Some properties of the a+b → C reaction-diffusion system with initially separated components

We study some properties of the A+BC reaction-diffusion system with initially separated components, first analyzed by means of an asymptotic scaling argument by Gàlfi and Ràcz. We show that, in contrast to the asymptotic result that predicts that the rate of production of C goes liket ^–1, at early times it is shown to increase ast ^1/2. Deviations from this behavior appear at times inversely proportional to the reaction constant. Analogous crossover properties appear in the kinetic behavior of the reaction front. A second part of the study is concerned with the same chemical reaction on a fractal surface. When the substrate is a percolation cluster at criticality, both the maximum production rate and the width of the reaction zone differ considerably from those for the homogeneous space.     

Hydrophobicity Control in Adaptive Crystalline Assemblies

An amphiphile based on polyethylene glycol (PEG) polymer and two molecular moieties (perylene diimide and C₇ fluoroalkyl, PDI and C₇F) attached to its termini assembles into crystalline films with long‐range order. The films reversibly switch from crystalline to amorphous above the PEG melting temperature. The adaptive behavior stems from the responsiveness of the PEG domain and the robustness of the PDI and C₇F assemblies. The hydrophobicity of the film can be controlled by heating, resulting in switching from highly hydrophobic to superhydrophilic. The long‐range order, reversible crystallinity switching, and the temperature‐controlled wettability demonstrate the potential of block copolymer analogues based on simple polymeric/molecular hybrids.     

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

Iron cations are essential for the high activity of nickel and cobalt‐based (oxy)hydroxides for the oxygen evolution reaction, but the role of iron in the catalytic mechanism remains under active investigation. Operando X‐ray absorption spectroscopy and density functional theory calculations are used to demonstrate partial Fe oxidation and a shortening of the Fe?O bond length during oxygen evolution on Co(Fe)O_xH_y. Cobalt oxidation during oxygen evolution is only observed in the absence of iron. These results demonstrate a different mechanism for water oxidation in the presence and absence of iron and support the hypothesis that oxidized iron species are involved in water‐oxidation catalysis on Co(Fe)O_xH_y.     

Crystal packing mediates enantioselective ligand recognition at the peripheral site of acetylcholinesterase

Recently, alkylene-linked heterodimers of tacrine (1) and 5-amino-5,6,7,8-tetrahydroquinolinone (2, hupyridone) were shown to exhibit higher acetylcholinesterase (AChE) inhibition than either monomeric 1 or 2. Such inhibitors are potential drug candidates for ameliorating the cognitive decrements in early Alzheimer patients. In an attempt to understand the inhibition mechanism of one such dimer, (RS)-(+/-)-N-9-(1,2,3,4-tetrahydroacridinyl)-N'-5-[5,6,7,8-tetrahydro-2'(1'H)-quinolinonyl]-1,10-diaminodecane [(RS)-(+/-)-3] bisoxalate, the racemate was soaked in trigonal Torpedo californica AChE (TcAChE) crystals, and the X-ray structure of the resulting complex was solved to 2.30 A resolution. Its structure revealed the 1 unit bound to the "anionic" subsite of the active site, near the bottom of the active-site gorge, as seen for the 1/TcAChE complex. Interestingly, only the (R)-enantiomer of the 2 unit was seen in the peripheral "anionic" site (PAS) at the top of the gorge, and was hydrogen-bonded to the side chains of residues belonging to an adjacent, symmetry-related AChE molecule covering the gorge entrance. When the same racemate was soaked in orthorhombic crystals of TcAChE, in which the entrance to the gorge is more exposed, the crystal structure of the corresponding complex revealed no substantial enantiomeric selectivity. This observation suggests that the apparent enantiomeric selectivity of trigonal crystals of TcAChE for (R)-3 is mainly due to crystal packing, resulting in preferential binding of one enantiomeric inhibitor both to its "host" enzyme and to its neighbor in the asymmetric unit, rather than to steric constraints imposed by the geometry of the active-site gorge.     

One-electron oxidation and demetalation of metalloporphyrins and chlorophyll a in dichloroethane solutions as studied by pulse radiolysis

Formation of cation radicals by pulse radiolysis of metalloporphyrins and chlorophyll a in 1,2-dichloroethane is reported Demetalation of the metalloporphyrin by radiolytically produced HC1 is also observed. Rate constants for demetalation of ZnTPP and Chl a are 1 1 × 10⁸ and ≈ 3 × 10⁸ M^?1 S^?1. Oxidation of Chl a by ZnTPP⁺ has a rate constant of ≈ 4× 10⁹ M^?1S^?1.     

Magnetic field dependence of electron spin polarization generated through radical-triplet interactions

The magnetic field dependence of electron spin polarization (ESP), generated in free radicals when they encounter photoexcited triplets, was measured experimentally and analyzed theoretically. The time-resolved electron paramagnetic resonance measurements were performed with a microwave setup consisting of low-loss dielectric ring resonators with tunable microwave frequencies and the corresponding magnetic fields. The ESP of the radical was found in the magnetic field range of 170-370 mT, and the results of the calculation based on the numerical solution of the stochastic Liouville equation were found to be in line with the experimental data showing that ESP decreases when the magnetic field increases.     

Unification of the quality assurance systems of public health laboratories conformed to ISO 17025, ISO 15189, and ISO 9000: a major organizational change

The Department of Public Health Laboratories consists of five major laboratories located across the country of Israel: four environmental laboratories performing microbiological and chemical testing of food and water products [accredited according to International Organization for Standardization (ISO) 17025 since 1999) and a fifth laboratory that is dedicated to virology testing (certified according to ISO 9000 since 2003). Historically, each laboratory operated independently and developed its own quality assurance (QA). On November 2004, an important strategic decision was made: to unify all five laboratories’ QA systems conformed to ISO 17025, ISO 15189, and ISO 9000—a transition from five laboratories operating independently in the field of QA toward establishing a multisite laboratory. This process was considered and visualized as a major organizational change and therefore raised some resistance among both QA managers and the professional laboratories’ management. Thus, it was necessary to overcome the resistance and at the same time induce thoughts of ways of reshaping and formatting the new and uniform quality manual and uniform standard operating procedures (SOPs). In September 2005, the first phase of the process was completed, and all four environmental public health laboratories successfully passed a reaccreditation audit using a uniform QA manual guide and partially uniform SOPs. We shall share our experience and discuss the major contributions of this process to overall laboratory management. Presented at the 3rd International Conference on Metrology, November 2006, Tel Aviv, Israel.     

Novel water-soluble bacteriochlorophyll derivatives for vascular-targeted photodynamic therapy: synthesis, solubility, phototoxicity and the effect of serum proteins

New negatively charged water-soluble bacteriochlorophyll (Bchl) derivatives were developed in our laboratory for vascular-targeted photodynamic therapy (VTP). Here we focused on the synthesis, characterization and interaction of the new candidates with serum proteins and particularly on the effect of serum albumin on the photocytotoxicity of WST11, a representative compound of the new derivatives. Using several approaches, we found that aminolysis of the isocyclic ring with negatively charged residues markedly increases the hydrophilicity of the Bchl sensitizers, decreases their self-association constant and selectively increases their affinity to serum albumin, compared with other serum proteins. The photocytotoxicity of the new candidates in endothelial cell culture largely depends on the concentration of the serum albumin. Importantly, after incubation with physiological concentrations of serum albumin (500-600 microM), WST11 was found to be poorly photocytotoxic (>80% endothelial cell survival in cell cultures). However, in a recent publication (Mazor, O. et al. [2005] Photochem. Photobiol. 81, 342-351) we showed that VTP of M2R melanoma xenografts with a similar WST11 concentration resulted in approximately 100% tumor flattening and >70% cure rate. We therefore propose that the two studies collectively suggest that the antitumor activity of WST11 and probably of other similar candidates does not depend on direct photointoxication of individual endothelial cells but on the vascular tissue response to the VTP insult.