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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]

B3LYP/aug-cc-pVDZ computations were carried out on polyamines with up to seven amino groups. The gas-phase proton affinities of these compounds are 219.6 (1-BuNH2), 238.6 (H2NCH2CH2CH2CH2NH2), 252.8 [(H2NCH2CH2CH2)2CHNH2], 261.3 [(H2NCH2CH2CH2)3CNH2 (1)], and 288.5 kcal mol-1 [(H2NCH2CH2CH(NH2)CH2CH2)3CNH2]. These results indicate that the tetraamine is near the top of the basicity scale and the heptaamine is more basic than any neutral organic compound which has been measured to date. A gas-phase equilibrium acidity determination between 1 and DBU also was carried out, and PA(1) = 256.2 ± 2.1 kcal mol-1 was obtained. This demonstrates that multiple intramolecular hydrogen bonds can greatly increase basicities, and represents a new motif for designing super bases. [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]