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The interactions between five amino acid based anions([AA]−(AA = Gly, Phe, His, Try, and Tyr)) and N7,N9-dimethylguaniniumcation ([dMG]+) have been investigated by the hybrid densityfunctional theory method B3LYP together with the basis set 6-311++G(d,p).The calculated interaction energy was found to decrease in magnitudewith increasing side-chain length in the amino acid anion. The interactionbetween the [dMG]+cation and [AA]−anionin the most stable configurations of ion pairs is a hydrogen bondinginteraction. These hydrogen bonds (H bonds) were analyzed by the quantumtheory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis. Finally, several correlations between electron densities inbond critical points of hydrogen bonds and interaction energy as wellas vibrational frequencies in the most stable configurations of ionpairs have been checked. [ABSTRACT FROM AUTHOR]

Like-charge ion pairing is commonly observed in protein structures and plays a significant role in biochemical processes. Density functional calculations combined with the conductor-like polarizable continuum model were employed to study the formation possibilities of doubly charged noncovalently linked complexes of a series of model compounds and amino acids in the gas phase and in solution. Hydrogen bond interactions were found to offset the Coulombic repulsion such that cation-cation clusters are minima on the potential energy surfaces and neither counterions nor solvent molecules are needed to hold them together. In the gas phase the dissociation energies are exothermic, and the separation barriers span from 1.7 to 15.6 kcal mol(-1). Liquid-phase computations indicate that the separation enthalpies of the cation-cation complexes become endothermic in water and nonpolar solvents with dielectric constants of ≥7 (i.e., the value for THF). These results reveal that electrostatically defying noncovalent complexes of like-charged ions can overcome their Coulombic repulsion even in low-polarity environments.

Understanding the nature of the interaction between metal nanoparticles and biomolecules has been important in the development and design of sensors. In this paper, structural, electronic, and bonding properties of the neutral and anionic forms of glutathione tripeptide (GSH) complexes with a Au(3) cluster were studied using the DFT-B3LYP with 6-31+G**-LANL2DZ mixed basis set. Binding of glutathione with the gold cluster is governed by two different kinds of interactions: Au-X (X = N, O, and S) anchoring bond and Au···H-X nonconventional hydrogen bonding. The influence of the intramolecular hydrogen bonding of glutathione on the interaction of this peptide with the gold cluster has been investigated. To gain insight on the role of intramolecular hydrogen bonding on Au-GSH interaction, we compared interaction energies of Au-GSH complexes with those of cystein and glycine components. Our results demonstrated that, in spite of the ability of cystein to form highly stable metal-sulfide interaction, complexation behavior of glutathione is governed by its intramolecular backbone hydrogen bonding. The quantum theory of atom in molecule (QTAIM) and natural bond orbital analysis (NBO) have also been applied to interpret the nature of interactions in Au-GSH complexes. Finally, conformational flexibility of glutathione during complexation with the Au(3) cluster was investigated by means of monitoring Ramachandran angles.