Electron Paramagnetic Resonance of High-Tc Superconducting Composites YBa2Cu3-xScxO6±δ

Microsolvation Effect, Hydrogen-Bonding Pattern, and Electron Affinity of the Uracil−Water Complexes U−(H2O)n (n = 1, 2, 3)

To achieve a systematic understanding of the influence of microsolvation on the electron accepting behaviors of nucleobases, the reliable theoretical method (B3LYP/DZP++) has been applied to a comprehensive conformational investigation on the uracil-water complexes U-(H(2)O)(n) (n = 1, 2, 3) in both neutral and anionic forms. For the neutral complexes, the conformers of hydration on the O2 of uracil are energetically favored. However, hydration on the O4 atom of uracil is more stable for the radical anions. The electron structure analysis for the H-bonding patterns reveal that the CH...OH(2) type H-bond exists only for di- and trihydrated uracil complexes in which a water dimer or trimer is involved. The electron density structure analysis and the atoms-in-molecules (AIM) analysis for U-(H(2)O)(n) suggest a threshold value of the bond critical point (BCP) density to justify the CH...OH(2) type H-bond; that is, CH...OH(2) could be considered to be a H-bond only when its BCP density value is equal to or larger than 0.010 au. The positive adiabatic electron affinity (AEA) and vertical detachment energy (VDE) values for the uracil-water complexes suggest that these hydrated uracil anions are stable. Moreover, the average AEA and VDE of U-(H(2)O)(n) increase as the number of the hydration waters increases.

Intermolecular Proton Transfer in Anionic Complexes of Uracil with Alcohols

A series of 18 alcohols (ROH) has been designed with an enthalpy of deprotonation in the gas phase (H(DP)) in the range 13.8-16.3 eV. The effects of excess electron attachment to the binary alcohol-uracil (ROH...U) complexes have been studied at the density functional level with a B3LYP exchange-correlation functional and at the second-order Møller-Plesset perturbation theory level. The photoelectron spectra of anionic complexes of uracil with 3 alcohols (ethanol, 2,2,3,3,3-pentafluoropropanol, and 1,1,1,3,3,3-hexafluoro-2-propanol) have been measured with 2.54 eV photons. For ROHs with deprotonation enthalpies larger than 14.8 eV, only the ROH...U- minimum exists on the potential energy surface of the anionic complex. For alcohols with deprotonation enthalpies in the range 14.3-14.8 eV, two minima might exist on the anionic potential energy surface, which correspond to the RO-...HU* and ROH...U- structures. For ROHs with deprotonation enthalpies smaller than 14.3 eV, the excess electron attachment to the ROH...U complex always induces a barrier-free proton transfer from the hydroxyl group of ROH to the O8 atom of U, with the product being RO-...HU*.

Interaction with Glycine Increases Stability of a Mutagenic Tautomer of Uracil. A Density Functional Theory Study

The most stable structures for the gas-phase complexes of minor tautomers of uracil (U) with glycine (G) were characterized at the density functional B3LYP/6-31++G level of theory. These are cyclic structures stabilized by two hydrogen bonds. The relative stability of isolated tautomers of uracil was rationalized by using thermodynamic and structural arguments. The stabilization energies for complexes between the tautomers of U and G result from interplay between the stabilizing two-body interaction energies and destabilizing one-body terms. The latter are related to the energies of (i) tautomerization of the unperturbed moieties and (ii) distortions of the resulting rare tautomers in the complex. The two-body term describes the interaction energy between distorted tautomers. The two-body interaction energy term correlates with perturbations of length of the proton-donor bonds as well as with deprotonation enthalpies and proton affinities of the appropriate monomer sites. It was demonstrated that the relative instability of rare tautomers of uracil is diminished due to their interactions with glycine. In particular, the instability of the third most stable tautomer (U(III)) is decreased from 11.9 kcal/mol for non-interacting uracil to 6.7 kcal/mol for uracil in a complex with the zwitterionic tautomer of glycine. A decrease of instability by 5.2 kcal/mol could result in an increase of concentration of U(III) by almost 5 orders of magnitude. This is the tautomer with proton donor and acceptor sites matching guanine rather than adenine. Moreover, kinetic characteristics obtained for the glycine-assisted conversion of the most stable tautomer of uracil (U(I)) to U(III) indicate that the U(I)<-->U(III) thermodynamic equilibrium could be easily attained at room temperature. The resulting concentration of this tautomer falls in a mutationally significant range.

Structural Criteria for the Rational Design of Selective Ligands: Convergent Hydrogen Bonding Sites for the Nitrate Anion

A large number of crystal structures are analyzed to characterize the structural aspects of hydrogen bonding interactions with the NO(3)(-) anion. Further insight is provided by the use of electronic structure calculations to determine stable geometries and interaction energies for NO(3)(-) complexes with several simple hydrogen bond donor groups, including water, methanol, N-methylform-amide, and methane. The results establish the existence of a clear set of structural criteria for the rational design of molecular receptors that complex the NO(3)(-) anion through hydrogen bonding interactions.