We reported an in-depth theoretical analysis of the IR spectroscopy of alanine in various forms. We applied the effective modes analysis, aiming to decompose the IR absorption spectra of alanine in the gas phase, i.e., in zwitterionic and neutral forms, and of hydrated and crystalline zwitterionic alanine. The decomposition relies on the identification of well-defined, maximally localized in frequency, absorption bands calculated as the power spectra of the effective modes, whose sum yields with good approximation the full IR spectrum of the molecule at hand.