Small-Angle X-ray Scattering and Single-Molecule FRET Spectroscopy Produce Highly Divergent Views of the Low-Denaturant Unfolded State

► smFRET results suggest that the unfolded state is collapsed at low denaturant. ► SAXS suggests that the low- and high-denaturant unfolded states are indistinguishable. ► Poor overlap between the data sets has historically obscured the significance of this discrepancy. ► Parallel studies on one protein under one set of conditions confirm the discrepancy.