Figure 4.

Aromatic region (a) and expansion of the dioxane signal (b) of the ¹H NMR spectra of the PI3-SH3 A state obtained with the PG-SLED sequence at different pulse field-gradient strengths. The signal of the small reference molecule dioxane decays much faster than the signals from the protein. (c) Dependence of the integral of the aromatic region between 6.6 and 7.7 ppm (▪) and of the dioxane (□) signal (3.75 ppm) on the strength of the field gradient, expressed as a percentage of the maximum strength used (≈60 G cm⁻¹). Data shown are derived from experiments performed on a PI3-SH3 sample (0.3 mM) at pH* 2.0 and 20°C. Analogous curves were obtained for the native protein and for the protein denatured in 3.5 M GuHCl. The solid lines in c correspond to fits to a Gaussian function (for the protein aromatic signals) or to a sum of two Gaussians (for the dioxane signal that resonates at a frequency where signals arising from the protein also resonate) (see ref. 26). Fitted decay rates are proportional to the diffusion coefficients for the protein and dioxane molecules, which are inversely proportional to their effective hydrodynamic radii.