The EPR spectrum of the Tyr–Cys· free radical in oxidized apoGAOX was investigated using power saturation analysis and isotopic labeling. Global fitting with evolving factor analysis revealed two distinct radical species, with the dominant low-power component assigned to the Tyr–Cys· side chain. EPR spectra of multiple isotopically labeled variants enabled determination of g‑values, hyperfine coupling constants, and linewidths through spectral simulation. The results indicate significant spin density on methylene protons, aromatic carbons, and sulfur and oxygen atoms, providing detailed structural insight into the Tyr–Cys· radical.
