We present a brief review for studies of the relative stability of canonical vs. zwitterionic forms of amino acids and dipeptides under the influence of gas phase hydration. Focus is given on how many water molecules are necessary to stabilize the zwitterionic conformer. Experimental and theoretical investigations for this interesting question are discussed. It is illustrated that the microsolvating properties of amino acids and dipeptides are strongly dependent on the characteristics (hydrophilicity, basicity etc.) of side chains, and also on the presence of metal cations or an excess electron. Besides the relative Gibbs free energies of various conformers to estimate their relative thermodynamic stability, the activation barriers of proton transfer processes between canonical and zwitterionic forms are emphasized to assess the kinetic stability of thermodynamically less favorable species in low temperature gas phase environment.
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