We present calculations for histidine-(H2O)(n) (n=0-6) to examine the effects of micro-hydrating water molecules on the relative stability of the zwitterionic vs. canonical forms of histidine. We calculate the structures and Gibbs free energies of the conformers at wB97XD/6-31 1++G(d,p) level of theory. We find that six water molecules are required to produce the thermodynamically stable histidine zwitterion. By calculating the barriers of canonical <-> zwitterionic transformation, we predict that both the most stable canonical and zwitterionic forms of histidine-(H2O)(6) may be observed in low temperature gas phase environment. (C) 2015 Elsevier B.V. All rights reserved.