As expected from their different structures, the IR absorption spectrum of gas phase neutral alanine is very different from that calculated for the zwitterionic form. By comparing Fig. and Fig. some major differences can be pointed out: (i) in the region 1000-1300 cm -1 , the low intensity band at 1150 cm -1 in zwitterionic alanine becomes more intense and splits into two contributions for the neutral form; (ii) in the region 1350- 1500 cm -1 , the large absorption band of zwitterionic alanine becomes less intense in the neutral form, and four main peaks appear to be better resolved; (iii) the region 1500-1900 cm -1 becomes more IR active for the neutral form of alanine that for the zwitterionic form, with the complete disappearance of absorption above 1800 cm -1 .