Low-energy (1-500 eV) ion-biomolecule interactions in the condensed phase
An understanding of all nascent events leading to radiolytic DNA damage is required to achieve a complete description of ionizing radiation effects on living cells. These early, sub-picosecond events, involve the production of numerous low-energy (E < 20 eV) secondary electrons, neutral radicals, and secondary ions. Secondary ions, created either by the primary radiation, or by secondary electrons via resonant processes, are produced all along the primary radiation track with energies from a few eV up to several hundreds eV depending on their formation mechanism. Although much recent attention has focused on DNA damage initiated by secondary electrons, the subsequent reactive damage induced by the equally reactive non-thermal ionic species remain virtually unexplored. This work focuses on the interaction dynamics of low energy (1-500 eV) ions (secondary ions or primary ions at track-end energies) with films of DNA components (thymine, deuterated thymine, uracil, bromouracil), or other biomaterials, to understand the basic mechanisms of low energy ion-induced DNA damage. A novel ultra-high vacuum ion beam system, adapted to the study of organic solids, has been specially designed and constructed to conduct these studies. The unique design of the apparatus allows irradiation of biomolecular films with highly focused, mass and energy selected atomic and molecular ion beams in the 1-500 eV energy range. We show that atomic and molecular, singly or doubly charged primary cation impact (e.g. Dn+ , n=1-3, He+ , N+ , N2+ , Ar+ , Ar++ ), at ion energies between 10-500 eV in 1-2 nm thick films of thymine completely fragment the molecules, even at energies as low as 15 eV (near 0.5 eV/amu for Ar+ ), and result in the formation of over 30 different secondary cation and anion fragments with energies below 5 eV. Many of these fragments react in the films before desorption, and result in formation of new products. We also show that fragment anions with energies below 5 eV, produced by secondary electrons, can lead to further physico-chemical damage in organic films. Our experiments also reveal strong reactive scattering channels for some projectiles, e.g. HeH+ formation and desorption for thymine films irradiated with 10-300 eV He+ ion beams. Our present work brings, for the first time ever, clear evidence that hyperthermal ion-interactions with DNA bases lead to severe damage, even at incident ion energies as low as 10-15 eV. The results of our preliminary studies, presented here, already challenge traditional models stipulating that heavy ion irradiation causes DNA damage via similar pathways as conventional ionizing radiation, by showing that primary and secondary ions incident on organic media induce significantly greater damage that reaches beyond single ionization, or free radical production. This has important implications for example in radiation therapy where the significant contribution of secondary ions and primary ions at track end energies cannot be forgotten anymore and left out of dose calculations.