SITE-DIRECTED MUTAGENESIS OF THE LH2 LIGHT-HARVESTING COMPLEX OF Rhodobacter sphaeroides: CHANGING βLys23 TO Gin RESULTS IN A SHIFT IN THE 850 nm ABSORPTION PEAK

The present study describes the construction of a Rhodobacter sphaeroides light-harvesting (LH2) mutant in which the charged residue βSLys23 is changed by site-directed mutagenesis to a Gin residue, and the characterization of the resulting mutant complex by a range of spectroscopic techniques. In the 77 K absorption spectrum of the mutant, the peak equivalent to the 850 nm peak in the wild-type membrane is blue-shifted by approximately 18 nm to 837 nm; except for this blue-shift, the 77 K. fluorescence excitation and emission spectra and the circular dichroism spectrum of the mutant are very similar to the equivalent spectra from the wild-type membranes, suggesting that the mutation βLys23 → Gin probably does not cause any major changes in the conformation or aggregation state of these membranes. Possible causes of the 18 nm blue-shift in the absorption spectrum are discussed.     

Effect of methanol concentration on substrate conversion in alkylation of toluene on zeolite catalysts

Influence of the concentration of toluene and methanol on the substrate conversion in toluene methylation reaction was studied by a pulse method. Increased methanol partial pressure gave better conversion to xylenes whereas toluene partial pressure did not affect the reaction. This behavior has been interpreted by supposing that the first step in toluene methylation is chemisorption of methanol on zeolite.

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An animal model for extracorporeal photochemotherapy based on contact hypersensitivity.

Recently, photopheresis was introduced as a specific immune suppressor in several T cell mediated disorders. In order to study photopheresis, animal models are indispensable. This report describes an easy to handle model for this purpose. It concerns the Wistar-derived rat with contact hypersensitivity (CHS), also a T cell mediated disorder that has already been studied extensively in several other fields of research. After subsequent exposure to 8-methoxypsoralen (8-MOP) and ultraviolet A radiation (UVA), white blood cells from CHS rats were intravenously injected into other syngeneic rats suffering from the same disorder. This treatment appears to be very efficacious in suppressing the immunological response against the applied contact allergen, 2,4-dinitrofluorobenzene (DNFB). Cells subsequently exposed to UVA and 8-MOP did not have any effect.     

Characterization of phosphate conversion layers by combined Auger electron spectroscopy and X-ray microanalysis

Combined high-resolution AES and EDX studies have been performed of a number of industrial phosphate conversion layers on steel in order to determine the importance of the surface composition for the corrosion protection of the phosphated steel.

The well-known Cr(III) rinse of zinc-iron phosphate which improves its long-term paint adhesion performance was found to result in an exchange of Zn²⁺ with Cr³⁺ ions in the surface layers of the crystals. Corrosion improvement is caused by a slower rate of attack of the Cr-containing crystals due to the low solubility of CrP0₄ in aqueous media.

A commercial calcium-zinc-iron phosphate system was studied for corrosion resistance in relation to its conditions of formation. Phosphate layers with optimum corrosion resistance were found to have a strongly enriched iron content in the surface layers.     

Structures and properties of self-assembled monolayers of bistable [2]rotaxanes on Au (111) surfaces from molecular dynamics simulations validated with experiment

Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)) (denoted as the ring), between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along a dumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is the most stable and thus designated the ground-state co-conformer (GSCC), whereas the other co-conformation with the ring surrounding the DNP ring system is less favored and so designated the metastable-state co-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs) of a bistable [2]rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics (MD) studies with a force field optimized from DFT calculations and we report several experiments that validate the predictions. On the basis of both the total energy per rotaxane and the calculated stress that is parallel to the surface, we find that the optimal packing density of the SAM corresponds to a surface coverage of 115 A(2)/molecule (one molecule per 4 x 4 grid of surface Au atoms) for both the GSCC and MSCC, and that the former is more stable than the latter by 14 kcal/mol at the optimum packing density. We find that the SAM retains hexagonal packing, except for the case at twice the optimum packing density (65 A(2)/molecule, the 3 x 3 grid). For the GSCC and MSCC, investigated at the optimum coverage, the tilt of the ring with respect to the normal is theta = 39 degrees and 61 degrees, respectively, while the tilt angle of the entire rotaxane is psi = 41 degrees and 46 degrees , respectively. Although the tilt angle of the ring decreases with decreasing surface coverage, the tilt angle of the rotaxane has a maximum at 144 A(2)/molecule (the 4 x 5 grid/molecule) of 50 degrees and 51 degrees for the GSCC and MSCC, respectively. The hexafluorophosphate counterions (PF(6)(-)) stay localized around the ring during the 2 ns MD simulation. On the basis of the calculated density profile, we find that the thickness of the SAM is 40.5 A at the optimum coverage for the GSCC and 40.0 A for MSCC, and that the thicknesses become less with decreasing surface coverage. The calculated surface tension at the optimal packing density is 45 and 65 dyn/cm for the GSCC and MSCC, respectively. This difference suggests that the water contact angle for the GSCC is larger than for the MSCC, a prediction that is verified by experiments on Langmuir-Blodgett monolayers of amphiphilic [2]rotaxanes.     

Hierarchical assembly of helicate-type dinuclear titanium(IV) complexes

The ligands 4-7-H(2) were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li(4)[(4-7)(6)Ti(2)] and Li(6)[(4/5a)(6)Ga(2)]. The X-ray crystal structures of Li(4)[(4)(6)Ti(2)], Li(4)[(5b)(6)Ti(2)], and Li(4)[(7a)(6)Ti(2)] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated. The differences between the Ti and Ga complexes can be attributed to different electronic properties and a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths bring together eleven individual components into one well-defined aggregate.     

Exact mass measurement of product ions for the structural confirmation and identification of unknown compounds using a quadrupole time-of-flight spectrometer: a simplified approach using combined tandem mass spectrometric functions

This article describes a simple method to perform lock mass corrected accurate mass measurements in tandem mass spectrometry (MS/MS) with a quadrupole time-of-flight (Q-TOF) mass spectrometer. The experimental approach consists of using the protonated molecule of a known compound, which is measured in a MS/MS function using low collision energy (no fragmentation), as mass calibrator. The unknown compound is acquired in MS/MS mode albeit using high collision energy. After the acquisition, the two MS/MS spectra of unknown and mass calibrator are combined, and the fragments of the unknown are lock mass corrected by using the protonated molecule of the mass calibrator. To prove this concept, 10 compounds were analyzed using this approach, the fragments interpreted and, where possible, related to structural data available in the literature. All the unequivocally assigned fragments were accurately mass measured with mass errors within appropriate limits, i.e. for m/z values <200 with a mass tolerance of 3 mDa while for m/z > 200 the mass tolerance is expressed as 10 ppm.     

Azepines.I : The formation and reactivity of the anion of 2-diethylamino-5-phenyl-3H-azepine

Deprotonation of 2-diethylamino-5-phenyl-3$\text{H}$-azepine with strong bases affords a highly coloured anion, which reacts with deuterium oxide or with an appropriate electrophile to give an azepine bearing the new substituent at C-3.     

Hexacyanoferrate(III) as a mediator in the determination of total iron in potable waters as iron(II)-1,10-phenanthroline at a single-use screen-printed carbon sensor device

Hexacyanoferrate(III) was used as a mediator in the determination of total iron, as iron(II)-1,10-phenanthroline, at a screen-printed carbon sensor device. Pre-reduction of iron(III) at −0.2 V versus Ag/AgCl (1 M KCl) in the presence of hexacyanoferrate(II) and 1,10-phenanthroline (pH 3.5-4.5), to iron(II)-1,10-phenanthroline, was complete at the unmodified carbon electrode surface. Total iron was then determined voltammetrically by oxidation of the iron(II)-1,10-phenanthroline at +0.82 V, with a detection limit of 10 μg l⁻¹.In potable waters, iron is present in hydrolysed form, and it was found necessary to change the pH to 2.5-2.7 in order to reduce the iron(III) within 30 s. A voltammetric response was not found at lower pH values owing to the non-formation of the iron(II)-1,10-phenanthroline complex below pH 2.5.Attempts to incorporate all the relevant reagents (1,10-phenanthroline, potassium hexacyanoferrate(III), potassium hydrogen sulphate, sodium acetate, and potassium chloride) into a modifying coated PVA film were partially successful. The coated electrode behaved very satisfactorily with freshly-prepared iron(II) and iron(III) solutions but with hydrolysed iron, the iron(III) signal was only 85% that of iron(II).