Chitta R. Paul

Radiation chemistry of d(TpApCpG) in oxygenated solution.

The radiation chemistry of the DNA tetranucleoside triphosphate d(TpApCpG) was investigated. The tetramer was X-irradiated in oxygenated aqueous solution and the various products separated by high-performance liquid chromatography. The principal modifications of the tetramer were analysed intact using nuclear magnetic resonance (NMR) spectroscopy and in some instances also by fast atom bombardment (FAB) mass spectrometry. The principal radiation-induced lesions of d(TpApCpG) were found to be a formamido modification derived from the thymine base, two stereoisomeric forms of a 1-carbamoyl-2-oxo-4,5-dihydroxyimidazolidine modification derived from the cytosine base and an 8-hydroxyguanine modification.

Radiation chemistry of 2'-deoxycytidylyl-(3'-5')-2'-deoxyguanosine and its sequence isomer in N2O- and O2-saturated solutions.

The radiation chemistry of the dinucleoside monophosphate d(CpG) and its sequence isomer, d(GpC), has been examined in aqueous solutions saturated with either N2O or O2. The products were isolated using HPLC, and the major products were identified using proton NMR spectroscopy and mass spectrometry. The major products include 5,6-dihydroxy-5,6-dihydrouracil (glycol) derivatives, 5- and 6-hydroxycytosine substitution products, 1-carbamoyl-2-oxo-4,5-dihydroxyimidazolidine products, and the 8-hydroxyguanine substitution product. Both trans stereoisomers of the imidazolidine derivatives are obtained from d(CpG) as well as from its sequence isomer. These are prominent products when the irradiation is carried out in the presence of oxygen, but they are not observed in the absence of oxygen.

Characterization of radiation-induced damage in d(TpApCpG).

The radiation chemistry of the oligomer d(TpApCpG) X-irradiated in aqueous solution containing glutathione was studied. Four products were isolated by HPLC and characterized by NMR spectroscopy. Two of the major products are isomers of a 5-hydroxy-5,6-dihydrothymine modification of d(TpApCpG). A dihydrothymine modification is also formed. The other major product is a result of strand scission. These products are different from the major products identified previously in a study of d(TpApCpG) X-irradiated in oxygenated solution. The effect of specific radiation-induced lesions on the behaviour of d(TpApCpG) as substrate to a spleen phosphodiesterase-micrococcal nuclease combination of enzymes and to nuclease P1 was studied. These enzymes are of interest because they are used in postlabelling assays of DNA damage.

Characterization of radiation and autoxidation-initiated damage in DNA model compounds

The damage caused by ionizing radiation and by autoxidation processes to the base moiety of deoxydinucleoside monophosphates d(TpA) and d(GpC) exposed in oxygenated solutions has been investigated. The same principal products are generated by both modalities. The products generated in largest yield have been identified as formamide and imidazolidine modifications derived from damaged thymine and cytosine moieties, respectively. These radiation-induced and autoxidation-induced lesions have been isolated by HPLC and characterized by 1H NMR spectroscopy including two-dimensional COSY spectroscopy. The possible biological significance of these lesions is discussed.