Folding and unfolding of a photoswitchable peptide from picoseconds to microseconds

Using time-resolved IR spectroscopy, we monitored the kinetics of folding and unfolding processes of a photoswitchable 16-residue alanine-based α-helical peptide on a timescale from few picoseconds to almost 40 μs and over a large temperature range (279–318 K). The folding and unfolding processes were triggered by an ultrafast laser pulse that isomerized the cross linker within a few picoseconds. The main folding and unfolding times (700 ns and 150 ns, respectively, at room temperature) are in line with previous T-jump experiments obtained from similar peptides. However, both processes show complex, strongly temperature-dependent spectral kinetics that deviate clearly from a single-exponential behavior. Whereas in the unfolding experiment the ensemble starts from a well defined folded state, the starting ensemble in the folding experiment is more heterogeneous, which leads to distinctly different kinetics of the experiments, because they are sensitive to different regions of the energy surface. A qualitative agreement with the experimental data-set can be obtained by a model where the unfolded states act as a hub connected to several separated “misfolded” states with a distribution of rates. We conclude that a rather large spread of rates (k1 : kn ≈ 9) is needed to explain the experimentally observed stretched exponential response with stretching factor β = 0.8 at 279 K.