Pump/probe self heterodyned 2D spectroscopy of vibrational transitions of a small globular peptide

Pump/probe self-heterodyne experiments on the amide I band of a small de no vo cyclic pentapeptide were utilized to demonstrate a novel form of two-dim ensional (2D) vibrational spectroscopy. Spectrally resolved cross peaks are observed, which measure the coupling between different peptide units and w hich can be related to the structure of the peptide, in analogy to 2D-NMR s pectroscopy. In contrast to our previous work, these experiments work in th e time domain in the semiimpulsive limit, employing two intense ultrashort infrared laser pulses. A theoretical formalism is presented in order to mod el the interstate coherent wave packet generated by the excitation pulse an d the resulting spectroscopic signal. The observed coherences provide an in dependent proof of excitonic coupling within the amide I manifold of the pe ptide backbone. (C) 2000 American Institute of Physics. [S0021-9606(00)0080 4-7].