Characterization of singlet oxygen-induced guanine residue damage after photochemical treatment of free nucleosides and DNA.

DNA and free nucleosides were photosensitized with the DNA-binding dyes methylene blue (MB) and meso-tetra(4-N-methyl-pyridyl) porphyrin (p-TMPyP) and the non-binding meso-tetra (4-sulphonatophenyl) porphyrin (TSPP). After light exposure DNA was enzymatically digested to nucleosides. Only the guanine residues were photodegraded. By measuring optical absorption, at least 20 photoproducts were detected. Singlet oxygen (1O2) was involved in induction of all these products since D2O enhanced their yields from 4 to 10 times. The photoproducts were the same for all sensitizers. However, several photoproducts were found only with DNA or only with free 2'-deoxyguanosine. Four of 20 photoproducts were induced both in DNA and free 2'-deoxy-guanosine. The yield of the photoproduct 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) relative to the degree of 2'-deoxy-guanosine degradation depended on which sensitizer was used and on whether nucleosides or DNA was exposed. Apparently, DNA structure affected the types of as well as the yields of photo-products induced by 1O2.