The binding site dependence of binding energy in both metalated and protonated diglycine and triglycine peptides

The dependence of binding energy (BE) on the binding sites of both protonated and various metalated (M+/2+ = Li+, Na+ or K+, and Be2+, Mg2+ or Ca2+) oligoglycine derivatives, GnH+Mx+/2+ (G = glycine and H+ = proton; n = 2, 3 and x = 1-3) in the gas phase has been determined at the B3LYP level with different basis sets. Results show that the BEs of these GnH+⋯Mx+/2+ complexes will change into negative values when n ⩾ 4 for GnH+Lix+, n ⩾ 5 for GnH+Nax+, n ⩾ 6 for GnH+Kx+, n ⩾ 2 for GnH+Mgx2+, and n ⩾ 3 for GnH+Cax2+, respectively. The BEs of both G2H+Bex2+ and G3H+Bex2+, however, are always negative values. The signs “n” and “x” denote the number of glycine residues of linking the two cations (H+ and M+/2+) and the serial number of site bound by a metal-ion, respectively. Moreover, the BEs (ΔE) decrease gradually along the increase of oligoglycine size (n) between two cations for all these different GnH+Mx+/2+ systems, i.e., ΔE(GH+Mx+/2+) > ΔE(G2H+Mx+/2+) > ΔE(G3H+Mx+/2+). Monohydration at the metal-ion almost keeps these BEs unchangeable. The continuum solvent effect, however, can change them into negative values. Interestingly, the different potential well depths (or activation energy) on the potential energy surface give different information for these monovalent and divalent metal-ion involved systems. In detail, electrostatic effect of monovalent metal-ion on the proton transfer of amino-terminus decreases stepwise along with the number increase of the glycine residues between two cations. Moreover, the well will disappear in the process of proton transfer assisted by a water molecule. Differently, the (N-)H of neighboring the metal-bound site is the most potential proton (hydrogen) to transfer in these divalent metal-ion involved systems.