title: Production of singlet oxygen-derived hydroxyl radical adducts during merocyanine-540-mediated photosensitization: analysis by ESR-spin trapping and HPLC with electrochemical detection.
abstract: Activated oxygen species produced during merocyanine 540 (MC540)-mediated photosensitization have been examined by electron spin resonance (ESR) spin trapping and by trapping reactive intermediates with salicylic acid using HPLC with electrochemical detection (HPLC-EC) for product analysis. Visible light irradiation of MC540 associated with dilauroylphosphatidylcholine liposomes in the presence of the spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) gave an ESR spectrum characteristic of the DMPO-hydroxyl radical spin adduct (DMPO/.OH). Addition of ethanol or methanol produced additional hyperfine splittings due to the respective hydroxyalkyl radical adducts, indicating the presence of free.OH.DMPO/.OH formation was not significantly inhibited by Desferal, catalase, or superoxide dismutase (SOD). Production of DMPO/.OH was strongly inhibited by azide and enhanced in samples prepared with deuterated phosphate buffer (PB-D2O), suggesting that singlet molecular oxygen (1O2) was an important intermediate. When MC540-treated liposomes were irradiated in the presence of salicylic acid (SA), HPLC-EC analysis indicated almost exclusive formation of 2,5-dihydroxybenzoic acid (2,5-DHBA), with production of very little 2,3-DHBA, in contrast to .OH generated by uv photolysis of H2O2, which gave nearly equimolar amounts of the two products. 2,5-DHBA production was enhanced in PB-D2O and inhibited by azide, again consistent with 1O2 intermediacy. 2,5-DHBA formation was significantly reduced in samples saturated with N2 or argon, and such samples showed no D2O enhancement. Ethanol had no effect on 2,5-DHBA production, even when present in large excess. Catalase and SOD also had no effect, and only a small inhibition was observed with Desferal. DMPO inhibited 2,5-DHBA production in a concentration-dependent fashion and enhanced formation of 2,3-DHBA. We propose that 1O2 reacts with DMPO to give an intermediate which decays to form DMPO/.OH and free.OH, and that the reaction between 1O2 and SA preferentially forms the 2,5-DHBA isomer. This latter process may provide the basis for a sensitive analytical method to detect 1O2 intermediacy. Singlet oxygen appears to be the principle activated oxygen species produced during MC540-mediated photosensitization.
authors.affiliation: Department of Radiology, Medical College of Wisconsin, Milwaukee 53226.
authors.forename: J B
authors.initials: JB
authors.lastname: Feix
authors.forename: B
authors.initials: B
authors.lastname: Kalyanaraman
mesh_terms: D002851:Chromatography, High Pressure Liquid; D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D006878:Hydroxides; D017665:Hydroxyl Radical; D010100:Oxygen / Q000737:chemistry*; D010777:Photochemistry; D011744:Pyrimidinones / Q000737:chemistry*; D011838:Radiation-Sensitizing Agents / Q000737:chemistry*; D012459:Salicylates / Q000737:chemistry
chemical_list: D003497:Cyclic N-Oxides; D005609:Free Radicals; D006878:Hydroxides; D011744:Pyrimidinones; D011838:Radiation-Sensitizing Agents; D012459:Salicylates; D017665:Hydroxyl Radical; C003954:merocyanine dye; C017245:5,5-dimethyl-1-pyrroline-1-oxide; D010100:Oxygen