Gas-phase FT-IR-spectra of natural amino acids

We present the first gas-phase Fourier transform infrared (FT-IR) spectra of uncapped (natural) amino acids using a fast thermal heating technique up to around 570 K. The experimental spectra of phenylalanine, proline, valine, leucine and isoleucine in the range 900–3700 cm−1 show no sign of decomposition and are in good agreement with ab initio calculated frequencies and intensities at the B3LYP/6-31G(d,p) level, if non-hydrogen bonded structures are assumed at the high temperatures of the experiment.