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

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.

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.

GMP and AMP as methyl radical traps in the reaction with pentaamminemethylcobalt(III)

C8 methylation of the title purine nucleotides was achieved in near neutral aqueous solution by reaction with (13)C enriched [Co(NH(3))(5)(CH(3))](2+). This cation decomposes upon dissolution in water by release of a methyl radical. The latter was identified by electron spin resonance (ESR) spectroscopy as the PBN/(13)C(.-)H(3) adduct (PBN=phenyl-N-tert-butylnitrone). When the purine nucleotide was used in large excess, the efficiency of the trapping by the C8 atom was determined by integration of the (13)C NMR signals to be 20-25% for GMP and 15-20% for AMP, respectively, at 37 degrees C.

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.