Conformationally Locked Noncovalent Networks Modulate Methylation in Functional Models of As(III) SAM Methyltransferase

Noncovalent interactions are central to catalysis, promoting ground-state bond activation and transition-state stabilization. Arsenite biomethylation by As^III S-adenosylmethionine methyltransferase (ArsM) is critical for detoxification, yet its variable efficiency remains mechanistically elusive. Here, we uncover the pivotal role of noncovalent interactions in modulating the methylation efficiency. Guided by MD simulations of SAM- and As-bound ArsM, which reveal stabilizing interactions from conserved Gly91 and Tyr70, we designed thioimidazolium-based methylating agents (<b>1</b>-<b>7</b>) to emulate the enzymatic process and incorporate tailored intramolecular interactions to modulate S-CH₃ bond activation. Notably, bis- (<b>4</b>) and tetra-thioimidazolium (<b>7</b>) derivatives exhibit dramatic enhancements in methylation rate─up to 80-fold─and remarkably high efficiencies compared to agent <b>1</b> (98% for <b>7</b> vs 16% for <b>1</b>), driven by cooperative and synchronized noncovalent interactions within conformationally locked frameworks that are entirely absent in <b>1</b>. In agent <b>7</b>, a closed-loop arrangement of four sulfur centers enables electron delocalization across S-CH₃ units, substantially lowering the methyl transfer activation barrier. Furthermore, the in situ generation of thione byproducts facilitates the reduction of pentavalent arsenic intermediates, completing the biomimetic methylation cycle. These findings establish a direct structure-function correlation between noncovalent interaction networks and methylation efficiency, offering a new strategy for designing efficient synthetic methylating agents.