Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of L-arginine is examined. The effects of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) and water on structures of Arg center dot M(H2O)(m), m = 0, 1 complexes have been determined theoretically by employing the density functional theories (DFT) and using extended basis sets. Of the three stable complexes investigated, the relative stability of the gas-phase complexes computed with DFT methods (with the exception of K+ systems) suggests metallic complexes of the neutral L-arginine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of arginine in the presence of the metal cations Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+ were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to arginine is exhibited by the Cu2+ cation. The computed Gibbs energies Delta G degrees are negative, span a rather broad energy interval (from -150 to -1500 kJ/mol), and are appreciably lowered upon hydration.