Three types of the cyclic peptides constructed of four sequences of glycine (CP1) and alanine (CP2) and two sequences of glycine-valine molecules (CP3) have been employed for selective complexation with Be2+, Mg2+, Ca2+, Sr2+ and Ba2+ metal ions. Obtained data, from the density functional theory, showed that all CPs have the most stable complexes with Be2+ ion, because of maximum interaction with oxygen atoms of the CPs. Moreover, the most stable complex is found for Be2+-CP2. All of the CPs make four-dentate stable complexes with Mg2+, Ca2+, Sr2+ and Ba2+ metal ions in the gas phase through the oxygen atoms of CO groups of CPs, while CP3 makes bidentate stable complexes with Mg2+, Ca2+ and Sr2+ ions in water. Natural bond orbital analysis indicates that charge transfers from oxygen and nitrogen atoms of CPs to the metal ions reveals that electrostatic interaction between the metal ion and CPs is an important factor in complexation. Quantum theory of atoms in molecule investigation confirmed that the most stable bond was found in Be2+ ion and oxygen atom of the CPs. These findings were in agreement with the molecular dynamic simulations.

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