Photocatalytic Hydrogen Evolution from Plastoquinol Analogues as a Potential Functional Model of Photosystem I.

The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH2) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H2) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH2) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Acr+-Mes) as a photoredox catalyst, and a cobalt(III) complex, CoIII(dmgH)2pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H2 and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H2 evolution from tetrachlorohydroquinone (Cl4QH2) with Acr+-Mes and CoIII(dmgH)2pyCl and H2O in deaerated MeCN was determined to be 10%. Photocatalytic H2 evolution is started by electron transfer (ET) from Cl4QH2 to the triplet ET state of Acr+-Mes to produce Cl4QH2•+ and Acr•-Mes with a rate constant of 7.2 × 107 M-1 s-1, followed by ET from Acr•-Mes to CoIII(dmgH)2pyCl to produce [CoII(dmgH)2pyCl]-, accompanied by the regeneration of Acr+-Mes. On the other hand, Cl4QH2•+ is deprotonated to produce Cl4QH•, which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [CoII(dmgH)2pyCl]- to produce a cobalt(III) hydride complex, [CoIII(H)(dmgH)2pyCl]-, which reacts with H+ to evolve H2, accompanied by the regeneration of CoIII(dmgH)2pyCl. The formation of [CoII(dmgH)2pyCl]- was detected by electron paramagnetic resonance measurements.

A new photosafety screening strategy based on in chemico photoreactivity and in vitro skin exposure for dermally-applied chemicals.

This study involved an attempt to establish a new photosafety screening system for dermally-applied chemicals consisting of a reactive oxygen species (ROS) assay and an in vitro skin permeation test. The ROS assay was undertaken to evaluate photoreactivity of six test compounds, acridine (ACD), furosemide (FSM), hexachlorophene (HCP), 8-methoxypsoralen (MOP), norfloxacin (NFX), and promethazine (PMZ), and the in vitro skin permeation test was conducted to obtain steady-state concentration (Css) values of test compounds in removed rat skin. All test compounds were photoreactive based on ROS generation under simulated sunlight exposure. In particular, ROS generation from ACD was high compared with other test compounds, and photoreactivity of ACD was deduced to be potent. The Css values of ACD, HCP, MOP, and PMZ were over 50 µg/mL, and skin exposure to FSM and NFX was found to be extremely low. Upon these findings, ACD was judged to be highly phototoxic. The rank for phototoxic risk of test compounds based on photoreactivity and in vitro skin exposure was mostly in agreement with outcomes on their in vivo phototoxicity in rats. The proposed strategy, an alternative to animal testing, would be efficacious for photosafety evaluation of drug candidates in early stages of pharmaceutical development.

Visible Light-Activatable Oxidase Mimic of 9-Mesityl-10-Methylacridinium Ion for Colorimetric Detection of Biothiols and Logic Operations.

In this work, 9-mesityl-10-methylacridinium ion (Acr+-Mes) is found to act as an effective photocatalyst mimicking the function of oxidase. Upon visible light illumination, the excited Acr+-Mes is able to exhibit superior enzymatic catalytic activity for small molecular substrates as well as protein biomacromolecule (cytochrome c). The experiment results demonstrate that the Acr+-Mes oxidase mimic shows higher affinity to 3,3',5,5'-tetramethylbenzidine (TMB) than natural horseradish peroxidase or the reported molecule oxidase mimic. The reaction mechanism is ascribed to the strong oxidation property of the long-lived electron-transfer state (Acr•-Mes•+) and the electron transfer from Acr•-Mes radical to dissolved oxygen to generate superoxide radicals, which can easily oxidize various substrates. On the basis of these observations, the light-activatable Acr+-Mes with an oxidase-like activity as the probe is utilized for cost-effective, sensitive, and highly selective colorimetric detection of two biothiols (L-cysteine and L-glutathione). The lowest detectable concentrations of L-Cys and L-GSH is 100 nM, which is lower than that of most of the reported methods for biothiols. Beyond this, we construct a series of visual molecular logic gates (AND, INH, and NOR) using the oxidase mimic-involved reaction systems.

Comparison of the toxicological impacts of carbamazepine and a mixture of its photodegradation products in Scrobicularia plana.

In the aquatic environment, pharmaceutical drugs are submitted to degradation processes, where photodegradation is one of the most important mechanisms affecting the fate, persistence and toxicity of the compounds. Carbamazepine, a widely used antiepileptic, is known to suffer photodegradation in water bodies and generate photoproducts, some of them with higher potential toxicity than the parent compound. Therefore, to evaluate the toxic effects of CBZ when combined with its photoproducts, an acute exposure (96h) with the edible clam Scrobicularia plana was performed using environmental concentrations of CBZ (0.00-9.00µg/L) irradiated (and non-irradiated) with simulated solar radiation. The analysis of the irradiated CBZ solutions by mass spectrometry revealed the formation of 5 photoproducts, including acridine (a compound known to be carcinogenic). Oxidative stress results showed that the exposure to CBZ photoproducts did not increase the toxicity to clams, by comparison with the parent compound. Lipid peroxidation levels, catalase and superoxide dismutase activities were the most responsive parameters to these stressors and lipid peroxidation results appeared to show the presence of an antagonistic effect resulting from the mixture of CBZ and its photoproducts.

A General Approach to Catalytic Alkene Anti-Markovnikov Hydrofunctionalization Reactions via Acridinium Photoredox Catalysis.

The development of methods for anti-Markovnikov alkene hydrofunctionalization has been a focal point of catalysis research for several decades. The vast majority of work on the control of regioselectivity for this reaction class has hinged on transition metal catalyst activation of olefin substrates. While progress has been realized, there are significant limitations to this approach, and a general solution for catalysis of anti-Markovnikov hydrofunctionalization reactions of olefins does not presently exist. In the past several years, this research lab has focused on alkene activation by single electron oxidation using organic photoredox catalysts to facilitate anti-Markovnikov hydrofunctionalization. By accessing reactive cation radical intermediates, we have realized a truly general approach to anti-Markovnikov olefin hydrofunctionalization reactions. We have identified a dual organic catalyst system consisting of an acridinium photooxidant, first reported by Fukuzumi, and a redox-active hydrogen atom donor that accomplishes a wide range of hydrofunctionalization reactions with complete anti-Markovnikov regiocontrol. This method relies on single electron oxidation of the alkene to reverse its polarity and results in the opposite regioselectivity for hydrofunctionalization. In 2012, we disclosed the anti-Markovnikov hydroetherification of alkenols employing an acridinium photocatalyst and a hydrogen atom donor that proceeds via interwoven polar and radical steps. This general catalyst system has enabled several important reactions in this area, including anti-Markovnikov alkene hydroacetoxylation, hydrolactonization, hydroamination, and hydrotrifluoromethylation reactions. More recently, we have also delineated conditions for intermolecular anti-Markovnikov hydroamination reactions of alkenes using either triflamide or nitrogen-containing heteroaromatic compounds such as pyrazole, indazole, imidazole, and 1,2,3-triazole. Further development led to a method for the anti-Markovnikov addition of mineral acids to olefins using lutidinium halide salts as convenient reagents to deliver the mineral acids. Acids including HCl, HF, H3PO4, and MeSO3H all participate in the hydrofunctionalization reactions, even with alkenes that are highly prone to polymerization. A combination of transient and steady-state absorption spectroscopy tools were employed to observe alkene cation radicals and the resultant acridine radical, lending support for an electron transfer mechanism. The origin of the anti-Markovnikov selectivity in these reactions is likely the result of a reversible addition of the nucleophile to the alkene cation radical resulting in a greater population of the more stable radical. Loss of a proton followed by reaction of the radical intermediate with the hydrogen atom donor completes the transformations. Again, by means of transient absorption spectroscopy, oxidative turnover of the acridine radical was observed to complete the dual catalytic cycle mechanistic picture.

Photo-inducible cytotoxic and clastogenic activities of 3,6-di-substituted acridines obtained by acylation of proflavine.

The cytotoxicity and photo-enhanced cytotoxicity of a series of 18 3,6-di-substituted acridines were evaluated on both tumour CHO cells and human normal keratinocytes, and compared to their corresponding clastogenicity as assessed by the micronucleus assay. Compounds 2f tert-butyl N-[(6-tert-butoxycarbonylamino)acridin-3-yl]carbamate and 2d N-[6-(pivalamino)acridin-3-yl]pivalamide displayed a specific cytotoxicity on CHO cells. These results suggested that the two derivatives could be considered as interesting candidates for anticancer chemotherapy and hypothesized that the presence of 1,1-dimethylethyl substituents was responsible for a strong nonclastogenic cytotoxicity. Compounds 2b and 2c, on the contrary, displayed a strong clastogenicity. They indicated that the presence of nonbranched aliphatic chains on positions 3 and 6 of the acridine rings tended to induce a significant clastogenic effect. Finally, they established that most of the acridine compounds could be photo-activated by UVA-visible rays and focussed on the significant role of light irradiation on their biological properties.

Direct detection of nucleotide radical cations produced by electron-transfer oxidation of DNA bases with electron-transfer state of 9-mesityl-10-methylacridinium ion and resulting efficient DNA cleavage without oxygen.

Photoinduced electron transfer of DNA as well as DNA bases with 9-mesityl-10-methylacridinium ion results in formation of all types of DNA base radical cations, which have been detected as the transient absorption spectra measurements, leading to efficient DNA cleavage in the absence of O2.

Photocatalytic oxygenation of pivalic acid with molecular oxygen via photoinduced electron transfer using 10-methylacridinium ions.

Photoirradiation of the absorption band of the 10-methylacridinium ion (AcrH+) with visible light in deaerated CH3CN/H2O (1:1 v/v) containing pivalic acid (Bu(t)COOH) and less than 1 equiv of NaOH results in the selective formation of 9-tert-butyl-9,10-dihydro-10-methylacridine (AcrHBu(t)). The same product is obtained in O2-saturated CH3CN/H2O under visible light irradiation. Photoirradiation of the absorption band of AcrHBu(t) with UV light in deaerated CH3CN/H2O (1:1 v/v) results in the formation of tert-butyl hydroperoxide (Bu(t)OOH), accompanied by regeneration of AcrH+. This cycle can be repeated several times. When AcrH+ is replaced by the 9-phenyl derivative (AcrPh+), AcrPh+ acts as an effective photocatalyst for the one-pot photooxygenation of Bu(t)COOH in the presence of less than 1 equiv of NaOH relative to Bu(t)COOH with O2 to yield Bu(t)OOH and Bu(t)H. The photocatalytic oxygenation mechanism is discussed based on the detection of radical intermediates by laser flash photolysis and ESR measurements as well as quantum yield determination.

Chemiluminescence of lucigenin is dependent on experimental conditions.

The aim of the study was to test the effect of experimental conditions such as light radiation and temperature on chemiluminescence (CL) of 10(-2)-10(-5) mol/L lucigenin dissolved in various types of solvents. Irradiation by UV light (280, 297, 313 or 400 nm) induced a significant increase in CL of lucigenin dissolved in borate buffer. This effect was the most obvious for 10(-2)-10(-3) mol/L lucigenin. All wavelengths used had a similar effect. UV irradiation did not induce changes in the CL activity of lucigenin dissolved in dH2O or in dimethyl sulphoxide (DMSO). Different results for various solvents were not dependent on pH. The CL activity of 10(-2) mol/L and 10(-3) mol/L lucigenin dissolved in borate buffer increased depending on the solution temperature (25 degrees C, 30 degrees C, 37 degrees C or 40 degrees C) already at the beginning of the analysis, with a further increase during 16 h incubation period. It can be summarized that temperatures higher than 25 degrees C and intensive light irradiation are among those factors which significantly affect the result of analysis when lucigenin is used as a luminophor.

Photolysis of N-hydroxpyridinethiones: a new source of hydroxyl radicals for the direct damage of cell-free and cellular DNA.

N-Hydroxypyridine-2-thione (2-HPT), known to release hydroxyl radicals on irradiation with visible light, and two related compounds, viz. N-hydroxypyridine-4-thione (4-HPT) and N-hydroxyacridine-9-thione (HAT), were tested for their potency to induce DNA damage in L1210 mouse leukemia cells and in isolated DNA from bacteriophage PM2. DNA single-strand breaks and modifications sensitive to various repair endonucleases (Fpg protein, endonuclease III, exonuclease III, T4 endonuclease V) were quantified. Illumination of cell-free DNA in the presence of 2-HPT and 4-HPT gave rise to damage profiles characteristic for hydroxyl radicals, i.e. single-strand breaks and the various endonuclease-sensitive modifications were formed in the same ratios as after exposure to established hydroxyl radical sources. In contrast, HAT plus light gave rise to a completely different DNA damage profile, namely that characteristic for singlet oxygen. Experiments with various scavengers (t-butanol, catalase, superoxide dismutase) and in D2O as solvent confirmed that hydroxyl radicals are directly responsible for the DNA damage caused by photoexcited 2-HPT and 4-HPT, while the damage by HAT plus light is mediated by singlet oxygen and type I reactions. The type of DNA damage characteristic of hydroxyl radicals was also observed in L1210 mouse leukemia cells when treated with 2-HPT plus light or with H2O2 at 0 degrees C. t-Butanol (2%) inhibited the cellular DNA damage by approximately 50%. A dose of 2-HPT plus light that generated single-strand breaks at a frequency of 5 x 10(-7)/bp was associated with 50% cell survival. No DNA damage and cytotoxicity was observed after treatment with 2-HPT in the dark. We propose that 2-HTP and 4-HTP may serve as new agents to study the consequences of DNA damage induced by hydroxyl radicals in cells. In addition, the data provide direct evidence that hydroxyl radicals are ultimately responsible for the genotoxic effects caused by H2O2 in the dark.

[Radiolysis and variation of anti-microbial activity of gamma-irradiated acrinol aqueous solution on radiosterilization].

The investigations about the radiolysis materials and their quantities, and, anti-microbial activities of gamma-irradiated (in the ranges of 0.516-2.064 kC/kg (2-8 MR] acrinol on liquid dosage form have been carried out to study the application of radiosterilization. About nine components were found as radiolysis materials. Most of them were also found in the UV-irradiated of Fenton's reagent-treated acrinol solution. Increase of anti-microbial activity was observed with gamma-irradiated acrinol solutions, but this phenomenon was not long-lasting. The micro-organism such as Pseudomonas aeruginosa or Staphylococcus aureus that infect at the lips of wound are highly sensitive to the gamma-irradiation. They are almost sterilized by the irradiation of 10 kGy (1.0 Mrad). At a low acrinol concentration, the decomposition rate of acrinol by the irradiation was relatively high. When 1.0% of acrinol solution was irradiated at a dose of 10 kGy (1.0 Mrad), the decomposition of the drug was less than 2% and the variation of anti-microbial activity was negligible.

[Radiolysis of acrinol in radio-sterilization of acrinol pharmaceuticals].

Acrinol in dry solid state after gamma irradiation with 80 kGy (8 Mrad) undergoes 8% decomposition, with G(-M)=30, and in wet solid state (10% moisture content) undergoes 15% decomposition, with G(-M)=56, as shown by absorbed spectrometric measurement. From these results, it is estimated that the irradiation dose of 25 kGy (2.5 Mrad) prefered by many countries as sterilization dose will achieve the radiolysis rate of 2.8% or 4.7% for dry solid state or wet solid state acrinol, respectively. The stickiness power of plaster tape made of raw rubber and polyterpene resin don't decrease with irradiation dose of less than 40 kGy (4 Mrad). Then, the radio-sterilization on the dose of 25 kGy (2.5 Mrad) may be applied to solid pharmaceuticals such as commercial rubber adhesive plaster with acrinol pad. Besides, acrinol in 0.1% aqueous solution after irradiation of 10 kGy (1 Mrad) at room temperature undergoes 20% decomposition, with G(-M)=0.5. The radiolysis rate in this state is estimated 45% with the dose of 25 kGy (2.5 Mrad). Then, the radio-sterilization with this dose must not be applied to liquid pharmaceuticals such as 0.1% acrinol aqueous solution. While, it was reported by Hosobuchi and Sato that the antimicrobial effect of irradiated acrinol to Staphlococcus aureus increased with irradiation dose. Then, it is expectable that some materials with antimicrobial activity are prodused by gamma irradiation.

Visible-light-induced OH radicals in DNA-proflavine complexes: an e.p.r. and spin trapping study.

Using the spin-trapping technique, irradiation with visible light of complexes between DNA and proflavine was shown to generate OH radicals. The characteristic spectra were not obtained when proflavine or DNA were irradiated alone, nor when oxygen was absent. Using DMPO as a spin trap we found that the intensity of the DMPO-OH e.p.r. signal was enhanced when the molar ratio between bound proflavine and the DNA phosphorus increased up to a value of 0.15 demonstrating the efficiency of the intercalated dye molecules. A strong decrease of the e.p.r. signal was observed in the presence of various OH. scavengers like t-butanol, isopropanol or sodium benzoate. The OH. production was also decreased when the irradiation was made in the presence of SOD, ceruloplasmin or catalase and after addition of Chelex 100 resin.

Mechanism for strand-break induction in DNA-proflavine complexes exposed to visible light.

Proflavine bound-superhelical phi XRFI DNA Molecules undergo single-strand scission upon irradiation with visible light at high fluence rate. As shown by agarose gel electrophoresis analyses, the nicking reaction is (i) oxygen-dependent, (ii) strongly inhibited by catalase and an electron scavenger such as cystamine, and (iii) totally suppressed by ceruloplasmin and radical scavengers such as t-butanol sodium benzoate. This indicates that H2O2, e-, O2 and OH, respectively, are involved in the cleavage process. NaN3, a singlet-oxygen quencher, has very little effect on strand-breakage but it prevents almost completely the alteration of guanine residues (a lesion already observed after irradiation at low fluence rate). Since, in the presence of NaN3, strand scission can occur and guanine (as the other bases) recovered intact, it follows that the radical intermediates produced during breakages are probably not involved in any permanent modification of the DNA bases.

Flavin-catalyzed photooxidation of an acridan drug: a reinvestigation.

Anaerobic irradiation of flavin derivatives (IIalpha and IIbeta) in the presence of 2-chloro-9-(3-dimethylaminopropyl)acridan phosphate (Ialpha) resulted in the reduction of the flavins to IValpha and IVbeta and oxidation of an equimolar amount of Ialpha to IIIalpha. This photoreduction occurred from the first excited triplet state of IIalpha and IIbeta and proceeded via a covalent intermediate (V) between flavin and the acridan derivative. If the N-10 of the acridan was blocked by a methyl group, e.g., Ic, V could be observed spectrophotometrically at -40 degrees, decomposing homolytically on warming. With N-10 unsubstituted acridan derivatives, e.g., Ialpha and Ibeta, V could not be observed because of fast heterolytic decomposition, yielding IValpha, IVbeta, and the acridine compounds IIIalpha and IIIbeta. All photoreactions showed a kinetic isotope effect between 1.50 and 2.20 when position 9 of the acridan compounds was substituted by deuterium instead of hydrogen.

Effect of gamma-irradiation on dye-DNA binding.

The effect of gamma-rays on the binding of proflavine and acridine orange to DNA was investigated by spectrophotometry. The effect of irradiation was observed on the buffered solutions of the free dye and free DNA. A dose of about 35 krad caused a hyperchromicity of 30-40 per cent to the DNA peak at 258 nm, while the same dose introduced a hypochromic effect to the monomer peaks of the dyes by 30 per cent. This implied that gamma-rays have an effect of decreasing the monomer concentration of free-day molecules in solution. From the results, we conclude that more dye is bound to the changed conformation of dye-bound DNA on irradiation. Scratchard-binding isotherms drawn for the unirradiated and irradiated complexes of Pf-DNA showed interesting differences. Similar isotherms could not be obtained for the acridine orange-DNA system.