Hydrogen exchange at equilibrium: a short cut for analysing protein-folding pathways?

Hydrogen exchange is an attractive method for observing small populations of partly unfolded states of proteins at equilibrium. It has been suggested that these represent folding intermediates so that hydrogen exchange can offer a short cut for studying protein-folding pathways. This cannot work in theory because it is not possible to tell whether they are intermediates or side reactions. Experimental studies of barnase and chymotrypsin inhibitor 2 show that there is no obvious relationship between hydrogen exchange at equilibrium and their folding pathways.