Pump/probe self-heterodyne experiments on the amide I band of a small de novo cyclic pentapeptide were utilized to demonstrate a novel form of two-dimensional (2D) vibrational spectroscopy. Spectrally resolved cross peaks are observed, which measure the coupling between different peptide units and which can be related to the structure of the peptide, in analogy to 2D-NMR spectroscopy. In contrast to our previous work, these experiments work in the time domain in the semiimpulsive limit, employing two intense ultrashort infrared laser pulses. A theoretical formalism is presented in order to model the interstate coherent wave packet generated by the excitation pulse and the resulting spectroscopic signal. The observed coherences provide an independent proof of excitonic coupling within the amide I manifold of the peptide backbone.

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