| Potassium cation involved in numerous biochemical functions, such as stabilization of protein structures and osmotic equilibrium of cells. For a better understanding of such interactions, information about the intrinsic binding modes and energies of K+ to simple model systems ligands is essential.;The present study addresses this subject by determining the geometries and energetics of K+ binding to: (a) small organic ligands, (b) aliphatic amino acids (glycine, alanine, valine, leucine and isoleucine), (c) linear dipeptides (glycylglycine, GG, and alanylalanine, AA), (d) dipeptides containing phenylalanine and (e) the basic amino acid, histidine, by quantum chemical molecular modeling methods. The theoretical studies were carried out by high-level density functional theory calculations at the B3-LYP/6-311+G(3df,2p)/B3-LYP/6-31G(d)) level of theory (abbreviated as the 'EP(K+) protocol'). Our study shows that theoretical protocol yields absolute affinities for 65 model organic ligands in excellent agreement with experimental affinities (mean absolute deviation of 4.5 kJ mol-1), in support of the reliability of the B3-LYP protocol adopted in our study.;Using this EP(K+) protocol, the most stable mode of all K+-aliphatic amino acids complexes involves a bidentate interaction between the cation and the O=C and --OH sites in the charge-solvated (CS) form. For the larger aliphatic amino acids, the zwitterionic (ZW) form is stabilized more than the CS mode. Factors affecting the relative K + affinities of these different ZW versus CS binding modes are discussed.;The most stable K+-GG/AA/FG/GF complexes are found to be in the charge-solvated (CS) form, with K+ bound to the two carbonyl oxygen atoms of the peptide backbone. For the FG/ GF dipeptides, K+ additionally bound to the phenyl pi-ring of phenylalanine are comparably stable. The key stabilizing factors in these complexes are the electrostatic global ion-dipole interaction, as well as the local ion-dipole interaction between K + and the amide carbonyl oxygen of the peptide bond. The usefulness of proton affinity as a criterion for estimating the relative stability of ZW versus CS binding modes is examined.;Theoretical studies on protonated and potassiated histidine ([His + H/K] +) and their mass spectrometric fragmentations in the gas phase have been conducted. |
