The gas-phase metal affinities of glycine and alanine for Li+, Na+ and Cu+ ions have been determined theoretically employing the hybrid B3LYP exchange-correlation functional and using extended basis sets. All computations indicate that the metal ion affinity (MIA) decreases on going from Cu+ to Li+ and Na+ for both the considered amino acids. The absolute MIA values are close to the experimental counterparts with the exception of lithium for which a deviation of about 7 kcal/mol at the B3LYP level is obtained. The optimized structures indicate that Li+, Na+ and Cu+ prefer a bidentate coordination, bonding with both nitrogen and oxygen atoms of amino acids.