Selective enhancement of (6-4) photoproduct formation in dithymine dinucleotides driven by specific sugar puckering.

Four dinucleotide analogs of thymidylyl(3'-5')thymidine (TpT) have been designed and synthesized with a view to increase the selectivity, with respect to CPD, of efficient UV-induced (6-4) photoproduct formation. The deoxyribose residues of these analogs have been modified to increase north and south conformer populations at 5'- and 3'-ends, respectively. Dinucleotides whose 5'-end north population exceeds ca. 60% and whose 3'-end population is almost completely south display a three-fold selective enhancement in (6-4) adduct production when exposed to UV radiation, compared to TpT. These experimental results undoubtedly provide robust foundations for studying the singular ground-state proreactive species involved in the (6-4) photoproduct formation mechanism.

Photocobilins integrate B12 and bilin photochemistry for enzyme control.

Photoreceptor proteins utilise chromophores to sense light and trigger a biological response. The discovery that adenosylcobalamin (or coenzyme B12) can act as a light-sensing chromophore heralded a new field of B12-photobiology. Although microbial genome analysis indicates that photoactive B12-binding domains form part of more complex protein architectures, regulating a range of molecular-cellular functions in response to light, experimental evidence is lacking. Here we identify and characterise a sub-family of multi-centre photoreceptors, termed photocobilins, that use B12 and biliverdin (BV) to sense light across the visible spectrum. Crystal structures reveal close juxtaposition of the B12 and BV chromophores, an arrangement that facilitates optical coupling. Light-triggered conversion of the B12 affects quaternary structure, in turn leading to light-activation of associated enzyme domains. The apparent widespread nature of photocobilins implies involvement in light regulation of a wider array of biochemical processes, and thus expands the scope for B12 photobiology. Their characterisation provides inspiration for the design of broad-spectrum optogenetic tools and next generation bio-photocatalysts.

Crystal structure reveals the binding mode and selectivity of a photoswitchable ligand for the adenosine A2A receptor.

Rational synthetic expansion of photoresponsive ligands is important for photopharmacological studies. Adenosine A2A receptor (A2AR) is stimulated by adenosine and related in Parkinson's disease and other diseases. Here, we report the crystal structure of the A2AR in complex with the novel photoresponsive ligand photoNECA (blue) at 3.34 Å resolution. PhotoNECA (blue) was designed for this structural study and the cell-based assay showed a photoresponsive and receptor selective characteristics of photoNECA (blue) for A2AR. The crystal structure explains the binding mode, photoresponsive mechanism and receptor selectivity of photoNECA (blue). Our study would promote not only the rational design of photoresponsive ligands but also dynamic structural studies of A2AR.

Rolling circle amplification assisted CRISPR/Cas12a dual-cleavage photoelectrochemical biosensor for highly sensitive detection of miRNA-21.

BACKGROUND: MicroRNA-21 has been determined to be the only microRNA overexpressed in 11 types of solid tumors, making it an excellent candidate as a biomarker for disease diagnosis and therapy. Photoelectrochemical (PEC) biosensors have been widely used for quantification of microRNA-21. However, most PEC biosensing processes still suffer from some problems, such as the difficulty of avoiding the influence of interferents in complex matrices and the false-positive signals. There is a pressing need for establishing a sensitive and stable PEC method to detect microRNA-21. RESULTS: Herein, a nicking endonuclease-mediated rolling circle amplification (RCA)-assisted CRISPR/Cas12a PEC biosensor was fabricated for ultrasensitive detection of microRNA-21. The p-p type heterojunction PbS QDs/Co3O4 polyhedra were prepared as the quencher, thus the initial PEC signal attained the "off" state. Furthermore, the target was specifically identified and amplified by the RCA process. Then, its product single-stranded DNA S1 activated the cis- and trans-cleavage abilities of CRISPR/Cas12a, leading to almost all of the PbS QDs/Co3O4 polyhedra to leave the electrode surface, the p-n semiconductor quenching effect to be disrupted, and the signal achieving the "super-on" state. This pattern of PEC signal changed from "off" to "on" eliminated the interference of false-positive signals. The proposed PEC biosensor presented a satisfactory linear relationship ranging from 1 fM to 10 nM with a detection limit of 0.76 fM (3 Sb/N). SIGNIFICANCE AND NOVELTY: With innovatively synthesized PbS QDs/Co3O4 polyhedra as the effective quencher for PEC signal, the CRISPR/Cas12a dual-cleavage PEC biosensor possessed excellent selectivity, stability and repeatability. Furthermore, the detection of various miRNAs can be realized by changing the relevant base sequences in the constructed PEC biosensor. It also provides a powerful strategy for early clinical diagnosis and biomedical research.

Dysphania ambrosioides (L.) Mosyakin and Clemants: bridging traditional knowledge, photochemistry, preclinical investigations, and toxicological validation for health benefits.

Dysphania ambrosioides L. (Chenopodiaceae) is a Moroccan medicinal plant known locally as "M'Khinza." It is widely used in traditional medicine to treat numerous ailments, such as diabetes, digestive disorders, fever, fertility problems, immune disorders, hypertension, bronchitis, respiratory conditions, pharyngitis, cough, and flu. As part of this review, comprehensive preclinical investigations, including in vitro, in vivo, and in silico studies, were conducted to better understand the mechanisms of action of D. ambrosioides. Additionally, the phytochemical profile of the plant was examined, highlighting the presence of certain bioactive secondary metabolites. The information was gathered from electronic data sources such as Web of Science, PubMed, Science Direct, Scopus, Springer Link, and Google Scholars. Numerous studies have mentioned the pharmacological properties of D. ambrosioides, including its antioxidant, anti-inflammatory, antiparasitic, antiviral, antibacterial, and antifungal activities. Furthermore, research has also suggested its potential as an anticancer, antidiabetic, and vasorelaxant agent. Phytochemical characterization of D. ambrosioides has revealed the presence of over 96 major bioactive compounds, including terpenoids, polyphenols, flavonoids, alkaloids, and fatty acids. As for the toxicity of this plant, it is dose-dependent. Furthermore, more in-depth pharmacological studies are needed to establish the mechanisms of action of this plant more accurately before considering clinical trials. In conclusion, this review highlights the traditional use of D. ambrosioides in Moroccan medicine and emphasizes its potential pharmacological properties. However, to fully harness its therapeutic potential, further research, both in terms of chemistry and pharmacology, is necessary. These future studies could help identify new active compounds and provide a better understanding of the mechanisms of action of this plant, thus opening new prospects for its pharmaceutical application.

Primary processes in photophysics and photochemistry of a potential light-activated anti-cancer dirhodium complex.

Photophysics and photochemistry of a potential light-activated cytotoxic dirhodium complex [Rh2(µ-O2CCH3)2(bpy)(dppz)](O2CCH3)2, where bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine (Complex 1 or Rh2) in aqueous solutions was studied by means of stationary photolysis and time-resolved methods in time range from hundreds of femtoseconds to microseconds. According to the literature, Complex 1 demonstrates both oxygen-dependent (due to singlet oxygen formation) and oxygen-independent cytotoxicity. Photoexchange of an acetate ligand to a water molecule was the only observed photochemical reaction, which rate was increased by oxygen removal from solutions. Photoexcitation of Complex 1 results in the formation of the lowest triplet electronic excited state, which lifetime is less than 10 ns. This time is too short for diffusion-controlled quenching of the triplet state by dissolved oxygen resulting in 1O2 formation. We proposed that singlet oxygen is produced by photoexcitation of weakly bound van der Waals complexes [Rh2…O2], which are formed in solutions. If this is true, no oxygen-independent light-induced cytotoxicity of Complex 1 exists. Residual cytotoxicity deaerated solutions are caused by the remaining [Rh2…O2] complexes.

Visible-Light Mediated Deoxygenation of Carboxylic Acid for Late-Stage Peptide Modification Targeting Dehydroalanine.

A visible-light mediated deoxygenative radical addition of carboxylic acids to dehydroalanines has been disclosed. The method can be used in ß-acyl alanine derivative synthesis, including those chiral and deuterated variants, and late-stage peptide modification with various functional groups, both in the homogeneous phase and on the resin in SPPS. It provides a new tool kit for rapid construction of bioactive peptide analogues, which has been demonstrated by modification of the antimicrobial peptide Feleucin-K3.

Rapid Release of Halocarbons from Saline Water by Iron-Based Photochemistry.

Methyl halides play important roles in stratospheric ozone depletion, but their formation mechanisms are not well defined. This study demonstrated that iron-based photochemistry significantly enhanced alkyl halide production by promoting the reaction of the representative monomer of lignin with halide ions in saline water under solar light irradiation. The methyl chloride (CH3Cl) emission from the light/Fe(III) process was 2 orders of magnitude higher than dark treatment and in the absence of iron. In addition, bromide and iodide showed better reactivity in the formation of the corresponding methyl bromide (CH3Br) and methyl iodide (CH3I). Alkyl halides identified from seawater, brackish water, and salt pan water under sunlight irradiation were positively correlated with the Fe(III) concentrations, indicating that iron-based photochemistry is ubiquitous. This work suggested that the photoinduced formation of methyl radical and redox cycling of iron triggered by the Fenton-like reaction are responsible for the enhanced release of alkyl halides. This study represents an abiotic formation pathway of alkyl halides, which accounts for a portion of the unidentified sources of halocarbons in the ocean.

Photochemistry of oil in marine systems: developments since the Deepwater Horizon spill.

Oil spills represent a major source of negative environmental impacts in marine systems. Despite many decades of research on oil spill behavior, photochemistry was neglected as a major factor in the fate of oil spilled in marine systems. Subsequent to the Deepwater Horizon oil spill, numerous studies using varied approaches have demonstrated the importance of photochemistry, including short-term impacts (hours to days) that were previously unrecognized. These studies have demonstrated the importance of photochemistry in the overall oil transformation after a spill and more specifically the impacts on emulsification, oxygenation, and microbial interactions. In addition to new perspectives, advances in analytical approaches have allowed an improved understanding of oil photochemistry after maritime spill. Although the literature on the Deepwater Horizon spill is extensive, this review focuses only on studies relevant to the advances in oil photochemistry understanding since the Deepwater Horizon spill.

Cell-surface photochemistry mediated calcium overload for synergistic tumor therapy.

Calcium (Ca2+) is essential for mitochondrial homeostasis and function coordination, particularly in cancer cells that metabolize frequently to sustain their growth. Photochemistry mediated calcium overload has attracted lots of attention as an effective way to achieve tumor suppression. Herein, we developed a photonanomedicine to synergistically induce calcium overload via cell-surface photochemistry and thus tumor suppression. Specifically, the photosensitizer, protoporphyrin IX (PpIX) was loaded onto upconversion nanoparticles (UCNP), which was subsequently modified by a polymer bearing photo-crosslinking cinnamate (CA) groups. The resulting nanoparticle was further functionalized by anti-CD20 aptamers (Apt), to give photonanomedicine. The interaction between CD20 receptors and anti-CD20 aptamers allowed photonanomedicine to accurately attach onto the Raji cell surface after an intravenous injection. Following the local application of a 980 nm NIR laser, the photonanomedicine was able to capture the NIR light and convert it into ultraviolet (UV) light. On one hand, the converted UV light led the crosslinking of cinnamate groups in photonanomedicine, further stimulating the clustering of CD20 receptors and causing Ca2+ influx. On the other hand, the UV light could simultaneously excited PpIX to generate reactive oxygen species (ROS) in situ to break down the integrity of cell membrane and lead to an influx of Ca2+. The synergistic Ca2+ overload mediated by photonanomedicine exhibited an enhanced and superior anti-tumor efficacy. We believe this photonanomedicine expands the toolbox to manipulate intracellular Ca2+ concentration and holds a great potential as an anti-tumor therapy.

Fabrication of near infrared light responsive photoelectrochemical immunosensor for in vivo detection of melanoma cells.

Effective and convenient detection of melanoma cells with high sensitivity is essential to identify malignant melanoma in its early stage. However, the existing detection methods, such as immunohistochemical analysis, are too complicated and time-consuming to realize the convenient in vivo and in situ detection. Herein, a near infrared responsive photoelectrochemical (PEC) immunosensor is proposed with plasmonic Au nanoparticles-photonic TiO2 nanocaves (Au/TiO2 NCs) as photon harvest and conversion transducer and antibody as cell recognition unit. The micro-antibody/Au/TiO2 NCs photoelectrode can easily in vivo distinguish melanoma cells and can realize sensitive detection of melanoma cells in short time of 1 min with a lowest limit of detection of 2 cell mL-1. The PEC immunosensor strategy not only allows us to pioneeringly implement sensitive in vivo bio-detection, but also opens up a new avenue for rational design of cell recognition units and micro-electrode for universal and reliable bio-detections.

Polyamine, 1,3-diaminopropane, regulates defence responses on growth, gas exchange, PSII photochemistry and antioxidant system in wheat under arsenic toxicity.

The metalloid arsenic (As) is extremely hazardous to all living organisms, including plants. Pollution with As is very detrimental to the photosynthetic machinery, cell division, energy generation, and redox status. In order to cope with stress, the use of growth regulators such as polyamines (PA), which strengthen the antioxidant system of plants, has become widespread in recent years. PAs can modulate the plant growth through basic mechanisms common to all living organisms, such as membrane stabilization, free radical scavenging, deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and protein synthesis, enzyme activities and second messengers. However, the effect of 1,3- diaminopropane (Dap), which is a product of PA catabolism, is not clear enough in plants exposed to As toxicity. In the current study, the different concentrations of 1,3-diaminopropane (0.1, 0.5 and 1 mM Dap) were hydroponically treated to wheat (Triticum aestivum) under arsenic stress (100 µM As) and then relative growth rate (RGR), relative water content (RWC), proline content (Pro), gas exchange parameters, PSII photochemistry, chlorophyll fluorescence kinetics, antioxidant activity and lipid peroxidation were assessed. RGR, RWC, osmotic potential and Pro content decreased in As-applied plants. The inhibition of these parameters could be reversed by Dap treatments. Besides, Dap applications mitigated the As toxicity-induced suppression on chlorophyll fluorescence (Fv/Fm, Fv/Fo and Fo/Fm) and the performance of PSII photochemistry. As impaired the balance on antioxidant capacity by decreased activities of catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and the contents of ascorbate (AsA) and glutathione (GSH) and then lipid peroxidation (TBARS content) increased. In the presence of Dap under As stress, the plants exhibited an increase in superoxide dismutase (SOD), POX, and GPX. Dap treatments contributed to the maintenance of cellular redox state (AsA/DHA and GSH/GSSG) by regulating the activities/contents of enzyme/non-enzyme involved in the AsA-GSH cycle. After Dap applications against stress, ROS accumulation (H2O2 content) and lipid peroxidation (TBARS) were effectively reduced. The findings showed that by eliminating As-induced oxidative damage and protecting the biochemical processes of photosynthesis, Dap treatments have a substantial potential to give resistance to wheat.

Photo-Uncaging by C(sp3)-C(sp3) Bond Cleavage Restores ß-Lapachone Activity.

ß-Lapachone is an ortho-naphthoquinone natural product with significant antiproliferative activity but suffers from adverse systemic toxicity. The use of photoremovable protecting groups to covalently inactivate a substrate and then enable controllable release with light in a spatiotemporal manner is an attractive prodrug strategy to limit toxicity. However, visible light-activatable photocages are nearly exclusively enabled by linkages to nucleophilic functional sites such as alcohols, amines, thiols, phosphates, and sulfonates. Herein, we report covalent inactivation of the electrophilic quinone moiety of ß-lapachone via a C(sp3)-C(sp3) bond to a coumarin photocage. In contrast to ß-lapachone, the designed prodrug remained intact in human whole blood and did not induce methemoglobinemia in the dark. Under light activation, the C-C bond cleaves to release the active quinone, recovering its biological activity when evaluated against the enzyme NQO1 and human cancer cells. Investigations into this report of a C(sp3)-C(sp3) photoinduced bond cleavage suggest a nontraditional, radical-based mechanism of release beginning with an initial charge-transfer excited state. Additionally, caging and release of the isomeric para-quinone, α-lapachone, are demonstrated. As such, we describe a photocaging strategy for the pair of quinones and report a unique light-induced cleavage of a C-C bond. We envision that this photocage strategy can be extended to quinones beyond ß- and α-lapachone, thus expanding the chemical toolbox of photocaged compounds.

Proteomic analysis reveals mechanisms underlying increased efficacy of bleomycin by photochemical internalization in bladder cancer cells.

Photochemical internalization (PCI) is a promising new technology for site-specific drug delivery, developed from photodynamic therapy (PDT). In PCI, light-induced activation of a photosensitizer trapped inside endosomes together with e.g. chemotherapeutics, nucleic acids or immunotoxins, allows cytosolic delivery and enhanced local therapeutic effect. Here we have evaluated the photosensitizer meso-tetraphenyl chlorine disulphonate (TPCS2a/fimaporfin) in a proteome analysis of AY-27 rat bladder cancer cells in combination with the chemotherapeutic drug bleomycin (BML). We find that BLMPCI attenuates oxidative stress responses induced by BLM alone, while concomitantly increasing transcriptional repression and DNA damage responses. BLMPCI also mediates downregulation of bleomycin hydrolase (Blmh), which is responsible for cellular degradation of BLM, as well as several factors known to be involved in fibrotic responses. PCI-mediated delivery might thus allow reduced dosage of BLM and alleviate unwanted side effects from treatment, including pulmonary fibrosis.

Selective Photodynamic Activity of Tetrakis(4-aminophenyl)porphyrins with and without Acetyl Protecting Groups on Cancer and Normal Cells.

Tetrakis(4-aminophenyl)porphyrin (1) and tetrakis(4-acetamidophenyl)porphyrin (2) were dissolved in water with the incorporation of a polysaccharide (λ-carrageenan (CGN)) as a water-solubilizing agent. Although the photodynamic activity of the CGN-2 complex was considerably lower than that of the CGN-1 complex, the selectivity index (SI; IC50 in a normal cell/IC50 in a cancer cell) of the CGN-2 complex was considerably higher than that of the CGN-1 complex. This is because the photodynamic activity of the CGN-2 complex was significantly affected by the intracellular uptakes by the normal and cancer cells. During in vivo experiments, the CGN-2 complex inhibited tumor growth under light irradiation with high blood retention compared with the CGN-1 complex and Photofrin, which exhibited lower blood retention. This study showed that the photodynamic activity and SI are influenced by substituent groups of arene in the meso-positions of porphyrin analogs.

Rapid Redox Cycling of Fe(II)/Fe(III) in Microdroplets during Iron-Citric Acid Photochemistry.

Fe(III) and carboxylic acids are common compositions in atmospheric microdroplet systems like clouds, fogs, and aerosols. Although photochemical processes of Fe(III)-carboxylate complexes have been extensively studied in bulk aqueous solution, relevant information on the dynamic microdroplet system, which may be largely different from the bulk phase, is rare. With the help of the custom-made ultrasonic-based dynamic microdroplet photochemical system, this study examines the photochemical process of Fe(III)-citric acid complexes in microdroplets for the first time. We find that when the degradation extent of citric acid is similar between the microdroplet system and the bulk solution, the significantly lower Fe(II) ratio is present in microdroplet samples due to the rapider reoxidation of photogenerated Fe(II). However, by replacing citric acid with benzoic acid, no much difference in the Fe(II) ratio between microdroplets and bulk solution is observed, which indicates distinct reoxidation pathways of Fe(II). Moreover, the presence of •OH scavenger, namely, methanol, greatly accelerates the reoxidation of photogenerated Fe(II) in both citric acid and benzoic acid situations. Further experiments reveal that the high availability of O2 and the citric acid- or methanol-derived carbon-centered radicals are responsible for the rapider reoxidation of Fe(II) in iron-citric acid microdroplets by prolonging the length of HO2•- and H2O2-involved radical reaction chains. The results in this study may provide a new understanding about iron-citric acid photochemistry in atmospheric liquid particles, which can further influence the photoactivity of particles and the formation of secondary organic aerosols.

The molecular mechanism of light-induced bond formation and breakage in the cyanobacteriochrome TePixJ.

Cyanobacteriochromes (CBCRs) are small and versatile photoreceptor proteins with high potential for biotechnological applications. Among them, the so-called DXCF-CBCRs exhibit an intricate secondary photochemistry: miliseconds after activation with light, a covalent linkage between a conserved cysteine residue and the light-absorbing tetrapyrrole chromophore is reversibly formed or broken. We employed time-resolved IR spectroscopy over ten orders of magnitude in time in conjunction with 2D-IR spectroscopy to investigate the molecular mechanism of this intriguing reaction in the DXCF-CBCR model system TePixJ from T. elongatus. The crosspeak pattern in the 2D-IR spectrum facilitated the assignment of the dominant signals to vibrational modes of the chromophore, which in turn enabled us to construct a mechanistic model for the photocycle reactions from the time-resolved IR spectra. Here, we assigned the time-resolved signals to several proton transfer steps and distinct geometric changes of the chromophore. We propose a model that describes how these events lead to the rearrangement of charges in the chromophore binding pocket, which serves as the trigger for the light-induced bond formation and breakage with the nearby cysteine.

Chemiexcitation: Mammalian Photochemistry in the Dark†.

Light is one way to excite an electron in biology. Another is chemiexcitation, birthing a reaction product in an electronically excited state rather than exciting from the ground state. Chemiexcited molecules, as in bioluminescence, can release more energy than ATP. Excited states also allow bond rearrangements forbidden in ground states. Molecules with low-lying unoccupied orbitals, abundant in biology, are particularly susceptible. In mammals, chemiexcitation was discovered to transfer energy from excited melanin, neurotransmitters, or hormones to DNA, creating the lethal and carcinogenic cyclobutane pyrimidine dimer. That process was initiated by nitric oxide and superoxide, radicals triggered by ultraviolet light or inflammation. Several poorly understood chronic diseases share two properties: inflammation generates those radicals across the tissue, and cells that die are those containing melanin or neuromelanin. Chemiexcitation may therefore be a pathogenic event in noise- and drug-induced deafness, Parkinson's disease, and Alzheimer's; it may prevent macular degeneration early in life but turn pathogenic later. Beneficial evolutionary selection for excitable biomolecules may thus have conferred an Achilles heel. This review of recent findings on chemiexcitation in mammalian cells also describes the underlying physics, biochemistry, and potential pathogenesis, with the goal of making this interdisciplinary phenomenon accessible to researchers within each field.

Hormetic activation of nano-sized rare earth element terbium on growth, PSII photochemistry, antioxidant status and phytohormone regulation in Lemnaminor.

Soils contaminated with rare earth elements (REEs) can damage agriculture by causing physiological disorders in plants which are evaluated as the main connection of the human food chain. A biphasic dose response with excitatory responses to low concentrations and inhibitory/harmful responses to high concentrations has been defined as hormesis. However, not much is clear about the ecological effects and potential risks of REEs to plants. For this purpose, here we showed the impacts of different concentrations of nano terbium (Tb) applications (5-10-25-50-100-250-500 mg L-1) on the accumulation of endogeneous certain ions and hormones, chlorophyll fluoresence, photochemical reaction capacity and antioxidant activity in duckweed (Lemna minor). Tb concentrations less than 100 mg L-1 increased the contents of nitrogen (N), phosphate (P), potassium (K+), calcium (Ca2+), magnesium (Mg2+), manganese (Mn2+) and iron (Fe2+). Chlorophyll fluorescence (Fv/Fm and Fv/Fo) was suppressed under 250-500 mg L-1 Tb. In addition, Tb toxicity affected the trapped energy adversely by the active reaction center of photosystem II (PSII) and led to accumulation of inactive reaction centers, thus lowering the detected level of electron transport from photosystem II (PSII) to photosystem I (PSI). On the other hand, 5-100 mg L-1 Tb enhanced the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), NADPH oxidase (NOX), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione S-transferase (GST). Tb (5-50 mg L-1) supported the maintenance of cellular redox status by promoting antioxidant pathways involved in the ascorbate-glutathione (AsA-GSH) cycle. In addition to the antioxidant system, the contents of some hormones such as indole-3-acetic acid (IAA), gibberellic acid (GA), cytokinin (CK) and salicylic acid (SA) were also induced in the presence of 5-100 mg L-1 Tb. In addition, the levels of hydrogen peroxide (H2O2) and lipid peroxidation (TBARS) were controlled through ascorbate (AsA) regeneration and effective hormonal modulation in L. minor. However, this induction in the antioxidant system and phytohormone contents could not be resumed after applications higher than 250 mg L-1 Tb. TBARS and H2O2, which indicate the level of lipid peroxidation, increased. The results in this study showed that Tb at appropriate concentrations has great potential to confer tolerance of duckweed by supporting the antioxidant system, protecting the biochemical reactions of photosystems and improving hormonal regulation.

Synthesis and photobiological evaluation of Ru(II) complexes with expanded chelate polypyridyl ligands.

Photoreactive Ru(II) complexes capable of ejecting ligands have been used extensively for photocaging applications and for the creation of "photocisplatin" reagents. The incorporation of distortion into the structure of the coordination complex lowers the energy of dissociative excited states, increasing the yield of the photosubstitution reaction. While steric clash between ligands induced by adding substituents at the coordinating face of the ligand has been extensively utilized, a lesser known, more subtle approach is to distort the coordination sphere by altering the chelate ring size. Here a systematic study was performed to alter metal-ligand bond lengths, angles, and to cause intraligand distortion by introducing a "linker" atom or group between two pyridine rings. The synthesis, photochemistry, and photobiology of five Ru(II) complexes containing CH2, NH, O, and S-linked dipyridine ligands was investigated. All systems where stable in the dark, and three of the five were photochemically active in buffer. While a clear periodic trend was not observed, this study lays the foundation for the creation of photoactive systems utilizing an alternative type of distortion to facilitate photosubstitution reactions.