Long-Lived Intermediates in a Cooperative Two-State Folding Transition.

Biomolecular folding often occurs through a cooperative two-state reactant ↔ product transition; the term cooperative does not convey that intermediate structures are nonexistent but rather that these states are not observable by existing experimental techniques. Because of this, few intermediates have been studied and characterized. Recently, ion mobility spectrometry (IMS) measurements revealed that the oligomer polyproline-13 (Pro13, which in propanol (PrOH) favors the right-handed helical PPI structure having adjacent pyrrolidine rings in a cis configuration) folds through six sequential long-lived intermediates as it converts to the all-trans-configured PPII structure that is favored in aqueous solutions. Here, we examine the PPI → PPII folding transition for a HisPro13 sequence, i.e., Pro13 having a single histidine residue added to the N-terminus. Remarkably, the IMS measurements show that, upon addition of histidine, all of the IMS peaks associated with intermediate structures disappear. Instead, HisPro13 folds via a cooperative two-state transition, delayed by a significant induction period. The induction period is temperature dependent-shifting the transition to longer times at lower temperatures. Equilibrium studies show that the HisPro13 PPI → PPII transition is endothermic but favored entropically. From these clues, we propose a sequential folding mechanism and develop a model that suggests that ∼13-17 long-lived intermediates are likely responsible for the induction period. In this model, intermediates are separated by average individual activation barriers of ∼90 kJ·mol, and are entropically favorable.