Stereo-structural prediction and IR-characteristic band assignment of amorphous protonated forms of homodipeptides (L)-Phe-(L)-Phe, (L)-Trp-(L)-Trp, (L)-Tyr-(L)-Tyr and the neutral - (L)-Trp-(L)-Trp. Ab initio approximation and solid-state linear-polarized IR-spectroscopy.