Experimental and computational investigations of the THz spectra of dipeptide nanotubes

Continuous wave THz spectroscopy has been used to obtain spectra for four isostructural dipeptide nanotubes at 4.2K from 2 cm(-1) to 100 cm(-1) (0.05 to 3 THz). Line-narrowing of spectral features by a factor of 2 to 4 is observed for the crystalline dipeptide films investigated by absorption spectroscopy using a plane parallel waveguide, compared to spectra from pressed disks of polyethylene-diluted samples. The x-ray determined crystal structures of these peptides formed the basis for a parallel computational investigation. Spectral predictions from the ab initio level computational package DMOL3 and the empirical force field-model CHARMM22 are compared to the experimentally obtained THz absorption spectra. The THz waveguide spectroscopy technique can provide information on the orientation-dependent dipole coupling of the vibrational modes, which can aid in validating computational models.