Metal Interactions in the Ni Hyperaccumulating Population of Noccaea caerulescens Monte Prinzera

Hyperaccumulation is a fascinating trait displayed by a few plant species able to accumulate large amounts of metal ions in above-ground tissues without symptoms of toxicity. Noccaea caerulescens is a recognized model system to study metal hyperaccumulation and hypertolerance. A N. caerulescens population naturally growing on a serpentine soil in the Italian Apennine Mountains, Monte Prinzera, was chosen for the study here reported. Plants were grown hydroponically and treated with different metals, in excess or limiting concentrations. Accumulated metals were quantified in shoots and roots by means of ICP-MS. By real-time PCR analysis, the expression of metal transporters and Fe deficiency-regulated genes was compared in the shoots and roots of treated plants. N. caerulescens Monte Prinzera confirmed its ability to hypertolerate and hyperaccumulate Ni but not Zn. Moreover, excess Ni does not induce Fe deficiency as in Ni-sensitive species and instead competes with Fe translocation rather than its uptake.

Epigenetic Control of Plant Response to Heavy Metals

Plants are sessile organisms that must adapt to environmental conditions, such as soil characteristics, by adjusting their development during their entire life cycle. In case of low-distance seed dispersal, the new generations are challenged with the same abiotic stress encountered by the parents. Epigenetic modification is an effective option that allows plants to face an environmental constraint and to share the same adaptative strategy with their progeny through transgenerational inheritance. This is the topic of the presented review that reports the scientific progress, up to date, gained in unravelling the epigenetic response of plants to soil contamination by heavy metals and metalloids, collectively known as potentially toxic elements. The effect of the microbial community inhabiting the rhizosphere is also considered, as the evidence of a transgenerational transfer of the epigenetic status that contributes to the activation in plants of response mechanisms to soil pollution.

Metal Detoxification in Land Plants: From Bryophytes to Vascular Plants. STATE of the Art and Opportunities

Potentially toxic elements are a widespread concern due to their increasing diffusion into the environment. To counteract this problem, the relationship between plants and metal(loid)s has been investigated in the last 30 years. In this field, research has mainly dealt with angiosperms, whereas plant clades that are lower in the evolutive scale have been somewhat overlooked. However, recent studies have revealed the potential of bryophytes, pteridophytes and gymnosperms in environmental sciences, either as suitable indicators of habitat health and elemental pollution or as efficient tools for the reclamation of degraded soils and waters. In this review, we summarize recent research on the interaction between plants and potentially toxic elements, considering all land plant clades. The focus is on plant applicability in the identification and restoration of polluted environments, as well as on the characterization of molecular mechanisms with a potential outlet in the engineering of element tolerance and accumulation.

Overexpression of ZNT1 and NRAMP4 from the Ni Hyperaccumulator Noccaea caerulescens Population Monte Prinzera in Arabidopsis thaliana Perturbs Fe, Mn, and Ni Accumulation

Metalliferous soils are characterized by a high content of metal compounds that can hamper plant growth. The pseudometallophyte Noccaea caerulescens is able to grow on metalliferous substrates by implementing both tolerance and accumulation of usually toxic metal ions. Expression of particular transmembrane transporter proteins (e.g., members of the ZIP and NRAMP families) leads to metal tolerance and accumulation, and its comparison between hyperaccumulator N. caerulescens with non-accumulator relatives Arabidopsis thaliana and Thlaspi arvense has deepened our knowledge on mechanisms adopted by plants to survive in metalliferous soils. In this work, two transporters, ZNT1 and NRAMP4, expressed in a serpentinic population of N. caerulescens identified on the Monte Prinzera (Italy) are considered, and their expression has been induced in yeast and in A. thaliana. In the latter, single transgenic lines were crossed to test the effect of the combined over-expression of the two transporters. An enhanced iron and manganese translocation towards the shoot was induced by overexpression of NcZNT1. The combined overexpression of NcZNT1 and NcNRAMP4 did perturb the metal accumulation in plants.

Comparative analysis identifies micro-RNA associated with nutrient homeostasis, development and stress response in Arabidopsis thaliana upon high Zn and metal hyperaccumulator Arabidopsis halleri

miRNAs have been found to be key players in mineral homeostasis, both in the control of nutrient balance and in the response to toxic trace elements. However, the effect of Zn excess on miRNAs has not been elucidated; moreover, no data are present regarding miRNAs in hyperaccumulator species, where metal homeostasis is tightly regulated. Therefore, expression levels of mature miRNAs were measured by RNA-Seq in Zn-sensitive Arabidopsis thaliana grown in control conditions and upon high Zn, in soil and in Zn-hyperaccumulator Arabidopsis halleri grown in control conditions. Differential expression of notable miRNAs and their targets was confirmed by real-time RT-PCR. The comparison in A. thaliana revealed a small subset modulated upon Zn treatment that is associated with stress response and nutrient homeostasis. On the other hand, a more consistent group of miRNAs was differentially expressed in A. halleri compared with A. thaliana, reflecting inherent differences in nutritional requirements and response to stresses and plant growth and development. Overall, these results confirm the involvement of miRNAs in Zn homeostasis and support the hypothesis of distinct regulatory pathways in hyperaccumulator species.

MYB59 transcription factor behaves differently in metallicolous and non-metallicolous populations of Arabidopsis halleri

In Arabidopsis thaliana (L.) Heynh., MYB59 transcription factor participates in regulating Ca homeostasis and signal transduction and is induced by Cd excess. To investigate its role in the facultative metallophyte Arabidopsis halleri ssp. halleri (L.) O'Kane and Al-Shehbaz, MYB59 expression was investigated under Cd treatment or Ca depletion in three populations belonging to distinct phylogeographic units (metallicolous PL22 and I16 and non-metallicolous I29), and compared with the expression in A. thaliana. In control conditions, MYB59 transcription in A. thaliana and the non-metallicolous population I29 follow a comparable trend with higher expression in roots than shoots, whereas in metallicolous populations I16 and PL22 its expression is similar in roots and shoots, suggesting a convergent evolution associated with adaptation to metalliferous environments. After 6 h of Ca depletion, MYB59 transcript levels were very high in I16 and PL22 populations, indicating that the adaptation to metalliferous environments requires tightly regulated Ca homeostasis and signalling. Cd treatment caused variability in MYB59 expression. In I29, MYB59 expression, as in A. thaliana, is likely associated to stress response, whereas its modulation in the two metallicolous populations reflects the different strategies for Cd tolerance and accumulation. In conclusion, MYB59 regulation in A. halleri is part of the network linking mineral nutrition and Cd tolerance/accumulation.

Enhancement of Zn tolerance and accumulation in plants mediated by the expression of Saccharomyces cerevisiae vacuolar transporter ZRC1

Transgenic Arabidopsis thaliana and Populus alba plants overexpressing the zinc transporter ScZRC1 in shoots exhibit Zn tolerance. Increased Zn concentrations were observed in shoots of P. alba, a species suitable for phytoremediation. Genetic engineering of plants for phytoremediation is worth to consider if genes leading to heavy metal accumulation and tolerance are expressed in high biomass producing plants. The Saccharomyces cerevisiae ZRC1 gene encodes a zinc transporter which is primarily involved in the uptake of Zn into the vacuole. The ZRC1 gene was expressed in the model species A. thaliana and P. alba (cv. Villafranca). Both species were transformed with constructs carrying ScZRC1 under the control of either the CaMV35S promoter for constitutive expression or the active promoter region of the tobacco Rubisco small subunit (pRbcS) to limit the expression to the above-ground tissues. In hydroponic cultures, A. thaliana and poplar ScZRC1-expressing plants accumulated more Zn in vegetative tissues and were more tolerant than untransformed plants. No differences were found between plants carrying the CaMV35::ScZRC1 or pRbcS::ScZRC1 constructs. The higher Zn accumulation in transgenic plants was accompanied by an increased superoxide dismutase (SOD) activity, indicating the activation of defense mechanisms to prevent cellular damage. In the presence of cadmium in addition to Zn, plants did not show symptoms of metal toxicity, neither in hydroponic cultures nor in soil. Zn accumulation increased in shoots, while no differences were observed for Cd accumulation, in comparison to control plants. These data suggest that ectopic expression of ScZRC1 can increase the potential of poplar for the remediation of Zn-polluted soils, although further tests are required to assay its application in remediating multimetal polluted soils.

Evolution of the metal hyperaccumulation and hypertolerance traits

To succeed in life, living organisms have to adapt to the environmental issues to which they are subjected. Some plants, defined as hyperaccumulators, have adapted to metalliferous environments, acquiring the ability to tolerate and accommodate high amounts of toxic metal into their shoot, without showing symptoms of toxicity. The determinants for these traits and their mode of action have long been the subject of research, whose attention lately moved to the evolution of the hypertolerance and hyperaccumulation traits. Genetic evidence indicates that the evolution of both traits includes significant evolutionary events that result in species-wide tolerant and accumulating backgrounds. Different edaphic environments are responsible for subsequent refinement, by local adaptive processes, leading to specific strategies and various degrees of hypertolerance and hyperaccumulation, which characterize metallicolous from non-metallicolous ecotypes belonging to the same genetic unit. In this review, we overview the most updated concepts regarding the evolution of hyperaccumulation and hypertolerance, highlighting also the ecological context concerning the plant populations displaying this fascinating phenomenon.

Heavy Metal Pollutions: State of the Art and Innovation in Phytoremediation

Mineral nutrition of plants greatly depends on both environmental conditions, particularly of soils, and the genetic background of the plant itself. Being sessile, plants adopted a range of strategies for sensing and responding to nutrient availability to optimize development and growth, as well as to protect their metabolisms from heavy metal toxicity. Such mechanisms, together with the soil environment, meaning the soil microorganisms and their interaction with plant roots, have been extensively studied with the goal of exploiting them to reclaim polluted lands; this approach, defined phytoremediation, will be the subject of this review. The main aspects and innovations in this field are considered, in particular with respect to the selection of efficient plant genotypes, the application of improved cultural strategies, and the symbiotic interaction with soil microorganisms, to manage heavy metal polluted soils.

The Arabidopsis thaliana transcription factor MYB59 regulates calcium signalling during plant growth and stress response

Transcription factor MYB59 is involved in plant growth and stress responses by acting as negative regulator of Ca signalling and homeostasis. The Arabidopsis thaliana transcription factor MYB59 is induced by cadmium (Cd) and plays a key role in the regulation of cell cycle progression and root elongation, but its mechanism of action is poorly understood. We investigated the expression of MYB59 and differences between wild-type plants, the myb59 mutant and MYB59-overexpressing lines (obtained by transformation in the mutant genotype) during plant growth and in response to various forms of stress. We also compared the transcriptomes of wild-type and myb59 mutant plants to determine putative MYB59 targets. The myb59 mutant has longer roots, smaller leaves and smaller cells than wild-type plants and responds differently to stress in germination assay. Transcriptomic analysis revealed the upregulation in the myb59 mutant of multiple genes involved in calcium (Ca) homeostasis and signalling, including those encoding calmodulin-like proteins and Ca transporters. Notably, MYB59 was strongly induced by Ca deficiency, and the myb59 mutant was characterized by higher levels of cytosolic Ca in root cells and showed a modest alteration of Ca transient frequency in guard cells, associated with the absence of Ca-induced stomatal closure. These results indicate that MYB59 negatively regulates Ca homeostasis and signalling during Ca deficiency, thus controlling plant growth and stress responses.

Phytoremediatory efficiency of Chrysopogon zizanioides in the treatment of landfill leachate: a case study

A common approach for waste management is their disposal in landfills, which is usually associated with the production of dangerous gases and of liquid leachate. Due to its toxicity, polluted liquid negatively impacts on the environment with the possible contamination of large volumes of soil, groundwater, and surface water. Leachate remediation is therefore subject of intensive research, and phytoremediation has been achieving increasing interest in recent decades. We describe here the suitability of vetiver grass for the remediation of two leachates collected in urban landfills of northern Italy, characterized by different composition. Our objective was measuring the accumulation/tolerance potential of this species and the evapotranspiration ability in a pot experiment, to evaluate applicability of vetiver plants for the reduction and decontamination of landfill leachate. Plants were grown for 4 months in pots with a zeolite growth bed and watered with either tap water (control) or undiluted landfill leachate. Plant growth and fitness and elemental content in shoots and roots were evaluated at the end of the experiment. In these experimental conditions, the high bioaccumulation of metals highlights the suitability of this species for its employment in phytoremediation; however, vetiver growth under leachate treatment was strongly dependent on leachate composition, making a case-to-case evaluation of plant tolerance necessary before large-scale application.

The potential of genetic engineering of plants for the remediation of soils contaminated with heavy metals

The genetic engineering of plants to facilitate the reclamation of soils and waters contaminated with inorganic pollutants is a relatively new and evolving field, benefiting from the heterologous expression of genes that increase the capacity of plants to mobilize, stabilize and/or accumulate metals. The efficiency of phytoremediation relies on the mechanisms underlying metal accumulation and tolerance, such as metal uptake, translocation and detoxification. The transfer of genes involved in any of these processes into fast-growing, high-biomass crops may improve their reclamation potential. The successful phytoextraction of metals/metalloids and their accumulation in aerial organs have been achieved by expressing metal ligands or transporters, enzymes involved in sulfur metabolism, enzymes that alter the chemical form or redox state of metals/metalloids and even the components of primary metabolism. This review article considers the potential of genetic engineering as a strategy to improve the phytoremediation capacity of plants in the context of heavy metals and metalloids, using recent case studies to demonstrate the practical application of this approach in the field.

The MTP1 promoters from Arabidopsis halleri reveal cis-regulating elements for the evolution of metal tolerance

In the hyperaccumulator Arabidopsis halleri, the zinc (Zn) vacuolar transporter MTP1 is a key component of hypertolerance. Because protein sequences and functions are highly conserved between A. halleri and Arabidopsis thaliana, Zn tolerance in A. halleri may reflect the constitutively higher MTP1 expression compared with A. thaliana, based on copy number expansion and different cis regulation. Three MTP1 promoters were characterized in A. halleri ecotype I16. The comparison with the A. thaliana MTP1 promoter revealed different expression profiles correlated with specific cis-acting regulatory elements. The MTP1 5' untranslated region, highly conserved among A. thaliana, Arabidopsis lyrata and A. halleri, contains a dimer of MYB-binding motifs in the A. halleri promoters absent in the A. thaliana and A. lyrata sequences. Site-directed mutagenesis of these motifs revealed their role for expression in trichomes. A. thaliana mtp1 transgenic lines expressing AtMTP1 controlled by the native A. halleri promoter were more Zn-tolerant than lines carrying mutations on MYB-binding motifs. Differences in Zn tolerance were associated with different distribution of Zn among plant organs and in trichomes. The different cis-acting elements in the MTP1 promoters of A. halleri, particularly the MYB-binding sites, are probably involved in the evolution of Zn tolerance.

Functional components of the bacterial CzcCBA efflux system reduce cadmium uptake and accumulation in transgenic tobacco plants

Cadmium (Cd) is a toxic trace element released into the environment by industrial and agricultural practices, threatening the health of plants and contaminating the food/feed chain. Biotechnology can be used to develop plant varieties with a higher capacity for Cd accumulation (for use in phytoremediation programs) or a lower capacity for Cd accumulation (to reduce Cd levels in food and feed). Here we generated transgenic tobacco plants expressing components of the Pseudomonas putida CzcCBA efflux system. Plants were transformed with combinations of the CzcC, CzcB and CzcA genes, and the impact on Cd mobilization was analysed. Plants expressing PpCzcC showed no differences in Cd accumulation, whereas those expressing PpCzcB or PpCzcA accumulated less Cd in the shoots, but more Cd in the roots. Plants expressing both PpCzcB and PpCzcA accumulated less Cd in the shoots and roots compared to controls, whereas plants expressing all three genes showed a significant reduction in Cd levels only in shoots. These results show that components of the CzcCBA system can be expressed in plants and may be useful for developing plants with a reduced capacity to accumulate Cd in the shoots, potentially reducing the toxicity of food/feed crops cultivated in Cd-contaminated soils.

Environmental photochemistry of fluoroquinolones in soil and in aqueous soil suspensions under solar light

The photodegradation fate of widely used fluoroquinolone (FQ) drugs has been studied both at the water-soil interface and in soil at actual concentrations (500 ng g(-1)) under natural solar light. Both human and veterinary drugs have been examined, namely ciprofloxacin, danofloxacin, enrofloxacin, levofloxacin, marbofloxacin and moxifloxacin. After spiking and irradiation, samples were submitted to microwave-assisted extraction and analyzed by high-performance liquid chromatography coupled to fluorescence detection (HPLC-FD). FQs degradation was faster in aqueous soil suspension than in neat soil (but lower than in "clean" water). A number of byproducts were identified by HPLC electrospray ionization tandem mass spectrometry after a post-extraction cleanup based on a molecularly imprinted polymer phase, for a more accurate detection. The distribution in the suspension was intermediate between those observed in soils and in aqueous solutions.

The Effect of Substituents on the Photochemical Reaction between Naphthalenedicarbonitrile and Methylbenzenes

The photochemical reaction between 1,4-naphthalenedicarbonitrile (NDN) and toluene in acetonitrile is extended to bibenzyl (4), 3- and 4-fluorotoluene (8, 7), and 3- and 4-m ethoxytoluene (9, 10). The product distribution obtained with com pounds 4, 7 and 10 is similar and includes 4-benzyl-1-naphthalencarbonitriles (1a -c), 1-benzyl-1,2-dihydro-1,4-naphthalenedicarbonitriles (5a-c), cis 2-benzyl-1,2-dihydro-1,4-naphthalenedicarbonitriles (2a-c) (with com pound 10 also the trans isomer 12), 12-substituted 6,11-dicyano-5,11-methano-5,6,11, 12-tetrahydrodibenzo[ a,e]cyclooctenes (3a, b) (not obtained with com pound 10), besides the hydrocarbons arising from the coupling o f the respective benzyl radicals (products 6a-c). With com pound 9 the only process observed is reduction of NDN to 1,2-dihydronaphthalene-1,4-dicarbonitrile (11). The results obtained fit into the previously proposed mechanistic scheme, involving electron and proton transfer follow ed either by in cage radical coupling to form products 2, 3, 5 and 12 or radical escape leading to products 1 and 6. The effect of substituents on the various phases of the reaction is discussed.

Recent advances in the analysis of metal hyperaccumulation and hypertolerance in plants using proteomics

Hyperaccumulator/hypertolerant plant species have evolved strategies allowing them to grow in metal-contaminated soils, where they accumulate high concentrations of heavy metals in their shoots without signs of toxicity. The mechanisms that allow enhanced metal uptake, root-to-shoot translocation and detoxification in these species are not fully understood. Complementary approaches such as transcriptomic-based DNA microarrays and proteomics have recently been used to gain insight into the molecular pathways evolved by metal hyperaccumulator/hypertolerant species. Proteomics has the advantage of focusing on the translated portion of the genome and it allows to analyze complex networks of proteins. This review discusses the recent analysis of metal hyperaccumulator/hypertolerant plant species using proteomics. Changes in photosynthetic proteins, sulfur, and glutathione metabolism, transport, biotic and xenobiotic defenses as well as the differential regulation of proteins involved in signaling and secondary metabolism are discussed in relation to metal hyperaccumulation. We also consider the potential contribution of several proteins to the hyperaccumulation phenotype.

Photodegradation of fluoroquinolones in surface water and antimicrobial activity of the photoproducts

The widespread presence of fluoroquinolone antibiotics (FQs) in natural ecosystems is a health hazard for humans and other living organisms. The role of sunlight in degrading FQs present in environmental waters has been studied. In particular, the photodegradation of four largely employed FQs, viz. Ciprofloxacin (CIP), Danofloxacin (DAN), Levofloxacin (LEV) and Moxifloxacin (MOX) has been studied in not tampered river water. Degradation rates have been investigated at ppb levels (20-50 μg L(-1)) under solar light, and the results have been commented critically. The products distribution has been studied by HPLC-ESI-MS/MS analysis and structures have been attributed on the basis of their mass fragmentation spectra. Importantly from the environmental point of view, the (potentially toxic) FQ nucleus remained intact over the early stages of the degradation. Indeed, the photoproducts were proved to possess residual antibacterial activity, as shown from in vitro antibacterial activity tests against different well characterized human and environmental bacterial strains, carried out on the above FQs, as well as for Enrofloxacin (ENR) and Marbofloxacin (MAR).

Sunlight-induced degradation of soil-adsorbed veterinary antimicrobials Marbofloxacin and Enrofloxacin

Marbofloxacin (MAR) and Enrofloxacin (ENR), two largely employed veterinary Fluoroquinolones (FQs), were found to be present at the micrograms per kilogram level in agricultural soils of South Lombardy (Italy) several months after manuring. Distribution coefficients (K(d)) from sorption experiments indicated a strong binding to the soil. Soil samples fortified with environmentally significant FQs amounts (0.5 mg kg(-1)) were exposed to solar light that promoted extensive degradation (80%) of both drugs in 60-150 h. Thus, photochemistry could be considered a significant depollution path in the soil, although it was two orders of magnitudes slower than in aqueous solution and a fraction of the drug (ca. 20%) remained unaffected. For MAR the photoprocess was the same as in solution, and involved cleavage of the tetrahydrooxadiazine ring. On the contrary, with ENR only some of the photoproducts determined in water (those arising from a stepwise oxidation of the piperazine side chain) were observed. Substitution of the 6-fluoro by a hydroxyl group and reduction did not occur in the soil, supporting the previous contention that such processes required polar solvation of FQs. Consistently with this rationalization, the irradiation of thin layers of solid drugs led to essentially the same products distribution as in the soil. From the environmental point of view it is important to notice that photodegradation mainly affects the side-chains, while the fluoroquinolone ring, to which the biological effect is associated, is conserved up to the later stages of the degradation.

Fluoroquinolones as potential photochemotherapeutic agents: covalent addition to guanosine monophosphate

The triplet aryl cation photochemically generated from fluoroquinolones bearing a fluoro atom at position 8 attacks guanosine monophosphate (k(r) > 10(9) M(-1)s(-1)) and forms covalent adducts. The reaction is a model for the implementation of oxygen-independent photochemotherapy.

Photochemical degradation of marbofloxacin and enrofloxacin in natural waters

The photochemical fate of Marbofloxacin (MAR) and Enrofloxacin (ENR), two Fluoroquinolones (FQs) largely used as veterinary bactericides known to be present in surface waters, was investigated in aqueous solution. The degradation of these pollutants (5-50 microg L(-1) starting concentration) was complete in about 1 h by exposure to solar light (summer) and obeyed a first-order kinetics. The structure of the primary photoproducts was determined. Those from ENR arose through three paths, namely, oxidative degradation of the piperazine side-chain, reductive defluorination, and fluorine solvolysis. More heavily degraded products that had been previously reported were rationalized as secondary photoproducts from the present ones. As for MAR, this underwent homolytic cleavage of the tetrahydrooxadiazine moiety to give two quinolinols. All of the primary products were themselves degraded in about 1 h. The photoreactions rates were scarcely affected by Ca(2+) (200 mg L(-1)), Mg(2+) (30 mg L(-1)), Cl(-) (30 mg L(-1)), and humic acid (1 mg L(-1)), but increased in the presence of phosphate (20 mg L(-1)). The fastest degradation of ENR occurred at pH about 8 where the zwitterionic form was present, while in the case of MAR the cationic form was the most reactive.

An exploratory and mechanistic study of the defluorination of an (aminofluorophenyl)oxazolidinone: S(N)1(Ar*) vs. S(R(+)N)1(Ar*) mechanism

The morpholinofluorophenyloxazolidinone 1 (the antibacterial drug linezolid) is found to undergo reductive defluorination upon irradiation in water (Phi 0.33), in some of the products accompanied by the simultaneous oxidative degradation of the morpholine side chain. In the presence of chloride, iodide and pyrrole, the fluorine is substituted by these groups (with pyrrole, in position 2). The defluorination is less efficient in methanol and mainly leads to reduction (Phi 0.053). These data can be accommodated through two different mechanisms, viz. either C-F bond heterolysis to give a phenyl cation [S(N)1(Ar*)], or ionization to give a radical cation [S(R(+)N)1(Ar*)]. Steady-state and time resolved data have been gathered for clarifying this issue. It is found that, indeed, ionization of 1 is efficient and proceeds from the singlet, but leads to no irreversible change. On the contrary, triplet (3)1 (lifetime 0.5 micros in MeOH, <0.1 micros in water) fragments and gives the corresponding triplet phenyl cation. The last intermediate explains well the observed hydrogen abstraction both inter- (from the solvent, when this is reducing) and intramolecularly (from the morpholine group), as well as addition to a charged anion or to a neutral pi nucleophile such as pyrrole. The rationalization is supported by the study of some related molecules. Thus, the only photochemical reaction from the non fluorinated analogue of linezolid (that ionizes just as 1) is an inefficient degradation of the morpholine chain (Phi 0.001), while a simple model such as N-(2-fluorophenyl)morpholine undergoes photosolvolysis in water and is not trapped by pyrrole.

Intramolecular electron transfer in the photochemistry of some nitrophenyldihydropyridines

4-Phenyl-1,4-dihydropyridine-3,5-dicarboxylates contain two pi chromophores separated by an sp3 carbon. The lowest singlet is localized on the dihydropyridine moiety (1PyH2-Ph) and emits a blue fluorescence (with close to unitary efficiency in glass at 77 K). In 3-nitrophenyl derivatives (PyH2-PhNO2, some of which are photolabile drugs) the fluorescence is completely quenched. Reasonably, this is due to intramolecular electron transfer between the close-lying donor and acceptor moieties to give the charge-separated species (PyH2*+-PhNO2*-). In EPA glass at 77 K, back-electron transfer gives the dihydropyridine-localized triplet (3PyH2-PhNO2), which emits a yellow phosphorescence. In solution, deprotonation from the radical cation on the dihydropyridine moiety initiates rearomatization, finally giving Py-PhNO2 with low quantum yield (5 x 10(-4) to 5 x 10(-3), increasing up to 0.013 by irradiation at 254 nm, where direct excitation of the nitrophenyl chromophore contributes). In the presence of triethylamine, the reaction changes to neat reduction of the nitro group. When a tethered alkylamino group is present, oxidative degradation of that moiety occurs, again via an electron-transfer intramolecular process. This has been found with the drug nicardipine, where photodegration is more efficient (phi 0.02 to 0.1). Donor-acceptor dyads of this type, easily available through the Hantzsch synthesis, may be useful for building new photoinduced electron-transfer systems.

Photochemistry of 4-(2-Nitrophenyl)-1,4-Dihydropyridines. Evidence for Electron Transfer and Formation of an Intermediate†

New evidence about the path followed in the photochemical reaction of 4-(2-nitrophenyl)-1,4-dihydropyridines such as the drugs nifedipine (Compound 1) and nisoldipine (Compound 2) to give the corresponding nitrosophenylpyridines has been found through determination of the steady-state photochemical parameters and a comparison of the photoreactions in solution and in matrix at 90 K. Additional support is given by comparison with the isomeric 4-(3-nitrophenyl)dihydropyridine as well as with simpler derivatives, such as the corresponding 4-methyldihydropyridine. In Compounds 1 and 2, the lowest lying singlet, localized on the dihydropyridine chromophore, is deactivated by (largely exothermic) electron transfer to the nitrobenzene moiety, as evidenced by the complete quenching of the blue fluorescence observed in analogues not containing the electron-accepting group. Intramolecular proton transfer ensues in the 2-nitrophenyl derivatives with a relatively medium-independent quantum yield of approximately 0.3 and leads to an aromatic zwitterion, which is detected in matrix at 90 K (photoionization of this intermediate takes place in 2-methyltetrahydrofuran secondary). The intermediate is smoothly converted into the end product upon melting the glass. The 3-nitrophenyl analog, for which such a path is not available, is less reactive by about three orders of magnitude at 366 nm, although the quantum yield arrives at approximately 0.01 by irradiation at 254 nm in MeOH, reasonably via the nitrophenyl localized triplet.

Photochemistry and phototoxicity of fluocinolone 16,17-acetonide

Fluocinolone 16,17-acetonide is a corticosteroid used topically to treat various inflammatory skin diseases. Its photoreactivity was studied under UV-A and UV-B light in aqueous buffer in the presence of oxygen. This drug is photolabile under UV-B light and, to a lesser extent, under UV-A light, which is absorbed far less. In phosphate buffer, approximately 80% of fluocinolone acetonide decomposes after 5 J/cm2 of UV-B irradiation, whereas under 30 J/cm2 of UV-A light approximately only 20% decomposes. Both the drug and its photoproducts have been evaluated through a battery of in vitro studies and found to cause photohemolysis and induce photodamage to proteins (erythrocyte ghosts, bovine serum albumin) and linoleic acid. In addition, one of the photoproducts (the 17-hydroperoxy derivative) is highly toxic in the dark. Therefore, both loss of therapeutic activity and light-induced adverse effects may be expected when patients expose themselves to sunlight after drug administration. A major mechanism for phototoxicity involves radicals forming from drug breakdown, at least under UV-B, although reactive oxygen species may play a role, particularly under UV-A.

General patterns in the photochemistry of pregna-1,4-dien-3,20-diones

The photochemistry of six pregna-1,4-dien-3,20-diones has been compared and found to involve both the cyclohexadienone moiety in ring A and the isolated ketone at C-20. The two reactions take place proportionally to the fraction of light absorbed by each chromophore. The cross-conjugated ketone absorbs predominantly or exclusively at both 254 and 366 nm and undergoes the "lumi" rearrangement to bicyclo[3.1.0]hex-3-en-2-one. The quantum yield of the reaction diminished somewhat with increasing lambda(exc), e.g., for prednisolone Phi(254) (nm) = 0.42, Phi(366) (nm) = 0.3. A much stronger lowering is caused by halogen substitution in position 9 (by a factor of 3 for F, >50 for Cl), apparently due to a shortened triplet lifetime caused by heavy atom effect. At 310 nm, both chromophores absorb to a comparable degree and both may react. The reaction at C(20) ketone involves either quite efficient alpha-cleavage (C(17)-C(20)) for compounds bearing an acetal or hydroxyl function at C(17) or less effective (by a factor of ca. 10) hydrogen abstraction from the 18-methyl group in the other cases (finally resulting in Norrish II fragmentation or Yang cyclization). The results allow generalizing how the substitution pattern surrounding each chromophore affects the photoreactivity at that site and the competition between the two modes, allowing predicting the photochemistry of this family of antiinflammatory drugs.

In Vitro Phototoxic Properties of Triamcinolone 16,17-acetonide and Its Main Photoproducts¶

The phototoxicity of triamcinolone 16,17-acetonide has been estimated through a panel of in vitro tests. The main target involved in phototoxicity induced by triamcinolone appeared to be the cell membrane. Oxygen-independent photohemolysis was observed. A photochemical study in water and buffered solutions supported the conclusion that this is related to the action of radicals formed upon UV irradiation (in particular UV-B) by Norrish Type-I fragmentation of the C-20 ketone group. Peroxy radicals were formed in the presence of oxygen and were the active species in that case. Three photoproducts, isolated from the photodegradation of the drug, were submitted to the same toxicity tests. Two of them were proved to possess toxic or phototoxic properties on erythrocytes, primarily induced by UV-B light, and may participate in the photosensitizing activity of triamcinolone 16,17-acetonide. Our in vitro results suggest that the drug can elicit weak photosensitizing properties in vivo.

Photostability of extended-release matrix formulations

The photostability of drugs has been widely studied while less attention is devoted to the possible modifications that UV light may induce on the excipients of a dosage form, in particular, on the functional polymers used to modulate drug delivery. In this work we have evaluated the effect of UV light on the release characteristics of extended-release matrix tablets containing hydroxypropylmethylcellulose (HPMC) or polyethylene oxide (PEO) as retarding polymers. Two different model drugs have been used: nifedipine (insoluble and photolabile drug) and diltiazem (soluble and photostable drug). Photodecomposition of nifedipine was evaluated and the formation of the photoproducts was followed during the dissolution process. Regarding the dissolution stability, the matrix tablets containing HPMC exposed to UV light have not shown significant differences in drug release profiles compared to the same non-irradiated formulation, while the matrix tablets containing PEO and exposed to the same conditions of UV light have shown a remarkable increase of drug release rate within the first minutes of the dissolution test (burst effect) which is particularly critical because it can cause the loss of the desired therapeutic control.