Figure 5.

Simulation of the values of k_obs vs. pH for a simple model (see text) in which only one ionizing group has pK_a values that differ in I, I^‡, and N. The pK_a value in I^‡ differs in the three cases shown: case 1, 6.6 (close to I); case 2, 5.15 (intermediate); case 3, 3.7 (close to N). The pK_a value in N is 3.5 and in I is 6.8 in all three cases. The fraction unfolded is shown on the right y axis, and the dotted line is the equilibrium unfolding curve (unfolding does not go to completion at low pH) calculated from the values of k_u and k_f vs. pH (see Experimental Procedures). Their values at pH 9 are k_u = 0.001 s⁻¹ and k_f = 6 s⁻¹.