Vibrational signatures of Ac-Phe-AA-NH2 dipeptides are recorded and analysed in the far IR/THz spectral domain (100-800 cm−1, 3-24 THz), with the 'AA' amino acid chosen within the series 'AA'=Gly, Ala, Pro, Cys, Ser, Val. Phe is for Phenylalanine. IR-UV ion dip experiments are conducted on the free electron laser FELIX and combined with DFT-based molecular dynamics simulations for the calculation of dynamical anharmonic vibrational spectra. The excellent agreements between the experimental and theoretical spectra for the Ac-Phe-AA-NH2 series allow to make detailed and unambigous mapping of the vibrational motions into three main domains: 700-800 cm−1 for C-H waggings, 400-700 cm−1for N-H waggings, with a one-to-one signature per amide N-H backbone group, 0-400 cm−1 for delocalized and large amplitude collective motions over the dipeptide backbone, with backbone torsional motions arising <100 −1.
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