Fig. 3.

Probing unfolded-state dynamics by FRET-FCS. (A) Autocorrelation of acceptor fluorescence, g_AA(τ); donor fluorescence, g_DD(τ); and cross-correlation of acceptor and donor fluorescence, g_AD(τ), at 3 M GdmCl; black lines show a global fit of the three correlations, with the reconfiguration time, τ_r, as a shared fit parameter. (B) FRET-FCS correlations g_AA(τ), g_DD(τ), and g_AD(τ) based on the MD trajectory of unfolded protein L (31) (SI Appendix). (C) τ_r from the global fit of g_AA(τ), g_DD(τ), and g_AD(τ) (red-filled circles); the internal friction contribution, τ_i, (purple line), is obtained by subtracting from the polynomial interpolation curve of τ_r (red line) the solvent component, τ_s (cyan line). The gray-filled circle at 0 M GdmCl represents the reconfiguration time of the chain from the MD simulation (SI Appendix). Error bars represent an average relative error of 15%, as estimated from the SD of multiple independent measurements. Confidence intervals (90%) of the fit are represented as shaded areas of the corresponding color. (Inset) τ_r at 2.2 M (empty squares) and 6.0 M GdmCl (empty circles) as a function of solution viscosity, η; linear extrapolation to zero viscosity (Eq. 1) allows τ_s (empty cyan circle and square in main panel) and τ_i (empty purple circle and square in main panel) to be estimated.