Additionally, we find that, compared to zwitterionic alanine in the gas phase, the band near 1300 cm -1 is slightly blue shifted in water and in the crystal, while, concomitantly, the band near 1900 cm -1 is red shifted when the molecule is not isolated. In order to analyze the points just discussed, we start by reporting in Fig. some of the effective modes giving rise to the IR absorption bands in the region 1000-1200 cm -1 . Note that all modes are very similar to each other, involving some kinds of wagging and twisting of the fragments CH 3 and NH 3 , mainly coupled to the vibrations of the CH bond that is not involved in CH 3 . Only mode 3 in the crystal form appears slightly different, as it clearly involves the umbrella mode in NH 3 .