Folding of a heterogeneous β-hairpin peptide from temperature-jump 2D IR spectroscopy

We provide a time-and structure-resolved characterization of the folding of the heterogeneous beta-hairpin peptide Tryptophan Zipper 2 (Trpzip2) using 2D IR spectroscopy. The amide I' vibrations of three Trpzip2 isotopologues are used as a local probe of the midstrand contacts, beta-turn, and overall beta-sheet content. Our experiments distinguish between a folded state with a type I' beta-turn and a misfolded state with a bulged turn, providing evidence for distinct conformations of the peptide backbone. Transient 2D IR spectroscopy at 45 degrees C following a laser temperature jump tracks the nanosecond and microsecond kinetics of unfolding and the exchange between conformers. Hydrogen bonds to the peptide backbone are loosened rapidly compared with the 5-ns temperature jump. Subsequently, all relaxation kinetics are characterized by an observed 1.2 +/- 0.2-mu s exponential. Our time-dependent 2D IR spectra are explained in terms of folding of either native or nonnative contacts from a common compact disordered state. Conversion from the disordered state to the folded state is consistent with a zip-out folding mechanism.