The IR absorption band of the fragment CO 2 is composed of two main bands: one band appears essentially in the region 1500-1600 cm -1 , but is coupled to smaller bands at (a bit less than) 1100 cm -1 and at 1300 cm -1 ; one band is very well defined and localized at around 1900 cm -1 . Therefore, also for this fragment, we find the participation of the bending motion in CO 2 (mode 1, depicted as arrows in the inset of Fig. ) to the large absorption region of zwitterionic alanine in the gas phase in the region 1300-1600 cm -1 (black line in the upper panel of Fig. ), while the asymmetric stretching in CO 2 (mode 2, in the inset of Fig. ) produces an absorption peak in the high frequency region of the analyzed spectrum. In Section III A, those modes will be referred to as δCO, indicating a bending motion where one or both CO bonds are involved, and as ν as COO, respectively. It is worth mentioning here that, in general, the CO stretch would be expected at lower frequencies, i.e., at around 1650-1700 cm -1 . Finding this mode near 1900 cm -1 is a consequence of the parametrization of the force field.