Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy

Femtosecond infrared (IR) pump probe and dynamic hole burning experiments were used to examine the ultrafast response of the modes in the 1600-1700 cm(-1) region (the so-called amide I modes) of N-methylacetamide (NMA) and three small globular peptides, apamin, scyllatoxin, and bovine pancreatic trypsin inhibitor (BPTI). A value of 16 cm(-1) was found for the anharmonicity of the amide I vibration. Vibrational relaxation of the amide I modes of all investigated peptides occurs in ca. 1.2 ps. An even faster value of 450 fs is obtained for NMA, a model for the peptide unit. The vibrational relaxation is dominated by intramolecular energy redistribution (IVR) and reflects an intrinsic property of the peptide group in any environment. Dynamic hole burning experiments with a narrow band pump pulse which selectively excites only a subset of the amide I eigenstates reveal that energy migration between different amide I states is slow compared with vibrational relaxation. Two-dimensional pump-probe (2D-IR) spectra that display the spectral response of the amide I band as a function of the frequency of the narrow band pump pulse show that the amide I states are nevertheless delocalized along the peptide backbone. A simple excitonic coupling model describes the nonlinear pump-probe spectrum, and it reproduces the experimental 2D-IR spectra. It is estimated that the accessible peptide excitons are delocalized over a length of ca. 8 Angstrom.