Search

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]

In view of better understanding interactions of amino acids and peptides with metallic cations in the isolated state, the model system histidine–M2+ (M2+ = Mg2+, Ca2+, Zn2+) has been studied theoretically. The computations have been performed with the help of the density functional theory (DFT) and the B3LYP functional. The extended basis set was the standard 6-311++G**. All the molecular complexes obtained by the interaction between several energetically low-lying tautomers/conformers/zwitterions of histidine and the cations on different binding sites were considered. Our study shows that complexes of histidine with Mg2+, Ca2+, Zn2+ are rather similar. In the isolated state, the most stable form corresponds to a tridentate complex in which the cation interacts with oxygen and two nitrogen atoms: one from the terminal NH2 and one from the imidazole ring. All computations indicate that the metal ion affinity (MIA) decreases on going from Zn2+ to Mg2+ and Ca2+, for the considered amino acid. This indicates that histidine prefers to bind to the transition metal cation rather than alkali earth metals. The influence of theses cations on the acidity of histidine were also considered. As expected, upon metal complexation, proton dissociation of histidine becomes much more favorable, that is, its acidity becomes much less endothermic. [ABSTRACT FROM AUTHOR]

Abstract: Histidine as a natural amino acid is found to be biologically important and is known to function as a nucleophile or enzyme co-factor, or in proton transfer process. The properties of gaseous aromatic amino acid histidine depend on the structural forms it may take in gas-phase. Ab initio method has been used to characterize the gas-phase conformer/tautomers of histidine. Wide range of possible structures for histidine was surveyed at the MM level, and then the geometries of the unique conformers were refined at the B3LYP/6-311++G (d,p) levels. At this theoretical level, 25 conformers were located for both tautomers of histidine i.e., His [NπH] and His [NτH]. The MM level provides a poor description of the relative energies. Calculations at the B3LYP/6-311++G (d,p) level represent a significant improvement. Ab initio dipole moments are reported for all the conformers/tautomers. The results are compared to previous studies of amino acids and are analyzed in terms of intramolecular hydrogen-bonding interactions and Newman projections. In addition, we calculated the infrared frequencies and intensities of the most stable structures in order to assist in the assignment of hydrogen-bonding. Furthermore, Ramachandran backbone potential energy surfaces (PES) of 25 conformers of His [NπH] and His [NτH] tautomers of histidine were considered. The stationary points characterizing the potential energy profile of the various conformers/tautomers of histidine were investigated by the same density functional theory B3LYP/6-311++G (d,p). Minima and transition states characterizing the energetic paths for the interconversion of various structures of histidine were explored in detail. [ABSTRACT FROM AUTHOR]