Photochemistry of aqueous solutions of benzazolo[3,2-a]quinolinium salts. A spin-trapping study using 17O-enriched water and oxygen

EPR Evidence for Mechanistic Diversity of Cu(II)/Peroxygen Oxidation Systems by Tracing the Origin of DMPO Spin Adducts

Electron paramagnetic resonance (EPR) has been extensively used for the identification of free radicals that are generated from advanced oxidation processes (AOPs) so as to establish the reaction mechanism. However, some misinterpretations or controversies on the identity of detected EPR signals remain in the literature. This study, with Cu(II)-based AOPs as examples, comprehensively investigated the origin of 5,5-dimethyl-l-pyrroline N-oxide (DMPO) adducts in Cu(II) alone, Cu(II)/H₂O₂, Cu(II)/peroxymonosulfate (PMS), and Cu(II)/peroxydisulfate (PDS) systems. In most Cu(II) systems, DMPO-OH signals can be detected even without any peroxygens, indicating the presence of other origins of this adduct in addition to the genuine spin trapping of ^•OH by DMPO. According to the formed secondary radical adducts (DMPO-OCH₃ from a nonradical process or DMPO-CH₂OH from a radical oxidation) derived from methanol quenching, we propose that CuO⁺, instead of free radicals, is involved in the Cu(II)/PMS system, while ^•OH is indeed generated in the Cu(II)/H₂O₂ and Cu(II)/PDS systems under neutral conditions. Notably, ¹⁷O-incorporation experiments demonstrate that -OH in the detected DMPO-OH adduct originates 100% from water in the Cu(II) alone system but the amount of -OH is over 99.8% from the oxidant while peroxygens are added. In addition, DMPO-O₂^- appears only in the Cu(II)/PDS system under highly alkaline conditions and H₂O is not involved in superoxide formation.

Photosensitized oxidation of hypoxanthine and xanthine by aluminum phthalocyanine tetrasulfonate. Role of the alkylating quinone 2,5-dichloro-diaziridinyl-1,4-benzoquinone

Photoirradiation of nitrogen-saturated aqueous solutions containing aluminum phthalocyanine tetrasulfonate (AlPcS4) at 675 nm in the presence of 2,5-dichloro-diaziridinyl-1,4-benzoquinone (AZDClQ) and hypoxanthine (HX) produces the oxidized HX derivatives, xanthine (X) and uric acid (UA). Concentrations of the AZDClQ semiquinone, X and UA increase at the expense of HX with an increase in irradiation time. Almost negligible decomposition of HX, as well as very low amounts of X, are detected if photolysis occurs under identical conditions but in the absence of AZDClQ. Addition of calf-thymus DNA produces quinone-DNA covalent adducts after photolysis of anaerobic samples containing quinone, DNA and AlPcS4, in the presence or absence of HX and at pH 5.5. However, larger amounts of quinone-DNA adducts are detected if HX is present. The results presented here could have applications in the photodynamic treatment of hypoxic tissues such as solid tumors, under conditions of high HX concentration, where Type-I pathways could be more important than singlet oxygen generation.

Photochemistry of water-soluble quinones. Production of a water-derived spin adduct

The photolyses of phosphate-buffered (pH 7) air- and nitrogen-saturated solutions containing the water-soluble quinones, 1,4-benzoquinone (BQ), 2-methyl-1,4-benzoquinone (MBQ), sodium 1,4-naphthoquinone-2-sulfonate (NQ2S), 9,10-anthraquinone-2-sulfonate (AQ2S) or 9,10-anthraquinone-1,5-disulfonate (AQDS), and the spin trap 5,5-dimethylpyrroline-1-oxide (DMPO) produce a DMPO-OH adduct. Electron paramagnetic resonance spectroscopy of the photolyzed samples in 17O-enriched water demonstrates that this adduct derives almost exclusively from water. With the exception of BQ, quantum yields for the formation of DMPO-OH are larger in air than in nitrogen-saturated samples, thus supporting the idea of the formation of air-oxidized intermediates that enhance the DMPO hydroxylation reaction rate. Evidence has been obtained which suggests that BQ and MBQ, but not AQDS, are able to photooxidize water, with the consequent production of the free OH radical.