Picosecond conformational transition and equilibration of a cyclic peptide

Abstract

Ultrafast IR spectroscopy is used to monitor the nonequilibrium backbone dynamics of a cyclic peptide in the amide I vibrational range with picosecond time resolution. A conformational change is induced by means of a photoswitch integrated into the peptide backbone. Although the main conformational change of the backbone is completed after only 20 ps, the subsequent equilibration in the new region of conformational space continues for times >16 ns. Relaxation and equilibration processes of the peptide backbone occur on a discrete hierarchy of time scales. Albeit possessing only a few conformational degrees of freedom compared with a protein, the peptide behaves highly nontrivially and provides insights into the complexity of fast protein folding.

Fig. 1.

Structure sketch of the S-tert-butylthio-protected linear precursor peptide linAMPB (a) and the disulfide-bridged cyclic peptide cycAMPB (b).

Fig. 5.

Ensemble of structures with the azobenzene unit in the cis (Lower Left) and trans conformation (Lower Right), as obtained from NMR structure analysis (25). After photoisomerizing the built-in azobenzene unit, the random distribution of cis structures is projected onto a new potential energy surface. During the driven phase (in red), the ensemble relaxes very quickly on a 6-ps time scale and entirely floods the bottom area of the potential energy surface. During the subsequent biased diffusion phase, the system equilibrates on a discrete hierarchy of time scales, extending from 20 ps to >16 ns, toward the much better defined global minimum of the trans conformation.

Fig. 2.

Absorption spectra of the cis (Top, solid line) and trans conformer (Top, dashed line), transient difference spectra at selected delay times (Middle) and stationary cis-trans difference spectrum measured in the FTIR spectrometer (Bottom) of the cyclic peptide cycAMPB. The FTIR difference and absorption spectra have been scaled to facilitate direct comparison with the time-resolved spectra. The transient responses with both parallel (filled circles) and perpendicular (open circles) polarization of pump and probe pulses are shown (both have been measured simultaneously and have not been scaled with respect to each other; see Materials and Methods). The scale of the time-resolved spectra is the same as in Fig. 3.

Fig. 3.

Absorption spectrum (Top), transient difference spectra at selected delay times (Middle), and stationary cis-trans difference spectrum measured in the FTIR spectrometer (Bottom) of the linear precursor peptide linAMPB. The responses with both parallel (filled circles) and perpendicular (open circles) polarization of pump and probe pulses are shown (both have been measured simultaneously and have not been scaled with respect to each other; see Materials and Methods). The FTIR difference and absorption spectra have been scaled to facilitate direct comparison with the time-resolved spectra. The scale of the time-resolved spectra is the same as in Fig. 2.

Fig. 4.

Polarization-dependent (parallel, perpendicular, and magic angle) transient absorbance change of the cycAMPB and linAMPB-peptide at selected probe frequencies. An example of the anisotropy is shown for 1,670 cm^–1.