| Summary: | A large number of secondary electrons with low energies (< 20 eV) is produced during irradiation of biological matter. It is known that such electrons lead to fragmentation of DNA via dissociative electron attachment (DEA). DEA happens via the formation of a resonance (transient negative ion). Studying the properties of the resonances (energy and lifetime) is the first step to describe and understand DEA. Although isolated DNA constituents have been studied broadly, DEA is likely to be affected by the environment. The goal of this research is a theoretical investigation of the influence of the environment on resonance formation. The object of this study is low-energy collision with pyridine (prototype of biological molecule) and thymine (one of the DNA nucleobases), both in gas-phase and surrounded by water (forming small clusters). We provide a comparison of pyridine and pyridine-H<sub>2</sub>O calculations for various standard scattering models (Static Exchange, Static Exchange plus Polarization and Close-Coupling). For pyridine-(H<sub>2</sub>O)<sub>n'</sub> n = 2, 3, 5, calculations have been performed using the Static Exchange method, whereas thymine and thymine-(H<sub>2</sub>O)<sub>n'</sub> n = 2, 3, 5, were studied using the Static Exchange and Static Exchange plus Polarization methods. This research gives an insight into how the water environment changes resonance properties. The presence of water does not have the same effects in all systems studied. We confirmed that the effects depend on the character of water as a proton donor or acceptor. We showed that the resonance shift depends on the water binding site to the molecule and that it does not shift all resonances in one system equally strongly. Additionally, isolated pyridine has been investigated in more detail, in order to compare it with diazines, studied previously in our group. We found that the resonance formation in both system is similar.
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