2'-Deoxyguanosine (DG) oxidation and strand-break formation in DNA by the radicals released in the photolysis of N-tert-butoxy-2-pyridone. Are tert-butoxyl or methyl radicals responsible for the photooxidative damage in aqueous media?

Reactivity and Product Analysis of a Pair of Cumyloxyl and tert-Butoxyl Radicals Generated in Photolysis of tert-Butyl Cumyl Peroxide

Alkoxyl radicals play important roles in various fields of chemistry. Understanding their reactivity is essential to applying their chemistry for industrial and biological purposes. Hydrogen-atom transfer and C-C β-scission reactions have been reported from alkoxyl radicals. The ratios of these two processes were investigated using cumyloxyl (CumO^•) and tert-butoxyl radicals (t-BuO^•), respectively. However, the products generated from the pair of radicals have not been investigated in detail. In this study, CumO^• and t-BuO^• were simultaneously generated from the photolysis of tert-butyl cumyl peroxide to understand the chemical behavior of the pair of radicals by analyzing the products and their distribution. Electron paramagnetic resonance and/or transient absorption spectroscopy analyses of radicals, including CumO^• and t-BuO^•, provide more information about the radicals generated during the photolysis of tert-butyl cumyl peroxide. Furthermore, the photoproducts of (3-(tert-butylperoxy)pentane-3-yl)benzene demonstrated that the ether products were formed in in-cage reactions. The triplet-sensitized reaction induced by acetophenone, which is produced from CumO^•, clarified that the spin state did not affect the product distribution.

Mechanism of coralyne-mediated DNA photo-nicking process

Previously, we reported that coralyne and UVA combination sensitized a wide range of human carcinoma cells regardless of their p53 status. The coralyne induced photosensitization of cancer cells may be clinically attractive, as mutation in the p53 gene is prevalent in many types of tumors. Coralyne mediated photosensitization of cancer cells is attributable to its ability to cause extensive DNA single strand breaks (SSB). However, the precise mechanism of coralyne induced DNA photo-damage is not yet known. The present study was aimed to understand the hitherto unknown mechanism of the coralyne-induced DNA photo-cleavage process. To this end, we compared the DNA photo-nicking properties of berberine, jatrorrhizine and coralyne, and deciphered involvement of the photochemical processes in the photo-nuclease action of coralyne using absorption and electron spin resonance spectroscopy, high performance liquid chromatography and mass spectroscopy (MS) techniques in conjunction with relevant in vitro studies with plasmid DNA. In association with UVA, coralyne, but not berberine and jatrorrhizine induced significant nicking of plasmid DNA via an O₂-independent photo-chemical process. The Job's plot of our spectrophotometric data suggested that one coralyne molecule remains intercalated with two DNA base pairs (i. e., 1:2) and starts forming aggregates beyond this molar ratio. The DNA photo-nicking by the combination of coralyne and UVA (designated as CUVA) was primarily caused by the coralyne aggregates without any significant contribution from the DNA-intercalated coralyne monomer.

Chemiluminescence of Cigarette Smoke: Salient Features of the Phenomenon

The study disclosed herein provides for the first time a detailed experimental support for the general mechanism of the cigarette-smoke-derived chemiluminescence, as an example par excellence of the excited-state generation in a chemically complex aerosol medium. The mechanism involves chemiexcitation in a unimolecular transformation of the smoke-borne free radical species. However, the concentration of these radicals, [r∙], obeys a bimolecular (second-order) kinetics and depends on a particulate-phase content (total particulate matter, TPM) of the cigarette smoke. The decrease in [r∙] with increasing the TPM amount manifests radical-scavenging propensity of the smoke particulate phase. Astonishingly, no energy transfer takes place from the primary excited light-emitting species to luminophoric molecules abundant in the smoke. The reported results build up fundamentals of a facile chemiluminescence assay for free radical properties of the smoke. The experimental approaches developed for this study are of general scope and may be used for mechanistic elucidation of the excited-state generation in chemical systems and environments of an arbitrary complexity.

Computer-aided design of promising photochemical alkoxy radical precursors

A computer-aided design of alkoxyl radical precursors is performed. The new precursors should combine the advantages of N-alkoxypyridine-2(1H)thiones (less reactive radicals) and N-alkoxythiazole-2(3H)thiones (stable with respect to daylight). Additionally, the radical liberation process should be initiated by light with a wavelength of around 350 nm. To find promising compounds, 18 test candidates were obtained by a systematic variation of the parent compound N-alkoxythiazole-2(3H)thione. The properties of the test molecules were computed by a protocol that was already successfully used to rationalize the photochemical behavior of N-alkoxypyridine-2(1H)thiones and N-alkoxythiazole-2(3H)thiones. The computations identify two promising new compounds. For N-methoxy-(1,3)dihydro-[1,3]azaphosphole-2-thione (6a), they predict that the fragmentation process will be initiated by an absorption at 348 nm. An analysis of its fragmentation process indicates that the free excess energy of the resulting radicals should more resemble the situation found for N-alkoxypyridine-2(1H)thiones. For N-methoxy-(1,3)dihydro-pyrrole-2-thione (3a), the excitation energy is somewhat higher (330 nm), but the computed fragmentation paths again indicate that the remaining excess energy of the released radicals is quite favorable. The test molecules also contained the experimentally well-known N-methoxypyridine-2(1H)one (1b). For this molecule, our computed data rationalizes nicely the experimental findings.

Cytotoxicity and genotoxicity induced by the photochemical alkoxyl radical source N-tert-butoxypyridine-2-thione in L5178Y mouse lymphoma cells under UVA irradiation

The cell-damaging effects of N-tert-butoxypyridine-2-thione (tBuOPT), which generates alkoxyl and thiyl radicals on photolysis, have been investigated in L5178Y mouse lymphoma cells. The UVA irradiation of 2.5 microM tBuOPT inhibits strongly cell growth and cell viability, causes pronounced membrane damage, and induces micronuclei. Without irradiation, tBuOPT does not cause any cell damage at 2.5 microM concentration. The phototoxicity of tBuOPT is effectively inhibited by the radical scavenger glutathione, while the photogenotoxicity (micronuclei induction) is not affected by this strong hydrogen-atom donor. Thus, for the cytotoxicity and genotoxicity different reactive species seems to be responsible. The cytotoxicity is presumably caused by oxyl radicals, which are derived from tert-butoxyl radicals generated by photocleavage of tBuOPT, while in the genotoxicity the less reactive pyridyl-2-thiyl radicals appear to play a role. These results demonstrate that N-alkoxypyridinethiones are useful photochemical sources of oxyl and thiyl radicals to elucidate biological effects caused by these free radicals.

Photobiological model studies on perester and pyridone tert-butoxyl radical sources (photo-Fenton-type reagents): 2'-deoxyguanosine modification by methyl radicals generated through competitive beta-cleavage in aqueous media

In aqueous solution, UV irradiation of the photo-Fenton-type reagents perester 1 and N-tert-butoxypyridone 2 leads to the formation of tert-butoxyl radicals and methyl radicals as confirmed by product studies and by spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), followed by EPR spectroscopy. The methyl radicals result from the initially released tert-butoxyl radicals through beta-cleavage, a fragmentation pathway, which dominates at lower DMPO concentrations. In the presence of 2'-deoxyguanosine (dG), both photochemical radical sources afford 8-MedG [(2.3 +/- 0.3)%] and N7-MedG [(0.27 +/- 0.05)%] as modified products, whereas the direct reaction of the tert-butoxyl radicals with dG cannot compete with this beta-cleavage process.