Protein stability parameters measured by hydrogen exchange.

The hydrogen exchange (HX) rates of the slowest peptide group NH hydrogens in oxidized cytochrome c (equine) are controlled by the transient global unfolding equilibrium. These rates can be measured by one-dimensional nuclear magnetic resonance and used to determine the thermodynamic parameters of global unfolding at mild solution conditions well below the melting transition. The free energy for global unfolding measured by hydrogen exchange can differ from values found by standard denaturation methods, most notably due to the slow cis-trans isomerization of the prolyl peptide bond. This difference can be quantitatively calculated from basic principles. Even with these corrections, HX experiments at low denaturant concentration measure a free energy of protein stability that rises above the usual linear extrapolation from denaturation data, as predicted by the denaturant binding model of Tanford.