For the quantum-chemical calculations of the harmonic vibrational frequencies and normal modes, we used MoViPac/SNF in combination with Turbomole, applying the same setup as for the geometry optimizations (i.e., including the electrostatic potential of the point charges, the corresponding interaction energy, and the interaction with the Lennard-Jones potential. All frequency calculations were performed for the N -deuterated peptides. For the localization of normal modes, we employed the LocVib package with the atomic-contribution localization measure.