(A) smFRET histogram of drkN SH3 in 6 M GdmCl, constructed from ∼10,000 intensity bursts. The histogram was fitted to four Gaussians and normalized to the area excluding the zero-FRET peak. The parametric fitting results are listed in Table 2. The inset shows a comparison between the high-FRET peaks measured in Tris buffer (blue bars) and 6 M GdmCl (red bars). (B) FRET histograms of drkN SH3 in 20% (v/v) formamide (black), 4 M GdmCl (red), and 50% (v/v) DMSO (blue). DMSO data were acquired with a different donor dye, i.e., A555 instead of BFL. To see this figure in color, go online.