Shown are HRMS analyses of dAdo control (A), simulated incorporation of one deuterium in dAdo (B), dAdo isolated from a reaction with Tyr-30 (ring-2,6-D2, 2-D) MftA (Cα-labeled) (C), dAdo isolated from a reaction with Tyr-30 (3,3-D2) MftA (Cβ-labeled) (D), and dAdo isolated from a reaction with Val-29 (3-D) MftA (Cβ-labeled) (E).