J. Cadet

Photosensitized reactions of nucleic acids

The main effects of near-ultraviolet and visible light on cellular DNA are reviewed with emphasis on base lesions, oligonucleotide single-strand breaks and DNA-protein cross-links. Model system photosensitization reactions of DNA are also discussed. This includes photodynamic effects, menadione-mediated photooxidation, photoionization of antibiotics, the photochemistry of 5-halogenopyrimidines and urocanic acid.

Sensitized Photo-oxidation of Thymidine by 2-methyl-1,4-naphthoquinone. Characterization of the Stable Photoproducts

The near ultraviolet photolysis of an aerated aqueous solution of thymidine containing 2-methyl-1,4-naphthoquinone gives rise to two main classes of photoproducts as a result of the initial formation of a pyrimidine radical cation. These photo-oxidation products have been separated by high performance liquid chromatography and further characterized by various spectroscopic techniques including fast atom bombardment mass spectrometry and high field 1H and 13C nuclear magnetic resonance analysis. This photoreaction constitutes an excellent model to study the chemical properties of the thymidine radical cation which is expected to be one of the primary consequences of the direct effects of ionizing radiation.

PHTHALOCYANINE AND NAPHTHALOCYANINE PHOTOSENSITIZED OXIDATION OF 2′‐DEOXYGUANOSINE

Abstract— The photodynamic properties of the di‐and tetrasulfonated zinc and aluminium phthalocyanines and a tetrasulfonated aluminium napththalocyanine were studied using 2′‐deoxyguanosine as a DNA model compound. The major photooxidation products of this nucleoside were identified and classified according to their formation through a radical mechanism (type I) or a singlet oxygen mediated mechanism (type II). The major type I product was obtained and identified as 2,2‐diamino [(2‐deoxy‐β‐d‐erythropentofuranosyl)‐4‐amino]‐5(2H)‐oxazolone. Two major type II products were characterized as the 4R* and 4S* diastereomers of 9‐(2‐deoxy‐β‐d‐erythropentofuranosyl)‐7,8‐dihydro‐4‐hydroxy‐8‐oxoguanine. In addition a third product, also resulting from a type II photooxidation, was identified as 8‐oxo‐7,8‐dihydro‐2′‐deoxyguanosine. Quantification of these products provided a means to estimate the contribution of type I and type II pathways during the phthalocyanine and naphthalocyanine mediated photooxidation of 2′‐deoxyguanosine, confirming the major role of singlet oxygen in these processes.

[52] Photodynarnic methods for oxy radical-induced DNA damage

Publisher Summary DNA damage in a cell associated with oxidative stress or exposure to ionizing radiation is in part induced by initial OH radical attack on DNA constituents. These reactions modify the chemical structure of DNA subunits, and they mark the onset of subsequent biochemical and biological effects observed in OH-generating systems. This chapter discusses the types of stable products that arise from the reactions of OH radicals with pyrimidine nucleosides of DNA and provides some insight into the rather complicated mechanism by which they are formed. Thymidine (dThd) and 2′-deoxycytidine (dCyd) are degraded into a mixture of products by photooxidation using 2-methyl-l,4-naphthoquinone (MQ) as a sensitizer and near-UV light (h > 320 nm). The same types of pyrimidine products are formed by this photochemical reaction as by OH radical-induced degradation in aqueous oxygenated solutions. However, the attack of OH radicals on nucleosides takes place at several sites on both the pyrimidine base (OH radical addition either at C-5 or C-6; H-abstraction from the C-5 methyl group of thymidine) and the deoxyribose moiety (H-abstraction from several sites), whereas in the photochemical reaction, the primary precursor of all products is generated specifically. Consequently, the product mixtures are cleaner and the products can be obtained in higher yields compared to using OH-generating systems. The chromatography techniques including high-performance liquid chromatography (HPLC) and thin layer chromatography (TLC) are presented in the chapter to provide analysis of these products.

MENADIONE SENSITIZED PHOTOOXIDATION OF NUCLEIC ACID and PROTEIN CONSTITUENTS. AN ESR and SPIN‐TRAPPING STUDY

The menadione photosensitized reactions of nucleic acid and protein constituents were studied by ESR and spin trapping. Thymine, thymidine, cytosine, 2′‐deoxycytidine,5′‐dCMP, uracil and several N‐acetyl amino acids and dipeptides were investigated. Photolysis at 335 nm was carried out in air‐saturated or Ar saturated DMSO : H2O (1 : 1, vol/vol) containing 10 3M menadione and 10‐2M 2‐methyl‐2‐nitrosopropane as the spin trap. The observed spin adducts were explained in terms of electron transfer from the substrate to the excited triplet state of menadione to form the radical cation of the substrate and the anion radical of menadione which was also detected by ESR.

Phthalocyanine and naphthalocyanine photosensitized oxidation of 2'-deoxyguanosine : distinct type I and type II products

Abstract

Reversed-phase high-performance liquid chromatography—thermospray mass spectrometry of radiation-induced decomposition products of thymine and thymidine

High-performance liquid chromatography-thermospray mass spectrometry was applied to the analysis of various radiation-induced decomposition products of thymidine including N-(2-deoxy-beta-D-erythro-pentofuranosyl)formamide and the various diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine, 5-hydroxy-5,6-dihydrothymidine and 5,6-dihydrothymidine. This method combines high sensitivity and product resolution, rendering it particularly useful for monitoring the formation of radiation-induced base damage within DNA.

Photo and radiation-induced formation of thymidine hydroperoxides☆

Abstract Several thymidine hydroperoxides have been found to be key intermediate products in two important free-radical processes. Radiolysis of oxygenated aqueous solutions of thymidine, which involves initially the reactions of hydroxyl radicals with thymidine, led to the formation of 13 hydroperoxides of which 6 were identified as 5(6)-hydroxy-6(5)-hydroperoxides. This finding can only be explained by the addition of hydroxyl radicals to C(5) and C(6) of the thymidine double bond. In contrast, menadione photosensitization of thymidine in aqueous oxygenated solutions, which initially generates the thymidine cation radical, led specifically to 4 isomeric 6-hydroxy-5-hydroperoxides of thymidine and 5-hydroperoxymethyl-2′-deoxyuridine. The lack of the isomeric 5-hydroxy-6-hydroperoxides and 18O labelling experiments suggests that the radical cation of thymidine is selectively hydrated at C(6) of thymidine.