Multidimensional infrared spectroscopy of a peptide intramolecular hydrogen bond

Stimulated three pulse photon echoes and heterodyned 2D IR echoes of acetylproline-NHMe in CHCl3 are reported. A global fitting of the pump/probe decays of the internally hydrogen bonded N-H stretching vibration of AcProNHMe in CHCl3 gives two population relaxation times of 400 and 1300 fs, which suggest the existence of different hydrogen bonded structures. Two different responses are clearly demonstrated in the 2D spectra, |S∼(τ,T,ωt)|; one of them appears to be a more homogeneous response than the other. This function peaks at τ-values which decrease with a sub-100 fs time constant to a plateau of ca. τ = 70 fs. This decay implies fast spectral diffusion and the persistent non-zero offset suggests that a large static inhomogeneity exists in the whole time range considered in this experiment, which implies the existence of a rigid hydrogen bonded structure on the time scale of many picoseconds. The spectral shape of the 2D IR could not be reproduced by conventional responses for independent oscillators having overlapping transitions. It is suggested that the two hydrogen bond configurations are exchanging rapidly and the shape of the 2D IR spectra is determined by the two chemically interchanging structures. In this paper multidimensional IR spectroscopy clearly shows the potential to disentangle congested spectra by spreading them into two dimensions and expose multiple structures, which are not seen distinctly in the FTIR spectra. The correlation function of the H-bonded N-H group is dominated by a ca. 100 fs component that is suggested to represent overdamped internal structure fluctuations.