Ab Initio Calculations of Proline Vibrations with and without Water: Consequences on the Infrared Spectra of Proline-Rich Proteins

Abstract

The infrared spectra of proline-rich proteins display a strong band in the 1450 cm⁻¹ region. In the literature, this band has been assigned either to the deformation modes of the CH₂ and CH₃ groups or to the CN stretching mode of proline residues. In order to establish the correct assignment of this band, the impact of proline vibrations in a polypeptide chain is studied and ab initio calculations are performed for a model molecule (I) containing a repeat unit of polyproline. A strong band is effectively calculated in the 1450 cm⁻¹ region and mostly assigned to CN stretching, whereas, due to the absence of the N–H bond, there is no amide II band. These results are in good agreement with the spectral features observed in the Fourier transform infrared (FT-IR) spectra of gliadins. Moreover, the spectral shifts calculated when a water molecule is complexed with (I) are consistent with the hydration effect observed in the experimental data.