亚氨基嘧啶互变异构形式代表必备的V结构。该模型经历了增量去质子化和MO分解。 4'-嘧啶基部分除了是提取C2质子的内部碱基外，还作为第二电子阱。因此，通过它们的HOMO的促进，噻唑鎓和嘧啶基亚胺系统的LUMO串联地稳定了这些瞬态结构中的显影电荷。此外，解释了通过NMR在（13）C 2标记的硫胺素-酶复合物中没有可检测量的C 2碳负离子。 NBO分析和QTAIM拓扑电子特性均表明H键合方案的运行会导致未积累的隐秘C2碳负离子的形成。 N4'-H氢去质子作用削弱了碳负离子对N4'-H氢键的屏蔽作用。因此，N1'质子先前返回附近的谷氨酸盐可能是用于将不稳定的C2碳负离子的定时释放流向进入的底物的开关。

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