In Fig. , we find that the selected effective modes responsible for the different spectral features of neutral alanine with respect to zwitterionic alanine clearly involve some vibrational motions of the fragments that mainly distinguish the two species: the OH bond in the COOH and the NH 2 fragments. Mode 2 of Fig. , centered around 1150 cm -1 is a seemingly wagging motion of the NH 2 moiety, which is absent in the zwitterionic form. Similarly, the wagging motion of NH 2 at 1200 cm -1 , which is coupled to the in-plane oscillations of the OH bond (in plane with COO), is totally absent in the absorption spectrum of zwitterionic alanine of Fig. . Mode 8, around 1486 cm -1 , is similar to mode 3 just described, but is coupled to a vibration in the CH 3 fragment that looks like an umbrella mode. Finally, we find that mode 12, slightly above 1700 cm -1 , involves solely the HNH bending motion in NH 2 , and was not so well defined in the analysis reported in Fig. .