Folding and unfolding of a photoswitchable peptide from picoseconds to microseconds

doi:10.1073/pnas.0607748104

Folding and unfolding of a photoswitchable peptide from picoseconds to microseconds

*Physikalisch-Chemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland;
^†Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto M5S 3H6, Canada;
^‡Faculty of Sciences, Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands

Edited by William A. Eaton, National Institutes of Health, Bethesda, MD, and approved January 2, 2007 (received for review September 6, 2006)

Abstract

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 : k_n ≈ 9) is needed to explain the experimentally observed stretched exponential response with stretching factor β = 0.8 at 279 K.

Footnotes

^§To whom correspondence should be addressed. E-mail: p.hamm{at}pci.unizh.ch
Author contributions: J.A.I., J.B., J.H., G.A.W., and P.H. designed research; J.A.I., J.B., R.P., J.H., L.C., G.A.W., and P.H. performed research; R.P., L.C., and G.A.W. contributed new reagents/analytic tools; J.A.I., J.B., I.H.M.v.S., and P.H. analyzed data; and J.A.I., G.A.W., and P.H. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.