(A) PDS-based spin-spin distance distributions between a flavin radical and a C-terminal nitroxide moiety appended to the CTT (red) by site-enzymatic ligation of a spin-labeled peptide. Distance distributions were obtained upon light-excitation (440 nm, 23.5 mW) with the nitroxide-labeled WT CRY (purple, 4 nm broad distance, undocked CTT) and the L405E/C416N mutant mimicking the dark-state (red, 3 nm narrow distance, docked CTT). (B) Representative distance distributions obtained after restraining to a one or two-state model of Gaussian function. ASQ forming variants were restrained to one-component (the WT parent, purple), while the NSQ forming variants were restrained to two components (mixture of docked (red)/undocked (purple) states).