Mapping gas phase dipeptide motions in the far-infrared and terahertz domain.

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 stands 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 the dynamical anharmonic vibrational spectra. The excellent agreements between the experimental and theoretical spectra of the Ac-Phe-AA-NH2 series allow us to make detailed and unambiguous mapping of the vibrational motions into three main domains: 700-800 cm-1 for C-H waggings, 400-700 cm-1 for 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 cm-1.