Zwitterion vs neutral structures of amino acids stabilized by a negatively charged site: Infrared photodissociation and computations of proline-chloride anion

Infrared photodissociation (IRPD) spectra are reported for a proline-chloride anion cluster along with its d₂- and d ₇-isotopomers. The spectral data indicate that proline is in its neutral form as opposed to a zwitterion, and computations are in agreement in that some neutral conformers are energetically low-lying and reproduce the observed spectra. Zwitterionic conformers are predicted to be essentially as stable as the neutral ones and should be significantly populated; however, there is no evidence for these structures in the IRPD spectra. An exploration of the potential energy surface for the loss of chloride anion, the observed fragmentation channel, reveals that it is 8.4 kcal mol^-1 more difficult to break apart the zwitterionic cluster ion. This is a reflection of the 15.8 kcal mol^-1 estimate for the gaseous proline zwitterion-neutral energy difference. Kinetic results suggest the presence of two photolabile populations in similar amounts (i.e., 56 vs 44%). The more abundant structure is also the more labile species, and the neutral form of proline is assigned to this cluster ion. The less abundant and slower fragmenting structure consequently is zwitterionic. As originally suggested by Evans et al.(33) in general, it appears that in this instance both spectral and kinetic data are needed to determine the structure of the proline-chloride anion cluster.