The reported decomposition of the spectra of alanine confirmed the capability of the effective modes analysis to unravel absorption spectra that can appear at first glance very broad and not well resolved. The comparison between the two gas phase forms of alanine allowed us to identify the modes responsible for the changes in the IR spectra: even though it is expected that the changes in the neutral form are produced by the fragments of the molecule that are not present in the zwitterionic form, it is interesting to find that those changes are confirmed based on a rigorously-derived numerical procedure. In addition, the comparison with the literature in the assignment of the absorption bands of the gas phase species allowed us to validate the new parametrization of the AMOEBA polarizable force field, which we used to predict the IR absorption spectroscopy of hydrated and crystal alanine.