STRUCTURE OF THE AMIDE-I BAND OF PEPTIDES MEASURED BY FEMTOSECOND NONLINEAR-INFRARED SPECTROSCOPY

Femtosecond infrared (IR) pump probe and dynamic hole burning experime nts were used to examine the ultrafast response of the modes in the 16 00-1700 cm(-1) region (the so-called amide I modes) of N-methylacetami de (NMA) and three small globular peptides, apamin, scyllatoxin, and b ovine pancreatic trypsin inhibitor (BPTI). A value of 16 cm(-1) was fo und for the anharmonicity of the amide I vibration. Vibrational relaxa tion 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 intramol ecular energy redistribution (IVR) and reflects an intrinsic property of the peptide group in any environment. Dynamic hole burning experime nts with a narrow band pump pulse which selectively excites only a sub set of the amide I eigenstates reveal that energy migration between di fferent amide I states is slow compared with vibrational relaxation. T wo-dimensional pump-probe (2D-IR) spectra that display the spectral re sponse of the amide I band as a function of the frequency of the narro w band pump pulse show that the amide I states are nevertheless deloca lized along the peptide backbone. A simple excitonic coupling model de scribes the nonlinear pump-probe spectrum, and it reproduces the exper imental 2D-IR spectra. It is estimated that the accessible peptide exc itons are delocalized over a length of ca. 8 Angstrom.