Fig. 4.
Dependence of the apparent radii of gyration (Rg) of the labeled protein segments on the concentration of GdmCl (filled circles) and urea (open circles), with (A) CspTm (yellow), (B) IN (red), (C) ProTαN (cyan), and (D) ProTαC (blue). Fits to a binding model for the urea dependence (Eq. 2, colored dashed lines), and to polyampholyte theory for the GdmCl dependence (Eq. 5, black solid lines) are shown. The two components of Eq. 5, corresponding to the contributions of GdmCl binding and electrostatic repulsion, are indicated as continuous and dashed gray lines, respectively. Note that the fits to Eq. 5 are performed based on thermodynamic activities, but plotted on a concentration scale. The colored squares in (A) and (D) indicate the values of Rg on addition of 1 M KCl (compare to Fig. 5). The gray squares indicate the expected values estimated with Eqs. 4 and 5, assuming the values for K, a, and ρ obtained from the fits of the urea dependencies (Table S4), the value of ν obtained from the fits of the GdmCl dependencies (Table S2), and calculating κ for an ionic strength of 1 M. The remaining difference between experimental and calculated values may be due to the preferential interaction of GdmCl with the polypeptide, leading to a higher local charge density than in the bulk solution and a correspondingly stronger charge shielding than for KCl.