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| - On the Enhanced Stability of the Guanine−Cytosine Base-PairRadical Cation
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| - Ab initio (UHF/6-31G*) and density functional(Becke3LYP/D95*) calculations have been used toinvestigate the structures and stabilities of the radical cations ofthe DNA bases and base pairs. The calculatedstructures of the base pairs show excellent agreement withcrystallographic data. The most easily oxidizablebase,guanine, forms a particularly stable radical cation base pair withcytosine, so that the calculated adiabatic ionizationpotential for the guanine−cytosine hydrogen-bonded complex is about0.75 eV lower than that of guanine itself.UBecke3LYP/D95*//UHF/6-31G* calculations show that the shift ofthe central hydrogen-bonded proton at N1 ofguanine to N3 of cytosine is only slightly endothermic (+1.6 kcalmol-1). The product of the correspondingprotonshift in the adenine−thymine system is unfavorable by +14.1 kcalmol-1. These results suggest that theguanine−cytosine radical cation represents even more of a thermodynamic sink inoxidized DNA than might be concludedfrom the ionization potentials of the individual bases, and that itenjoys about 7.3 kcal mol-1 extrastabilization fromthe central low-barrier hydrogen bond.
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