A Theoretical Study of Potassium Cation Binding to Glycylglycine (GG) and Alanylalanine (AA) Dipeptides

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Abstract
By combining Monte Carlo conformational search technique with high‐level density functional calculations, the geometry and energetics of K+ interaction with glycylglycine (GG) and alanylalanine (AA) were obtained for the first time. The most stable K+‐GG and K+‐AA complexes are in the charge‐solvated (CS) form with K+ bound to the carbonyl oxygens of the peptide backbone, and the estimated 0 K binding affinities (ΔH0) are 152 and 157 kJ mol−1, respectively. The K+ ion is in close alignment with the molecular dipole moment vector of the bound ligand, that is, electrostatic ion–dipole interaction is the key stabilizing factor in these complexes. Furthermore, the strong ion–dipole interaction between K+ and the amide carbonyl oxygen atom of the peptide bond is important in determining the relative stabilities of different CS binding modes. The most stable zwitterionic (ZW) complex involves protonation at the amide carbonyl oxygen atom and is approximately 48 kJ mol−1 less stable than the most stable CS form. The usefulness of proton affinity (PA) as a criterion for estimating the relative stability of ZW versus CS binding modes is examined. The effect of chain length and the nature of metal cations on cation–dipeptide interactions are discussed. Based on results of this study, the interaction of K+ with longer peptides consisting of aliphatic amino acids are rationalized.