High T c superconductors as ionic metals and the role of phonons in high T c superconductivity

We have investigated a number of zone center and zone boundary (X-point) modes in La₂CuO₄ using the full potential Linearized Augmented Plane Wave (LAPW) method and find excellent agreement with experiment for phonon frequencies, showing that the local density approximation (LDA) is an excellent approximation for the total energies, charge density, and static density response in high T _c superconductors. Results are compared with those of a non-empirical ionic model for La₂CuO₄ and YBa₂Cu₃O₇. Self-consistent calculations show that the ionic nature of these materials leads to large changes in the potential with atomic displacements. The ionic contributions to the electron-phonon matrix elements, which were neglected in previous studies, are large and possibly sufficient to explain high T _c .     

Eine neue Methode zur Bestimmung von Essigs?ure in Acetaten

Ohne Zusammenfassung     

Muonium Chemistry at Diiron Subsite Analogues of [FeFe]‐Hydrogenase

The chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu^.), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu^. undergoes the same chemical reactions as H^. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu^. into three models of the [FeFe]‐hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems.     

Construction of self-dual normal bases and their complexity

Recent work of Pickett has given a construction of self-dual normal bases for extensions of finite fields, whenever they exist. In this article we present these results in an explicit and constructive manner and apply them, through computer search, to identify the lowest complexity of self-dual normal bases for extensions of low degree. Comparisons to similar searches amongst normal bases show that the lowest complexity is often achieved from a self-dual normal basis.     

Structural characterization of the zinc site in protein farnesyltransferase

X-ray absorption spectroscopy has been used to determine the structure of the Zn site in protein farnesyltransferase. Extended X-ray absorption fine structure (EXAFS) data are consistent with a Zn site that is ligated to three low-Z (oxygen or nitrogen) ligands and one cysteine sulfur, as predicted from the crystal structures that are available for farnesyltransferase. However, in contrast with the crystallographic results the EXAFS data do not show evidence for significant distortions in the Zn-ligand distances. The average Zn-(N/O) and Zn-S distances are 2.04 and 2.31 A, respectively. Addition of a farnesyl diphosphate analogue causes no detectable change in the structure of the Zn site. However, addition of peptide substrate causes a change in ligation from ZnS(N/O)(3) to ZnS(2)(N/O)(2), consistent with ligation of the C-terminal cysteine to the Zn. There is no significant change in Zn-ligand distances when a substrate binds, demonstrating that the Zn remains four-coordinate. Addition of both peptide and farnesyl diphosphate to give the product complex causes the Zn to return to ZnS(N/O)(3) ligation, indicating that the product thioether is not tightly coordinated to the Zn. These spectroscopic experiments provide insight into the catalytic mechanism of FTase.     

On carboxylate as a leaving group at the active site of Mo nitrogenase: electrochemical reactions of some MO and W carboxylates, formation of mono-, di- and tri-hydrides and the detection of an MoH2(N2) intermediate

A carboxylate ligand can function as a leaving group at a reduced molybdenum dihydride centre and this exposes a site at which dinitrogen or other small molecules can bind ; dihydrogen loss occurs after substrate binding ; protic attack intercepts carboxylate loss affording a trihydride intermediate which can partake in a hydrogen evolution cycle : it is suggested that carboxylate loss from the reduced Mo centre of nitrogenase might similarly lead to substrate binding and thence obligatory evolution of H₂.