Dual Stabilization of <i>S</i> -Adenosylmethionine for Enzymatic DNA Labeling

<i>S</i>-Adenosyl-l-methionine (AdoMet) analogues are a powerful tool for site-specific biomolecular labeling via methyltransferase (MTase)-catalyzed transfer reactions. However, their utility is often limited by their poor chemical stability under enzymatic reaction conditions. Here, we report a new class of stabilized AdoMet analogues, featuring a conformationally constrained proline side chain in place of homoalanine. This substitution inhibits intramolecular cyclization, which is a major decomposition pathway. Combination with selenonium modification, which suppresses depurination, yields analogues with up to a 90-fold increase in half-life relative to AdoMet. These cofactors retain activity with DNA MTases and allow sequence-specific labeling of plasmid DNA using both two- and single-step approaches with fluorescent dyes.