Heterodyned Two-Dimensional Infrared Spectroscopy of Solvent-Dependent Conformations of Acetylproline-NH₂

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 experiments has also been performed utilizing three-pulse photon echoes. In these three-pulse experiments, the cross-peak intensities and polarizations are found to change with the waiting time between the second and third pulses. By comparison to the changes in the diagonal peaks, these effects are attributed to population and coherence transfer processes. The vibrational dynamics and inhomogeneous distributions at the acetyl and amino ends of acetylproline-NH2 are observed to be different.