Topological Analysis of the Electron Density in Model Azolium Systems for Thiamin Structure−Function: Sulfur Is the Electron Sink and Positively Polarized Carbanions Act as Nucleophiles

Abstract

The mechanism of thiamin diphosphate-dependent enzyme reactions requires two carbanion intermediates 1a and 1b. Neither has been isolated nor detected, but resonance stabilization is assumed to occur with the thiazolium quaternary nitrogen being the electron sink. We have questioned the electronic nature of these carbanion intermediates and, in a broader sense, the role of sulfur in the thiazolium moiety. To address these issues the theory of atoms in molecules (AIM) was used to acquire quantitative electron distributions in thiazolium 2, oxazolium 3, and imidazolium 4 as cations and zwitterions. Among the heteroatoms, only sulfur acts as an electron sink. This is corroborated by a similar behavior in phosphorothioates. Further, the formal carbanion at C2 and Calpha of the intermediates are positively charged and their nucleophilic character is explained with AIM theory by comparison with the sigmaC(-) of model 5a and piC(-) of model 6a. C2 of 2a excels in lone-pair coverage in the sigma-plane, surpassing the sigmaC(-) in acyclic 5a and other cyclic ylidenes, and hence, is a more effective nucleophile. The piC(-) of 6a reveals a depletion area centered in the sigma-plane but shows lone-pair concentration above and below the plane. Unlike 6a, the AIM properties, bond length, and bond order of 2b indicate no lone-pair on Calpha but essentially a double bond across C2-Calpha. Thus, the nucleophilic behavior at Calpha of 1b is based on the enamine chemistry induced by an electrophile.