Influence of the addition of water molecules (n = 1–6) on the interaction energy between Li ? , Na ? , K ? cations and indole molecule as tryptophan amino acid resi-due is considered at MP2(FULL)/6-311??G(d,p)//B3LYP/ 6-311??G(d,p) levels of theory. The calculations suggest that the size of cation and the number of water molecules are two important factors that affect the interaction energy between the hydrated metal cation and indole molecule. The strength of cation–p interactions get substantially reduced when the metal ion is solvated or the size of metal cation increases. Quantum theory of atoms in molecules analysis of cation–p interaction indicates that there is a correlation between the electron density (q(r)) in the cage critical points generated upon complexation and the distance between metal cation and centroid of phenyl ring in indole molecule.