Hoffman develops and applies electron-nuclear double resonance (ENDOR) spectroscopy -- a combination of EPR and NMR -- to resolve individual hyperfine-coupled nuclei at metalloenzyme active sites with atomic-scale precision, work that has revealed mechanisms of nitrogenase nitrogen fixation, radical-SAM enzyme catalysis, and copper/methane monooxygenase chemistry. The technique pushes magnetic-resonance spectroscopic resolution well past what conventional EPR can resolve, in a manner methodologically continuous with molecular spin-qubit sensing.