RADAM - BIOCLUS · The effects of radiation on biomolecular clusters: fragmentation and reactions incited by proton impact at velocities close to the Bragg peak

Final Activity Report Summary - RADAM - BIOCLUS (The effects of radiation on biomolecular clusters: fragmentation and reactions incited by proton impact at velocities close to the Bragg peak)

An original apparatus was constructed for the production, acceleration, and mass-selection of charged clusters of biologically important molecules. The system was developed notably for the production of cluster ions comprising a deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) base molecule and a fixed number of water molecules as model systems for experiments to explore the processes by which radiation could cause damage at local sites on key biological macromolecules. A proton accelerator was assembled for cross beam experiments on the cluster ions and a novel detection system was designed for the kinetic energy analysis of all the neutral and ionic fragments produced in the collisions. The technical innovations developed in this part of the project provided an exceptional platform to probe the inter-molecular reactions initiated by radiative energy deposition in a biologically relevant nano-system with key implications for the elucidation of the dose in biomedicine. A major program of proton impact experiments was carried out on isolated biomolecules utilising the developed experimental system. Through event-by-event acquisition, individual detected ions could be distinguished according to whether the removed electron had neutralised the incident proton (electron capture) or not. This information was of major importance for the understanding of radiation induced damage in biological material as free electron production was demonstrated to cause DNA strand breaks (Boudaiffa et al., Science, 287, 2000, 1658). The first measurement of the percentage of uracil ionisation events involved electron capture as a function of proton impact energy. Complementary vapour jet characterisation experiments enabled the first absolute cross-sections to be derived for nucleobase ionisation by proton impact, opening the possibility for the first quantitative comparisons with other types of irradiation. The most fundamentally important result that emerged from the project was the lack of energy-dependence in the relative production of fragment ions following proton impact ionisation of uracil. This was contrary to the widely-accepted generality that smaller impact parameters, with the projectile approaching the target more closely, led to increased molecular fragmentation. Whereas the impact parameter for the capture of an electron from a uracil molecule by an incident proton was observed to fall by a factor of 25 with increasing collision energy (20-150 keV), no evidence for any corresponding increase in uracil fragmentation upon ionisation was observed. This result highlighted the limitations of the standard association of smaller impact parameters with increased fragmentation when applied to ion interactions with electronically complex molecules. Indeed, the typical complexity of biomolecules combined with the notoriously difficult theoretical treatment of ion-molecule interactions at intermediate velocities (of the order of those of the electrons within a molecule) meant that modelling the collisions observed in the present work represented a major challenge. Experiments of the kind performed in this project were thus essential to elucidate the ion-induced fragmentation dynamics of biomolecules.

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