Figure 4.

(a) Protection factors for the main-chain amide protons of α₃D determined from hydrogen–deuterium exchange at 15°C, pH* 5.0 (in 50 mM deuterated acetate). The protection factors were calculated from the observed exchange rates and the intrinsic exchange rates (39). Absence of a bar indicates residues with amide protons exchanging in the dead time of the experiment (18 minutes) or a lack of a well resolved amide proton cross peak. (b) Assuming an EX2 exchange mechanism, the apparent free energy, ΔG_HX, was calculated from the protection factors, P_i, as RTln(P_i) (39). The dashed line represents the ΔG_u at 15°C determined calorimetrically by extrapolation of the Gibbs-Helmoltz equation, ΔG_T = ΔH_Tm (1 − T/T_m) + (T − T_m) ΔC_p − TΔC_pln(T/T_m). The shaded region denotes the estimated standard error in this parameter of ±0.5 kcal⋅mol⁻¹.