Infrared multiple photon dissociation spectroscopy of gas-phase doubly charged alkaline earth complexes of histidine reveals a transition from dominance of the zwitterion (salt bridge, SB) conformation with Ba2+ to substantial presence of the canonical (charge-solvated, CS) conformation with Ca2+. This result is a clear illustration of the importance of metal-ion size in governing the delicate balance between these two modes of complexation of gas-phase amino acids. The two conformational motifs are clearly distinguished by characteristic spectral features, confirmed by density functional theory simulated IR spectra of the low-energy conformers. As a further illustration of histidine complexation possibilities, the spectrum of the Na(+)His complex shows purely CS character and emphasizes the greater tendency toward SB character induced by the higher charge in the alkaline earth complexes. Calculation of the complete series of alkaline earth/histidine complexes confirms the increasing stability of the SB conformations relative to CS with increasing metal ion size, as well as showing that among SB conformations the most highly chelated conformation (SB3) is favored for small metals, whereas the most extended conformation (SB1) is favored for large metals. A decomposition of the binding thermochemistry shows that these thermochemical trends versus metal-ion size are due to differences in electrostatic binding energies, with relatively little contribution from the deformation and rearrangement energy costs of distorting the ligand framework.