The Initial Ionization Event

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The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. 

In dried air or nitrogen with ionization using a Ni source, the initial ionization event produces N+, NJ, O+, and O J which lead, through a series of rapid ion/mol-ecule reactions with the ubiquitous trace amounts of water, to the hydronium ion (H2OXH + [20], The subscript n denotes a range of values that depends on both the water concentration and the temperature. The equilibrium of the hydronium ion is shown in Equation 13.10,... 

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