Multiple intermediates and transition states during protein unfolding.

Rapid kinetic studies of the unfolding of the small protein barstar by urea have been used to demonstrate the presence of at least two unfolding intermediates on two competing unfolding pathways. One intermediate has native-like secondary structure but has a partially solvated hydrophobic core, while the other is devoid of considerable secondary structure but has an intact hydrophobic core. It is shown that the transition states on the two pathways are very dissimilar structurally, but very similar energetically.