Transient 2D-IR Spectroscopy:  Snapshots of the Nonequilibrium Ensemble during the Picosecond Conformational Transition of a Small Peptide

The technique of transient two-dimensional infrared (T2D-IR) spectroscopy is introduced, which extends the advantage of 2D-IR spectroscopy to the investigation of a transient species with picosecond time resolution. The conformational change of a small cyclic peptide is studied in the amide-I spectral range, which is induced by means of a photoswitch integrated into the peptide backbone. Substantial changes are found in the transient 2D-IR spectra at times when the transient 1D spectra show only a minor time dependence, illustrating the information gain accessible from 2D-IR spectroscopy. In contrast to 1D spectroscopy, 2D-IR can distinguish between homogeneous and inhomogeneous broadening. The homogeneous contribution to the total width of the amide-I band changes during the course of the conformational transition, a result that is interpreted in terms of the manner in which the peptide samples its conformational space.