Strange temperature dependence of the folding rate of a 16-residue β-hairpin

The folding/unfolding kinetics of a 16-residue β-hairpin that undergoes cold denaturation at ambient temperatures were investigated by time-resolved infrared spectroscopy coupled with the laser-induced temperature jump (T-jump) initiation method. We found that the relaxation kinetics of this β-hairpin following a T-jump, obtained by probing the amide I′ band of the peptide backbone, show strange temperature dependence. At temperatures below approximately 35 °C where this β-hairpin mainly exhibits cold denaturation, the T-jump induced relaxation rate is ∼5 μs−1, whereas at temperatures where heat denaturation takes place, the relaxation rate increases to ∼1 μs−1. These results cannot be readily explained by a two-state folding model that has been used to describe the folding thermodynamics of this β-hairpin. In addition, these results suggest that the folding free energy barrier separating the cold-denatured state from the folded state is different from that separating the heat-denatured state from the folded state, coinciding with the idea that the mechanism leading to cold denaturation is different from that leading to heat denaturation.