Damage induced in DNA and RNA components by low energy heavy ions, electrons and photons
Ion beams have recently emerged as a promising radiotherapy technique for treatment of large, solid, deep seated tumours. As the primary beam of heavy particles traverses through the cell, secondary charged and neutral species are generated. In particular secondary ions with energies up to 100 eV are produced in large numbers along the ionization track. Ionization and fragmentation of DNA components is a key step in radiobiological damage to the cell. An ultrahigh vacuum (UHV) ion beam system has been used to study the ionization and fragmentation pathways of DNA bases by ion irradiation in the condensed phase and electron impact in the gas phase. The apparatus consists of a low energy ion source, beam line, biomolecular film preparation system and a reaction chamber with high-resolution mass spectrometer to monitor desorbing ion yields. Solid condensed films of DNA bases were prepared in vacuo by sample evaporation from an oven and were subsequently irradiated with (1-100 eV) Ar+ ions. Upon bombardment, desorbing positive and negative fragments were collected using a Quadrupole Mass Spectrometer (QMS). Ionization and fragmentation of Adenine (A), Guanine (G), Cytosine (C) and 5-aminouracil bases has been observed by low energy (10-100 eV) Ar + ions in the condensed phase and 70 eV electrons in the gas phase. These observations demonstrate fragmentation mechanisms involve site specific concerted dissociation reactions, deamination of Ade, Gua, Cyt in the condensed phase and the gas phase as well as amination of Adenine in the condensed phase. Of significant features of the mass spectra of all four bases are the production of NH 4+ (18 amu) and CH3+ (15 amu) fragments with high intensities relative to the most intense peak in each spectrum (HCNH+ , 28 amu). Utilizing isotopically labelled Ade, Gua and Cyt common purine and pyrimidine bond cleavage pathways and fragment origin sites were identified. Experiments performed with 5-aminouracil confirmed the deamination hypothesis of A, G and C bases by low energy Ar + ion impact in the condensed phase as well as the low energy electron impact in the gas phase.