Photoactivation analysis of impurities in zirconium oxide

The authors have developed an instrumental analytical method based on photoactivation with a 25 MeV betatron for the determination of F and K impurities in ZrO₂. The fluorine content is determined by irradiating the sample at 11.90 MeV and detecting the annihilation radiation with a NaI(Tl) scintillation counter. The potassium content is determined by an irradiation at 25 MeV followed by the measurement of the 2166.8 keV gamma ray with a Ge(Li) detector. The problem of interferences is thoroughly discussed and the minimum detectable masses are computed.     

Dynamic Ultralong Organic Phosphorescence by Photoactivation

Smart materials with ultralong phosphorescence are rarely investigated and reported. Herein we report on a series of molecules with unique dynamic ultralong organic phosphorescence (UOP) features, enabled by manipulating intermolecular interactions through UV light irradiation. Our experimental data reveal that prolonged irradiation of single‐component organic phosphors of PCzT, BCzT, and FCzT under ambient conditions can activate UOP with emission lifetimes spanning from 1.8 to 1330 ms. These phosphors can also be deactivated back to their original states with short‐lived phosphorescence by UV irradiation for 3 h at room temperature or through thermal treatment. Additionally, the dynamic UOP was applied successfully for a visual anti‐counterfeiting application. These findings may provide unique insight into dynamic molecular motion for optical processing and expand the scope of smart‐response materials for broader applications.     

Steric Effects Govern the Photoactivation of Phytochromes

Phytochromes constitute a superfamily of photoreceptor proteins existing in two forms that absorb red (Pr) and far‐red (Pfr) light. Although it is well‐known that the conversion of Pr into Pfr (the biologically active form) is triggered by a Z→E photoisomerization of the linear tetrapyrrole chromophore, direct evidence is scarce as to why this reaction always occurs at the methine bridge between pyrrole rings C and D. Here, we present hybrid quantum mechanics/molecular mechanics calculations based on a high‐resolution Pr crystal structure of Deinococcus radiodurans bacteriophytochrome to investigate the competition between all possible photoisomerizations at the three different (AB, BC and CD) methine bridges. The results demonstrate that steric interactions with the protein are a key discriminator between the different reaction channels. In particular, it is found that such interactions render photoisomerizations at the AB and BC bridges much less probable than photoisomerization at the CD bridge.     

Photoactivation analysis of ores using short-lived radionuclides

The results of applying photoactivation for express nondestructive multicomponent analysis of apatite-nepheline, gold-bearing, polymetal and lapparite ores are presented. The experiments were carried out at the IPP microtron with bremsstrahlung of 10 to 15 MeV electrons. A setup with mixed gamma-neutron field was built and used for elemental analysis. Experiments show that such elements as P, Al, Ti, Ba, and Ce can be determined with a sensitivity of approximately 0.1 mass% and a statistical error of a few percents. These values are wholly satisfactory to modern demands of industry. Gold and silver can be successfully determined in gold-bearing ores on the level of grams per ton.     

Photoactivation of fluorescence of rhodamine dyes in the presence of haloalkanes

Organic halogen derivatives, viz., hexachloroethane, 2,2,2-trichloroethanol, 2,2,2-tribromoethanol, butyl trichloroacetate, 2,2,2-trichloroethyl methacrylate, can efficiently activate fluorescence of lactone forms of rhodamine dyes by UV irradiation through the UFS-1 light filter in both solutions and polymer poly(methyl methacrylate) films. This ability is also typical of the copolymer of methyl methacrylate and 2,2,2-trichloroethyl methacrylate, which is promising for the development of detecting media for archive optical data storage with fluorescent readout.     

Photoactivation of the photosynthetic reaction center of Blastochloris viridis in the crystalline state

Photoactivation in crystals of the bacterial reaction center of Blastochloris viridis was investigated by near-infrared spectroscopy. The bleaching of the special pair absorption at 970 nm and the simultaneous rise of the special pair cation absorption at 1300 nm were measured in response to transient irradiation by a HeNe laser over 5 orders of magnitude in laser power. The resulting power-saturation curve can be used to estimate the true extent of photoactivation achieved in a prior time-resolved crystallographic experiment (Baxter et al. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5982-5987). The overall extent of photoactivation was 50%, which demonstrates that the time-resolved crystallographic method can be applied to the optically dense reaction center crystals. Measurement of the charge-recombination rate, however, suggests the presence of a long-lived P+ state within the crystal.     

Semiconducting Polymer Nanobiocatalysts for Photoactivation of Intracellular Redox Reactions

An organic semiconducting polymer nanobiocatalyst (SPNB) composed of a semiconducting polymer core conjugated with microsomal cytochrome P450 (CYP) has been developed for photoactivation of intracellular redox. The core serves as the light‐harvesting unit to initiate photoinduced electron transfer (PET) and facilitate the regeneration of dihydronicotinamide adenine dinucleotide phosphate (NADPH), while CYP is the catalytic center for intracellular redox. Under light irradiation, the semiconducting core can efficiently catalyze the generation of NADPH with a turnover frequency (TOF) 75 times higher than the reported nanosystems, ensuring the supply of the cofactor for intracellular redox. SPNB‐mediated intracellular redox thus can be efficiently activated by light in living cells to convert the model substrate and also to trigger the bioactivation of anticancer drugs. This study provides an organic nanobiocatalytic system that allows light to remotely control intracellular redox in living systems.     

Solid‐State NMR Spectroscopy to Probe Photoactivation in Canonical Phytochromes

The photoreceptor phytochrome switches photochromically between two thermally stable states called Pr and Pfr. Here, we summarize recent solid‐state magic‐angle spinning (MAS) NMR work on this conversion process and interpret the functional mechanism in terms of a nano‐machine. The process is initiated by a double‐bond photoisomerization of the open‐chain tetrapyrrole chromophore at the methine bridge connecting pyrrole rings C and D. The Pr‐state chromophore and its surrounding pocket in canonical cyanobacterial and plant phytochromes has significantly less order, tends to form isoforms and is soft. Conversely, Pfr shows significantly harder chromophore–protein interactions, a well‐defined protonic and charge distribution with a clear classical counterion for the positively charged tetrapyrrole system. The soft‐to‐hard/disorder‐to‐order transition involves the chromophore and its protein surroundings within a sphere of at least 5.5 Å. The relevance of this collective event for signaling is discussed. Measurement of the intermediates during the Pfr → Pr back‐reaction provides insight into the well‐adjusted mechanics of a two‐step transformation. As both Pr → Pfr and Pfr → Pr reaction pathways are different in ground and excited states, a photochemically controlled hyper‐landscape is proposed allowing for ratchet‐type reaction dynamics regulating signaling activity.     

Photoactivation of alkene oxidation by molecular oxygen in the presence of palladium

A catalytic cycle which leads to α,-β ethylenic ketones and methyl ketones from terminal alkenes in the presence of palladium trifluoroacetate,oxygen and U.V. light is described.     

Photoactivation of pheophorbide a induces a mitochondrial-mediated apoptosis in Jurkat leukaemia cells

The mechanism of cell death by pheophorbide a (Pba) which has been established to be a potential photosensitizer was examined in experimental photodynamic therapy (PDT) on Jurkat cells, a human lymphoid tumor cell line. In 30-60 min after irradiation, Pba treated cells exhibited apoptotic features including membrane blebbing and DNA fragmentation. Pba/PDT caused a rapid release of cytochrome c from mitochondria into the cytosol. Sequentially, activation of caspase-3 and the cleavage of poly ADP-ribose polymerase (PARP) were followed. Meanwhile, no evidence of activation of caspase-8 was indicated in the cells. In experiments with caspase inhibitors, it was found that caspase-3 alone was sufficient initiator for the Pba-induced apoptosis of the cells. Pba specific emission spectra were confirmed in the mitochondrial fraction and the light irradiation caused a rapid change in its membrane potential. Thus, mitochondria were entailed as the crucial targets for Pba as well as a responsible component for the cytochrome c release to initiate apoptotic pathways. Taken together, it was concluded that the mode of Jurkat cell death by Pba/PDT is an apoptosis, which is initiated by mitochondrial cytochrome c release and caspase-3-pathways.     

Photoactivation of aromatic molecules on the surface of magnesium oxide

The influence of ultraviolet radiation on the state of molecules of aromatic hydrocarbons adsorbed from the gaseous phase onto magnesium oxide has been studied from the electronic absorption spectra (in diffuse reflected light) and electron paramagnetic resonance. Depending on the temperature of the preliminary vacuum treatment of the polycrystalline magnesium oxide, at least three types of molecular photoproducts were observed: paramagnetic products (for 600 C), nonparamagnetic products (for 400 and 900 C), and products identified as singly-charged radical-anions, converted into negative dimers in an excess of the hydrocarbon. A bathochromic shift of the absorption band maxima of the radical-anions is observed in the series BeO, MgO, CaO, and BaO. The mechanism of the photoformation of molecular ions and the nature of the surface defects on magnesium oxide are discussed.This work was carried out under the direction of Academician A. N. Terenin.     

Photoactivation of dichloro(ethylenediamine)platinum(II)

Photolysis of dimethylsulfoxide (dmso) solutions of the compound Pt(en)Cl₂, where en=ethylene-1,2-diamine, leads to solvolysis of the complex and formation of Pt(en)(dmso)Cl⁺. The reaction follows clean pseudo-first-order kinetics with parallel photolytically activated and thermally activated paths. Both paths are first-order in both Pt(en)Cl₂ and solvent. Eyring analysis of the rate constants for 25 °C≤T≤55 °C yielded a Gibbs energy of activation of 96 kJ mol⁻¹ for the thermal pathway and no measurable activation barrier for the photochemical pathway. The quantum yield for the photochemical path is 0.22, as determined using ferrioxalate actinometry.     

Photoactivation of a substrate for cell adhesion under standard fluorescence microscopes

Cell-culturing substrates where cell adhesion can be switched on by external stimuli during cell cultivation are useful scaffolds for tissue engineering, cell-based drug screening, and fundamental cellular studies. Here, we show a new strategy for photoactivation of a substrate for cell adhesion under standard fluorescence microscopes. A glass substrate chemically modified with an alkylsiloxane having a photocleavable 2-nitrobenzyl group was coated with bovine serum albumin to prevent cell adhesion. Upon irradiation under a fluorescence microscope, the protein was replaced with fibronectin, which made the irradiated region cell-adhesive. Subsequent seeding of HEK293 or COS7 cells produced patterns corresponding to the irradiated patterns. We succeeded for the first time in positioning single cells in proximity to cultivating single cells. The present method provides a general strategy for positioning single cells of same or different types at any locations on the substrate and will be useful for studying cell-cell interactions.     

Photoactivation Analysis for Stoichiometry of REE Alloys, Magnetic and Superconducting Compounds

Nondestructive photoactivation analysis for stoichiometry of Nb-Ge and Zr-Nb alloys, diluted antiferromagnets Mn _x Zn_1-x F₂ and CsNi_1-x F₂ and CsNi_1-x Mg _x Br₃, and high-temperature superconductors LaSrCuO has been developed at a 30 MeV microtron. It is shown that it is possible to obtain an accuracy of elemental content determination on the level of 1% or even less by using the method of internal standards. In particular, the method has been applied for inspection of the quality of single crystals grown for physical research purposes.