While matter is irradiated with highly-energetic particles, it may become chemically modified. Thereby, the reactions of free low-energy electrons (LEEs) formed as secondary particles play an important role. It is unknown to what degree and by which mechanism LEEs contribute to the action of electron-affinic radiosensitisers applied in radiotherapy of hypoxic tumours. Here we show that LEEs effectively cause the reduction of the radiosensitiser nimorazole via associative electron attachment with the cross-section exceeding most of known molecules. This supports the hypothesis that nimorazole is selectively cytotoxic to tumour cells due to reduction of the molecule as prerequisite for accumulation in the cell. In contrast, dissociative electron attachment, commonly believed to be the source of chemical activity of LEEs, represents only a minor reaction channel which is further suppressed upon hydration. Our results show that LEEs may strongly contribute to the radiosensitising effect of nimorazole via associative electron attachment.

Atomic and Molecular Collisions Laboratory CEFITEC Department of Physics Universidade NOVA de Lisboa 2829 516 Caparica Portugal

Institut für Chemie und Biochemie Physikalische und Theoretische Chemie Freie Universität Berlin Takustrasse 3 14195 Berlin Germany

Institut für Ionenphysik und Angewandte Physik and Center for Biomolecular Sciences Innsbruck Leopold Franzens Universität Innsbruck Technikerstrasse 25 A 6020 Innsbruck Austria

J Heyrovský Institute of Physical Chemistry v v i The Czech Academy of Sciences Dolejškova 3 18223 Prague Czech Republic

Université de Lyon; Université Claude Bernard Lyon1; Institut de Physique Nucléaire de Lyon CNRS IN2P3 UMR 5822 69622 Villeurbanne Cedex France

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