Abstract
Heterodyned femtosecond infrared two-pulse and three-pulse photon echoes of the dipeptide acetylproline-NH2 in D2O and CDCl3 have been measured and the results have been compared with force field calculations of the peptide structures. The heterodyned two-dimensional infrared (2D IR) spectra obtained from the measurements exhibit diagonal peaks and cross-peaks that are determined by the structures and vibrational dynamics of the acetylproline-NH2 molecule. The two-pulse measurements are analogous to 2D COSY experiments in NMR spectroscopy. In CDCl3, the 2D IR spectra from the two-pulse experiments resolve two acetyl amide I bands and two amino amide I bands that are not resolved in the linear spectrum. Thus, acetylproline-NH2 must have at least two structures in CDCl3. The angles between the amide I transition dipoles of the structures were determined to be <20° and 35° from polarized 2D IR measurements. A single structure is found in D2O with an angle of <20°. The infrared analogue to 2D NMR NOESY experi...
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