Alkylation of anionic DNA bases by styrene 7,8-oxide

Styrene 7,8-oxide (SO), a reactive epoxide that has been classified as a probable human carcinogen, was allowed to react under alkaline conditions with 2′-deoxyguanosine and thymidine monophosphates as well as with the dinucleotide dGpdT. The reaction products were separated by high-performance liquid chromatography and were characterised by UV and electrospray mass spectroscopy, the latter showing the ability to differentiate the isomerism of the hydroxyphenylethyl moiety of the adduct. The main alkylation products of the deoxyguanosine monophosphates at high pH were those reacted at the 1- (through the β-carbon of the epoxide), N2- (α-carbon) and 7-positions (β- and α-carbons) of guanine followed by alkylation of the phosphate group. The formation of a novel diastereomeric pair of N 2-guanine adducts connected to the β-carbon of SO was identified and characterised. Furthermore, two different geometrical isomers of 1-guanine were detected. For thymidine nucleotide, base alkylation under neutral conditions was almost negligible, but at pH 10.5, alkylation at the 3-position was very prominent. The same base-adducts and the pH-effects were observed in the case of dGpdT alkylation. However, no phosphate alkylation was detected in the case of the dinucleotide.