Fig. shows an overlay of the anharmonic one-dimensional infrared spectra of the dipeptide systems in the α-helix and β-strand regions. The mean values of the frequencies of the four modes as well as the standard deviations over the 25 structures considered in each case are listed in the Supporting Information (Table and Fig. ). Overall, the plots show that the vibrational frequencies and the intensity patterns strongly depend on the secondary structure. Normal mode 4 (dominated by the in-phase combination of the two classical amide III vibrations) in the α-helix structure region is shifted by about 10 cm -1 to lower wavenumbers than the corresponding peaks in the β-strand region. The position of normal mode 3 (dominated by the out-of-phase combination of the two classical amide III vibrations) differ largely, with the peaks in the α-helix region shifted by about 23 cm -1 to higher wavenumbers compared to the β-strand region. For both normal modes, similar standard deviations are found. For normal mode 2 (dominated by C α -H bending), the peaks for the structures in the α-helix region are downshifted by about 28 cm -1 compared to those for the strucutures in the β-strand region, and their standard deviation in the β-strand region is almost twice as large as in the α-helix region. Finally, for normal mode 1 (also dominated by C α -H bending), the peaks appear at about the same frequency for both regions, but they differ substantially in their standard deviations. For the α-helix region, the peak position shows the lowest standard deviation overall, whereas for the β-strand region the peaks are broadly scattered. While for the structures in the α-helix region the intensities of the C α -H bending vibrations (normal modes 1 and 2) are comparably small, they are significantly larger for those in the β-strand region, but in the latter case also the intensities vary widely between different structures.