Analysis of radiation induced nucleobase-peptide crosslinks by electrospray ionization mass spectrometry.

Upon exposure to ionizing radiation, DNA undergoes a variety of modifications including the production of a covalent bond between the nucleobase thymine and the amino acid tyrosine. These crosslinked lesions, produced in cells exposed to ionizing radiation, if unrepaired are thought to result in cell death. We have used electrospray ionization mass spectrometry (ESI-MS) to study a model system consisting of the peptide angiotensin, a 10 amino acid peptide containing only one tyrosine residue, irradiated in the presence of the nucleobase thymine. The presence of the covalently crosslinked species has been determined by ESI-MS, by the appearance of additional species in the irradiated samples which correspond to the adduction of thymine as well as a hydrated species containing thymine and water (5-hydroxy-6-hydrothymine). The formation of 5-hydroxy-6-hydrothymine adduct is reversible and the relative abundance of the thymine and 5-hydroxy-6-hydrothymine adducts is dependent on the pH of the spray solution. High resolution experiments using Fourier transform ion cyclotron resonance mass spectrometry confirms the presence of the thymine and hydrated thymine adducts. The high resolution nature of these experiments also allows the detection of a 5,6-dihydrothymine adduct.