The ars operon of the Escherichia coli plasmid R773 that confers arsenical and antimonial resistance is negatively regulated by the ArsR repressor. ArsR residues Cys-32 and Cys-34 were previously identified as involved in induction by arsenite and antimonite, suggesting coordination between As(III) and the two cysteine thiolates. However, in small molecule thiolate-As(III) complexes, arsenic is frequently three-coordinate. A site-directed mutagenic approach was employed in a search for a third arsenic ligand. ArsR proteins with C32G, C34G, and C32G/C34G substitutions were active repressors, but were not inducible in vivo. In vitro, the altered repressor-ars DNA complexes could not be dissociated by inducers. Alteration of Cys-37 and Ser-43, residues located in or near the putative helix-turn-helix DNA-binding region of the protein, had no effect on the inducibility of the operon. While these results indicated that neither the thiolate of Cys-37 nor the hydroxyl oxygen of Ser-43 is required for induction, they did not eliminate either atom as a potential arsenic ligand. Another approach involved reaction with an alternative inducer, phenylarsine oxide, which can form only two coordinations. Phenylarsine oxide was shown to be as effective as or more effective than arsenite or antimonite in induction in vivo. In vitro, the organic arsenical was more effective than either arsenite or antimonite in dissociating the repressor-promoter complex. Thus, two ArsR-arsenic bonds are sufficient for induction. The interaction of ArsR proteins with As(III) was examined using a phenylarsine oxide affinity resin. ArsR proteins containing any two of the three cysteine residues Cys-32, Cys-34, and Cys-37 bound to the resin. Alteration of any two of the three resulted in loss of binding. Arsenic X-ray absorption spectroscopy of ArsR treated stoichiometrically with arsenite confirmed the average arsenic coordination as AsS3 These results suggest that all three cysteine thiolates are arsenic ligands, but binding to only two, the Cys-32 and Cys-34 thiolates, is required to produce the conformational change that results in release of the repressor from the DNA and induction.

The a metallosubunit of carbon monoxide dehydrogenase from Clostridium thennoaceticum was isolated by subjecting native enzyme to low concentrations of the detergent sodium dodecyl sulfate, followed by anaerobic preparative native polyacrylamide gel electrophoresis. The isolated a subunit absorbs in the 400 nm region and contains one Ni and four Fe ions. The irons are organized into an [Fe4S4I2+'l+ cluster, the reduced form of which exhibits EPR features between g = 6 and 3, and a weak (0.1 spin/a) gav = 1.94 signal. The reduced cluster appears to exist in an S = 3/2 : S = '/2 spin-state mixture and to be predominantly S = 3/2 at 10 K. The Ni center is EPR silent and presumably Ni(II). X-ray absorption edge and EXAFS spectra reveal that the Ni center has a distorted square-planar geometry with two S donors at 2.19 A and two N/O donors at 1.89 A. Comparison of the Ni edge spectrum with those of structurally characterized Ni(II)N& model compounds suggests a DU distortion with a dihedral angle of about 20-30'. The Ni center does not appear to be incorporated into the cluster, and it may or may not be bridged to the cluster. These centers may be decompositional relatives of the A-cluster, the active site for acetyl-coenzyme A synthesis. JA950685G

Abstract:The optically generated collective electron density waves on metal-dielectric boundaries known as surface plasmons have been of great scientific interest since their discovery. Being electromagnetic waves on gold or silver nanoparticle's surface, localised surface plasmons (LSP) can strongly enhance the electromagnetic field. These strong electromagnetic fields near the metal surfaces have been used in various applications like surface enhanced spectroscopy (SES), plasmonic lithography, plasmonic trapping of particles, and plasmonic catalysis. Resonant coupling of LSPs to fluorophore can strongly enhance the emission intensity, the angular distribution, and the polarisation of the emitted radiation and even the speed of radiative decay, which is so-called plasmon enhanced fluorescence (PEF). As a result, more and more reports on surface-enhanced fluorescence have appeared, such as SPASER-s, plasmon assisted lasing, single molecule fluorescence measurements, surface plasmoncoupled emission (SPCE) in biological sensing, optical orbit designs etc. In this review, we focus on recent advanced reports on plasmon-enhanced fluorescence (PEF). First, the mechanism of PEF and early results of enhanced fluorescence observed by metal nanostructure will be introduced. Then, the enhanced substrates, including periodical and nonperiodical nanostructure, will be discussed and the most important factor of the spacer between molecule and surface and wavelength dependence on PEF is demonstrated. Finally, the recent progress of tipenhanced fluorescence and PEF from the rare-earth doped up-conversion (UC) and down-conversion (DC) nanoparticles (NPs) are also commented upon. This review provides an introduction to fundamentals of PEF, illustrates the current progress in the design of metallic nanostructures for efficient fluorescence signal amplification that utilises propagating and localised surface plasmons.

A highly efficient oxidation of cyclohexane to cyclohexanol and cyclohexanone is accomplished over calcined Au/ZSM-5 molecular sieve catalyst with oxygen as oxidant.