pH dependence of ΔGop. The GdmCl concentration-dependence of the free energy of opening to HX is shown for Phe 12, Val 16, and Lys 18, which are the fast-exchanging amide proton probes from helix 1, at pH 7 (open symbols) and at pH 5.7 (filled symbols), respectively. The solid and the dotted lines represent the free energies of global unfolding of Trx, as a function of GdmCl concentration, at pH 5.7 and at pH 7, respectively, and are given by the equations ΔGu = 10.7 − 3.5 [GdmCl] and ΔGu = 9.6 − 3.8 [GdmCl], respectively.