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(PP026) Photo-excitation of guanine radical cation in the near UV-VIS produces sugar radicals in DNA.
Adhikary, Amitava*,1, Pazdro, Robert 1, Malkhasian, Aramice 1, 2, Collins, Sean1, Becker , David *,1, Sevilla, Michael*,1, 1 Department of Chemistry, Rochester, Mi, USA2 Department of Chemistry, East Lansing, Mi, USA
ABSTRACT- It has been proposed that photo-excitation of guanine-radical cation (G.+) results in delocalization of a significant fraction of spin and charge onto the sugar moiety followed by rapid deprotonation to form C1′. (Shukla et al., Radiation Research, 2004, 161, 582-590). Here we present evidence that confirms and elucidates this mechanism. We find that in -irradiated DNA formation of C1′. from excited G.+ depends upon the wavelength of the incident light with effective formation observed in the range 310-460 nm at 77 K. Excitation using Nd-Yag laser at 521 nm as well as using light at wavelengths ≥ 480 nm do not produce C1′. at 77 K. Photo-excitation of G.+ in 2′-deoxyguanosine (2′-dG), and its 3′- and 5′-nucleotides (i.e., 3′-dGMP and 5′-dGMP) results in formation of C1′., C3′. and C4′./C5′. sugar-radicals. These species were confirmed by selective deuteration at C5′ and at C3′ atoms in 2′-dG. Variation of temperature, pH, concentration and wavelength of excitation provide important information about mechanism of formation of these sugar radicals: (a) unlike DNA, excitation at higher temperatures than 77K increases the extent of production of sugar radicals up to 95% from 10-20% at 77K, (b) at pH > 9 where the N-1 guanine proton is lost (pKa = 9.3), excitation does not lead to sugar radical formation, (c) hydration of G.+ to 8-OHG. does not occur during photo-excitation of G.+ in 2′-dG and 3′-dGMP by deuteration at C-8 in the guanine moiety, (d) unlike DNA, the extent of C1′. and C3′. formation from excited G.+ does not depend upon wavelength of excitation in the UVA-visible range, (e) phosphate groups deactivate sites toward radical production so that no C4′. /C5′. is found in 5′-dGMP and a little C3′. is found in 3′-dGMP. TD-DFT and CASSCF calculations show that spin and charge of excited G.+ delocalize considerably to the sugar moiety upon excitation in the range 300-600 nm.
Key words: excited guanine radical cation, sugar radical
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