(A–F) FRET provides evidence for compaction. (A–F) FRET provides evidence for compaction. Ribbon diagrams illustrating the location of the FRET pairs are shown together with the distance distributions. Red, unfolded state in 10 M urea; green, unfolded state in 1 M urea; black, folded state in 1 M urea. The folded and unfolded distributions in 1 M urea were extracted from the global fit to the FRET data. The unfolded-state distance distribution was modeled using a wormlike chain with a parameter for diffusion between donor and acceptor during the fluorescence lifetime, while a Gaussian distribution was used to model the folded-state distribution. Ivan Peran et al. PNAS doi:10.1073/pnas.1818206116 ©2019 by National Academy of Sciences