Gas-phase peptide structures unraveled by far-IR spectroscopy: Combining IR-UV ion-dip experiments with born-oppenheimer molecular dynamics simulations

Vibrational spectroscopy provides an important probe of the three-dimensional structures of peptides. With increasing size, these IR spectra become very complex and to extract structural information, comparison with theoretical spectra is essential. Harmonic DFT calculations have become a common workhorse for predicting vibrational frequencies of small neutral and ionized gaseous peptides.1 Although the far-IR region (<500 cm^-1) may contain a wealth of structural information, as recognized in condensed phase studies,2 DFT often performs poorly in predicting the far-IR spectra of peptides. Here, Born-Oppenheimer molecular dynamics (BOMD) is applied to predict the far-IR signatures of two γ-turn peptides. Combining experiments and simulations, far-IR spectra can provide structural information on gas-phase peptides superior to that extracted from mid-IR and amide A features. The use of low-frequency modes (towards 100 cm^-1) for structural assignment of peptides is explored. This far-IR region possibly contains detailed information on the secondary structure. The use of Born-Oppenheimer molecular dynamics simulations is discussed to calculate the far-IR signature of peptides.