Metal-specific allosteric activation and deactivation of a diamine

The reaction of a potentially tetradentate bis(pyridyl-thiazole) ligand with acetone is allosterically activated upon complexation with Cd(II) but deactivated by reaction with Cu(I), demonstrating metal-specific allosteric controlled reactivity.     

Identification of a potential general acid/base in the reversible phosphoryl transfer reactions catalyzed by tyrosine recombinases: Flp H305

Flp provides a unique opportunity to apply the tools of chemical biology to phosphoryl transfer reactions. Flp and other tyrosine recombinases catalyze site-specific DNA rearrangements via a phosphotyrosine intermediate. Unlike most related enzymes, Flp's nucleophilic tyrosine derives from a different protomer than the remainder of its active site. Because the tyrosine can be supplied exogenously, nonnatural synthetic analogs can be used. Here we examine the catalytic role of Flp's conserved H305. DNA cleavage was studied using a peptide containing either tyrosine (pKa congruent with 10) or 3-fluoro-tyrosine (pKa congruent with 8.4). Religation was studied using DNA substrates with 3'-phospho-cresol (pKa congruent with 10) or 3'-para-nitro-phenol (pKa congruent with 7.1). In both cases, the tyrosine analog with the lower pKa specifically restored the activity of an H305 mutant. These results provide experimental evidence that this conserved histidine functions as a general acid/base catalyst in tyrosine recombinases.     

Comparison of dissolution methods for the determination of potassium in rocks and minerals by atomic absorption spectrometry

Lithium metaborate fusion and hydrofluoric acid dissolution methods used before the flame emission or atomic absorption spectrometric determination of potassium in rocks and minerals are compared with regard to accuracy and precision. Possible contamination by potassium in borosilicate glassware was avoided by the use of platinum or polyethylene vessels; solutions were prepared and measured by weight instead of volume. Separation of potassium by cation-exchange chromatography enabled potassium in all solutions of a given sample to be measured against one set of standard solutions. When the various dissolution methods were used carefully, the results did not differ in precision. Significant systematic errors were not detected; in particular, potassium was not lost by volatilization during fusion for 15 min with lithium metaborate at 9OO°C.     

Synthesis, characterisation and theoretical study of ruthenium 4,4'-bi-1,2,3-triazolyl complexes: fundamental switching of the nature of S1 and T1 states from MLCT to MC

The series of complexes [Ru(bpy)(3-n)(btz)(n)][PF(6)](2) (bpy = 2,2'-bipyridyl, btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, 2n = 1, 3n = 2, 4n = 3) have been prepared and characterised, and the photophysical and electronic effects imparted by the btz ligand were investigated. Complexes 2 and 3 exhibit MLCT absorption bands at 425 and 446 nm respectively showing a progressive blue-shift in the absorption on increasing the btz ligand content when compared to [Ru(bpy)(3)][Cl](2) (1). Complex 4 exhibits a heavily blue-shifted absorption spectrum with respect to those of 1-3, indicating that the LUMO of the latter are bpy-centred with little or no btz contribution whereas that of 4 is necessarily btz-centred. DFT calculations on analogous complexes 1'-4' (in which the benzyl substituents are replaced by methyl) show that the HOMO-LUMO gap increases by 0.3 eV from 1'-3' through destabilisation of the LUMO with respect to the HOMO. The HOMO-LUMO gap of 4' increases by 0.98 eV compared to that of 3' due to significant destabilisation of the LUMO. Examination of TDDFT data show that the S(1) states of 1'-3' are (1)MLCT in character whereas that of 4' is (1)MC. The optimisation of the T(1) state of 4' leads to the elongation of two mutually trans Ru-N bonds to yield [Ru(κ(2)-btz)(κ(1)-btz)(2)](2+), confirming the (3)MC character. Thus, replacement of bpy by btz leads to a fundamental change in the ordering of excited states such that the nature of the lowest energy excited state changes from MLCT in nature to MC.     

Determination of chlortetracycline residues in tissues using high-performance liquid chromatography with fluorescence detection

A method is described for the determination of chlortetracycline residues in tissue samples. The samples were extracted into a hydrochloric acid - glycine solution and the extracts concentrated and purified on cyclohexyl-bonded reversed-phase cartridges. Any chlortetracycline present was converted to iso-chlortetracycline at pH 12, which was then separated from interfering compounds on a reversed-phase polymeric column using high-performance liquid chromatography with fluorescence detection. The detection and determination limits of the assay were 20 and 50 ng g-1, respectively, making it suitable for statutory residue testing purposes.     

First direct evidence for a magnetic phase transition in the heavy-electron system UCd11

Muon-spin-rotation and relaxation measurements performed on polycrystalline UCd₁₁ give first direct evidence for an antiferromagnetic phase transition at 5 K. As far as probed by the positive muon the behaviour close to the phase transition is clearly contrasted to that in other magnetic heavy-electron materials investigated by μ⁺SR, e.g. U₂Zn₁₇ and UCu₅, because in UCd₁₁ longitudinal field measurements reveal dynamic relaxation effects which are attributed to the critical slowing down of the uranium 5f-moments. No signal could be observed in the ordered regime because of a large dipolar field spread.     

Mechanically reliable ceramics: Needs and opportunities to understand and control fracture

The approaches used to obtain mechanically reliable ceramics are first reviewed, with the focus on their brittle failure because this is the major problem. After outlining needs to improve these reliability approaches, more basic issues of (1) crack formation, detection, characterization and propagation; and (2) development of more reliable materials by developing tougher or new materials are addressed. While the focus is on bulk ceramics, some specific discussion of ceramic coatings is also presented. Needs and opportunities noted for understanding of cracks range from many of the specifics of their occurrence and behavior on both a continuum and especially microstructural scale to very fundamental issues such as fracture on an atomic scale and particle and electromagnetic emission during fracture. Similarly needs and opportunities for understanding to obtain more reliable materials range from specifics of a variety of toughening mechanisms to the development of material property estimation techniques so that unique new materials, e.g. high temperature, analogs of jade, can be more effectively sought.