The electronic excitations caused by DNA when exposed to ion radiation is essential to DNA damage. In this paper, we investigated the energy deposition and electron excitation process of DNA with reasonable stretching range upon proton irradiation based on time-dependent density functional theory. Stretching changes the strength of hydrogen bonding between the DNA base pairs, which in turn affects the Coulomb interaction between the projectile and DNA. As a semi-flexible molecule, the way of energy deposition is weakly sensitive to the stretching rate of DNA. However, the increase of stretching rate causes the increase of charge density along the trajectory channel, sequentially resulting in an increase in proton resistance along the intruding channel. The Mulliken charge analysis indicates that the guanine base and guanine ribose are ionized, meanwhile the cytosine base and cytosine ribose are reduced at all stretching rates. In a few femtoseconds, there exists an electron flow passing through the guanine ribose, guanine, cytosine base and the cytosine ribose in turn. This electron flow increases electron transfer and DNA ionization, promoting the side chain damage of the DNA upon ion irradiation. Our results provide a theoretical insight for deciphering the physical mechanism of the early stage of the irradiation process, and are also of great significance for the study of particle beam cancer therapy in different biological tissues.
Keywords: DNA damage; DNA stretching; TDDFT; energy deposition; ion beam therapy.
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