MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS-II. pH DEPENDENCE OF ELECTRON ABSTRACTION RATE CONSTANT OF THE DYES IN THEIR TRIPLET STATE

Abstract— The pH dependence of the apparent reactivity of thiazine dyes in their triplet states has been studied in aqueous solutions, using as electron donor HY^-3, the trianionic species of ethylene diamine tetraacetic acid (EDTA), in the pH range 4–8. The pH dependence is found to be related to a change in the degree of protonation of the triplet excited dye. The apparent reactivity and lifetime of two differently protonated forms of thionine, azur B and methylene blue were determined by classical and dye-laser flash techniques, making it possible to evaluate the rate constant for electron abstraction of these molecules in their triplet states. It is found that: (a) protonation on the ring nitrogen increases the electron-abstraction rate constant of the triplet-state species about twenty-fold, and (b) methylation on the side amino groups decreases it.     

PHOTOSENSITIZATION BY ANTIMALARIAL DRUGS

Abstract The antimalarial drugs, chloroquine, hydroxychloroquine, quinine, quinacrine, amodiaquine and primaquine and the local anaesthetic, dibucaine, were tested for in vitro photosensitizing capability by irradiation with 365 nm UV light in aqueous solutions. The ability of these compounds to photosensitize the oxidation of 2,5-dimethylfuran, histidine, tryptophan or xanthine, and to initiate the free radical polymerization of acrylamide was examined in the pH range 2-12. Chloroquine and hydroxychloroquine show maximal photooxidative behaviour when in the monocation form at pH 9, in contrast to quinine which is extremely efficient as the dication below pH 4. This pattern appears to relate to the fluorescence yield as a function of pH. Chloroquine in the monocation or neutral form was found to undergo dechlorination upon irradiation, and this correlates directly with its ability to initiate photo-polymerization of acrylamide. Quinine also gives rise to small polymerization rates, attributed to photo-ionization in the quinoline ring, yielding a cation radical. Amodiaquine, primaquine and quinacrine do not have significant photochemical activity in aqueous solution. Dibucaine exhibits a strong photosensitizing capability at low pH, similar to quinine.     

Synthesis of time-optimal control of a second-order nonlinear process

The time-optimal control of a soft spring is discussed. The domain of controllability and the switching locus are described. It is shown that the switching locus changes in a discontinuous manner as the nonlinearity varies continuously.This research was partially supported by the Joint Services Electronics Program, Grant No. (4711) N00014-67-A-0226 ARPA 1129, and by NASA, Grant No. NGR 15-005-021.     

Photooxidation of lens proteins with xanthurenic acid: a putative chromophore for cataractogenesis.

The tryptophan metabolite, xanthurenic acid (Xan), is produced through a transamination reaction in high concentrations in human lenses with age and has been isolated from aged human cataractous lenses. It has appreciable absorption between 300 and 400 nm (lambda max = 334 nm), the range absorbed by the human lens. Our recent studies have shown that unlike most tryptophan metabolites in the eye, Xan is photochemically active, producing both superoxide and singlet oxygen. To determine if Xan could act as a photosensitizer and photooxidize cytosolic lens proteins, alpha-, beta- and gamma-crystallins were irradiated (lambda > 300 nm, 12 mW/cm2) in the presence and absence of Xan. Upon irradiation and in the presence of Xan, lens proteins polymerized in the order alpha > beta > gamma as assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Further analysis of the photolyzed alpha-crystallin by mass spectrometry indicated that histidine, tryptophan and methionine residues were oxidized at specific positions in a dose-dependent (irradiation time) manner. In alpha A-crystallin two forms of oxidized histidine 154 were observed, 2-imidazolone and 2-oxohistidine. Our results suggest that naturally occurring Xan is a chromophore capable of photosensitization and photooxidation of lens proteins. Furthermore, this compound could play a role in age-related cataractogenesis.     

Strong Photoluminescence Enhancement from Colloidal Quantum Dot Near Silver Nano-Island Films

We present the fabrication and optical investigation of highly random self-assembled, nano-scale films, probing their influence on the luminescence properties of near surface CdSe/ZnS colloidal quantum dots. When compared to quantum dots distributed on unstructured quartz substrates, the average luminescence intensity is found to be enhanced by a factor of 160×. The silver nanoparticles are prepared using slow thermal evaporation on quartz substrates and post-deposition annealing to produce a randomly-arranged layer of smooth nano-islands. Clear polarization dependent hot spots are observed. Such hot spots deliver a maximal enhancement of the emission intensity of 240× and have a spatial density of (0.050±0.002) μm^− 2. The results show that silver nano-island films strongly enhance the optical efficiency of near quantum dots emitters.     

The57Fe magnetic hyperfine field in Au-Fe alloys (evidence favoring the two Fe-environment model of Au-Fe alloys)

Conclusions The results of our analysis strongly support our two Fe environment hypothesis. The critical concentration at about 12 at.% suggests that the dependence of H(0,x) on x undergoes a fundamental change at this concentration. Furthermore, the so-called critical concentration at about 17 at.% apparently has little to do with any fundamental alloy property. Rather, it reflects the 12 at.% critical composition when properties are analyzed in terms of the average rather than the local Fe concentration. The two Fe environments appear to have differing configurations.This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.     

Laser Machined Plastic Laminates: Towards Portable Diagnostic Devices for Use in Low Resource Environments

Despite significant progress in development of bioanalytical devices cost, complexity, access to reagents and lack of infrastructure have prevented use of these technologies in resource‐limited regions. To provide a sustainable tool in the global effort to combat infectious diseases the diagnostic device must be low cost, simple to operate and read, robust, and have sensitivity and specificity comparable to laboratory analysis. In this mini‐review we describe recent work using laser machined plastic laminates to produce diagnostic devices that are capable of a wide variety of bioanalytical measurements and show great promise towards future use in low‐resource environments.