An EPR study of thermally and photochemically generated oxygen radicals on hydrated and dehydrated titania surfaces

The formation of a series of oxygen-centred radicals on different TiO₂ samples (P25 and two different rutile materials) under various conditions was investigated using X-band c.w. Electron Paramagnetic Resonance (EPR) spectroscopy. The radicals were formed either on thermally-reduced TiO₂, or by UV irradiation of the oxide under an oxygen atmosphere. The nature and stability of the radicals was also explored as a function of surface hydration. On thermally reduced TiO₂, containing surface and bulk Ti³⁺ centres, oxygen adsorption at 300 K results in the preferential formation and stabilisation of O₂ ^- anions on the P25 surface, but O^- and O₃ ^- anions are generated on the rutile surfaces. Superoxide anions (O^-) and trapped holes (O₂ ^-) were also identified after photo-irradiation of the thoroughly dehydrated TiO₂ samples under oxygen. The O^- anions were only visible at low temperatures under continuous irradiation, while the O₂ ^- anions were stable for days at 300 K. By comparison, on fully hydrated surfaces, no stable oxygen centred radicals could be detected on P25, while O₂ ^- anions were easily observed on the rutile surfaces. On partially hydrated P25, the O^-, O₂ ^- and HO₂ anions were detected after UV irradiation at 77 K; all radicals decayed upon warming to 298 K. On partially hydrated rutile, the O^- and O₂ ^- anions were detected and, unlike the case for P25, were found to be stable for days under the same conditions. The results illustrate the varied formation and stability of the oxygen centred radicals on TiO₂ surfaces depending on the pretreatment conditions.     

A computational study of the effectiveness of the frontier molecular orbital formalism in predicting conformational isomerism in (p-RC6H4NC)2W(dppe)2

Ab initio electronic structure calculations on a series of ligands, p-RC6H4NC:, indicate, that the energy of the LUMO correlates with the electron-withdrawing/donating capabilities of the substituent group, which determines the relative pi-acidity of the ligand. Depending on the nature of the para substituent group on the aryl isocyanide ligand, bis(aryl isocyanide) complexes of tungsten-containing bulky bidentate arylphosphine ligands adopt either cis or trans conformations. The frontier molecular orbital formalism predicts that strong pi-acids, which contain electron-withdrawing groups, tend to polarize sufficient charge density away from the metal center to effect the formation of the sterically less favorable but electronically stabilized cis conformer. Density functional theory calculations on similar complexes containing phosphines which do not impose severe steric contraints indicate that the balance between steric and electronic stabilization can be effectively predicted by comparing the relative energies of the ligand LUMOs.     

Investigation of aromatic-backbone amide interactions in the model peptide acetyl-Phe-Gly-Gly-N-methyl amide using molecular dynamics simulations and protein database search.

Weakly polar interactions between the side-chain aromatic rings and hydrogens of backbone amides (Ar-HN) are found in unique conformational regions. To characterize these conformational regions and to elucidate factors that determine the conformation of the Ar-HN interactions, four 4-ns molecular dynamics simulations were performed using four different low-energy conformations obtained from simulated annealing and one extended conformation of the model tripeptide Ac-Phe-Gly-Gly-NH-CH(3) as starting structures. The Ar(i)-HN(i+1) interactions were 4 times more frequent than were Ar(i)-HN(i+2) interactions. Half of the conformations with Ar(i)-HN(i+2) interactions also contained an Ar(i)-HN(i+1) interaction. The solvent access surface area of the Phe side chain and of the amide groups of Phe1, Gly2, and Gly3 involved in Ar-HN interactions was significantly smaller than in residues not involved in such interactions. The number of hydrogen bonds between the solvent and Phe1, Gly2, and Gly3 amide groups was also lower in conformations with Ar-HN interactions. For each trajectory, structures that contained Ar(i)-HN(i), Ar(i)-HN(i+1), and Ar(i)-HN(i+2) interactions were clustered on the basis of similarity of selected torsion angles. Attraction energies between the aromatic ring and the backbone amide in representative conformations of the clusters ranged from -1.98 to -9.24 kJ mol(-1) when an Ar-HN interaction was present. The most representative conformations from the largest clusters matched well with the conformations from the Protein Data Bank of Phe-Gly-Gly protein fragments containing Ar-HN interactions.