Formation of DNA-protein cross-links involving the initial formation of a guanine radical cation was investigated. For this purpose, riboflavin-mediated photosensitization of a TGT oligonucleotide in aerated aqueous solution in the presence of the KKK tripeptide was performed. We have shown that the nucleophilic addition of the c-amino group of the central lysine residue of KKK to the C8 atom of either the guanine radical cation or its deprotonated form gives rise to the efficient formation of a N epsilon-(guanin-8-yl)-lysine cross-link. Interestingly, the time course of formation of the above-mentioned cross-link was found to be not linear with the time of irradiation, and its formation rapidly reached a plateau. This is explained by secondary decomposition of the initially generated cross-link which could be further oxidized more efficiently than starting TGT oligonucleotide. One-electron oxidation of the initially generated cross-link was found to produce mainly two diastereomeric cross-links exhibiting a spiroimino-trilysine-dihydantoin structure as inferred from enzymatic digestion, CD, UV, NMR and mass spectrometry measurements. In addition, other minor crosslinks, for which formation was favored at acidic pH, were assigned as lysine-guanine adducts in which the modified guanine base exhibits a guanidino-trilysine-iminohydantoin structure. A proposed mechanism for the formation of the different detected oligonucleotide-peptide cross-links is given. The high yield of formation of the detected cross-links strongly suggests that a DNA-protein cross-link involving a lysine residue linked to the C8 position of guanine could be generated in cellular systems if a lysine is located in the close vicinity of a guanine radical cation.