- Generation and Characterization of Psoralen Radical Cations
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Radical cations of psoralen, 8-methoxypsoralen (8-MOP) and 5-methoxypsoralen have been generated by photosensitized electron transfer in acetonitrile and aqueous buffer/acetonitrile (1:1) and have absorption maxima at 600, 650 and 550 nm, respectively. The radical cations have lifetimes of ～5 us under these conditions, are unreactive toward oxygen and show behavior typical of arylalkene radical cations in their reactivity toward nucleophiles and the precursor psoralens. Direct 355 nm excitation of 8-MOP in aqueous buffer at physiological pH results in monophotonic photoionization to give 8-MOP.+ with a quantum yield of 0.015. The 8-MOP.+ reacts with both guanosine and adenosine mononucleotides (k = 2.5 × 109 and 3.4 × 107 M-1 s-1, respectively) via electron transfer to give the purine radical cations, but does not react with pyrimidine mononucleotides. These results suggest that reactions of psoralen radical cations generated by electron transfer or photoionization may be involved in psoralen/UVA therapy.
- Wood, Paul D.,Johnston, Linda J.
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p. 642 - 648
(2007/10/03)
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- Ionization of purine nucleosides and nucleotides and their components by 193-nm laser photolysis in aqueous solution: Model studies for oxidative damage of DNA 1
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The effect of 20-ns pulses of 193-nm laser light on aqueous solutions of purine bases, (2′-deoxy)nucleosides, and (2′-deoxy)nucleotides was investigated, and monophotonic ionization was observed. Although (deoxy)ribose and (deoxy)ribose phosphates are also ionized by 193-nm light, the photoionization of the (deoxy)nucleosides and -tides takes place predominantly (90%) at the purine moiety, due to the much higher extinction coefficients at 193 nm of the bases as compared to the (deoxy)ribose phosphates. The quantum yields of photoionization (φPl) of the purines are in the range 0.01 to 0.08, based on φ(Cl-) at 193 nm of 0.46. As shown by comparison with data obtained from pulse radiolysis, the ionized purines, i.e., the radical cations, deprotonate in neutral solution, yielding neutral radicals. The radical cation of 1-methylguanosine, produced by photoionization in oxygen-saturated aqueous solution, deprotonates with the rate constant 3.5 × 105 s-1. In the absence of oxygen, the hydrated electrons resulting from the photoionization react with the untransformed purine derivatives to yield the corresponding radical anions. As these are rapidly protonated by water (as concluded from pulse radiolysis), the photoionization in deaerated neutral solution results in two different neutral radicals: a deprotonated radical cation and a protonated radical anion.
- Candeias,Steenken
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p. 699 - 704
(2007/10/02)
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- Photochemical Reactions of Triplet Acetone with Indole, Purine, and Pyrimidine Derivatives
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The photochemical reactions of triplet acetone with indole, indole derivatives (1-methyl-, 2-methyl-, 3-methyl-, 5-methyl-, and 7-methylindole, and tryptophan), purine derivatives (caffeine, 7-methylguanine, adenine, adenosine, and guanosine), and a pyrimidine derivative (thymine) have been studied in aqueous solutions by using a KrF or ArF laser.The quenching processes of triplet acetone by indoles, being diffusion controlled, occur via the following paths: triplet-triplet energy transfer, electron transfer, photoaddition of triplet acetone to the 2-carbon atom of the indole ring, and deactivation without a chemical change.The yields of energy transfer, electron transfer, and photoaddition were determined from absorbance measurements.The transient absorptions due to triplet states were observed for caffeine, 7-methylguanine, and thymine, while weak transient absorptions which showed apparent second-order decays were observed for adenine, adenosine, and guanosine.Triplet acetone is quenched mainly via T-T energy transfer in caffeine, 7-methylguanine, and thymine.The weak absorptions may be attributed to neutral radicals in adenine and adenosine and to a cation radical in guanosine.
- Kasama, Kunihiko,Takematsu, Akiko,Arai, Shigeyoshi
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p. 2420 - 2427
(2007/10/02)
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