Hydrogen exchange of chymotrypsin inhibitor 2 has been measured in the presence of low concentrations of GdmCl and at different temperatures. The study of exchange at different temperatures allows us to obtain the activation enthalpies for the local exchange processes, and the change in enthalpy between the closed, exchange-incompetent, forms and the open, exchange-competent, forms. From the GdmCl dependence of exchange, an m-value, which is a measure of the new surface area exposed to solvent in the equilibrium between open and closed forms, can be determined for individual protons. This parameter therefore provides information about the structural nature of the opening reactions. In the absence of denaturant, exchange from native and native-like states dominates. As GdmCl concentration is increased, opening reactions that involve global unfolding are selectively promoted for the majority of amide protons. Three classes of protons emerge: for one set of protons, there is a linear and weak dependence on denaturant, indicating that the dominant opening reaction is the same throughout the range of GdmCl concentrations and involves local fluctuations with exposure of little new surface. For another set of protons, the most slowly exchanging residues, a linear, but much stronger, denaturant dependence is observed. For these protons, global unfolding dominates, and the m-values are similar to that obtained by equilibrium GdmCl denaturation measured by fluorescence under identical conditions. For the remaining protons, the GdmCl-dependence is weak at low GdmCl concentrations and increases at higher GdmCl concentrations. No segment of sub-global unfolding could be identified. Rather, all protons appear to merge together at high GdmCl concentrations to the global unfolding reaction.
