The analysis of IR spectra based on the identification of effective modes has been previously applied to a variety of molecular systems where diverse MD techniques were employed to generate the trajectories. In the present work, we focus on the IR spectroscopy of the alanine amino acid , a chiral molecule that some of the authors have recently studied based on ab initio and classical MD simulations in different phases . The effective modes analysis is used here to (i) identify the spectral signatures of neutral and zwitterionic alanine in the gas phase in the region 1000-2000 cm -1 as well as to (ii) determine the effect of the environment on the modes and on the absorption bands in the same spectral region by studying the hydrated form of alanine and its crystal form.