Water-Induced Zwitterionization of Glycine: Stabilization Mechanism and Spectral Signatures

Not only the question of how many water molecules are required to stabilize the physiologically relevant charge-separated zwitterionic form of amino acids upon solvation but also the stabilization mechanism is still under debate. It is well known that a water bridge connecting the carboxyl with the amino group must be established. Here, we show that this is not yet a sufficient condition to stabilize the zwitterion. Instead, the formation of a bifurcated H-bonded water wire that connects the two charged groups turns out to be the key, which explains why an unexpectedly large number of water molecules of about nine is required to enable zwitterionization of microsolvated glycine. Moreover, this bifurcated wire allows one to pinpoint a frequency window that enables the detection of zwitterionization by spectroscopy. These findings will be relevant to probe and rationalize microsolvation-induced zwitterionization of not only amino acids but of other acid/base reactions that involve somewhat distant such functional groups within the same molecule.