Experimental and Theoretical Studies on the Selectivity of GGG Triplets toward One-Electron Oxidation in B-Form DNA

The selectivity of 5‘-TGGGT-3‘ and 5‘-CGGGC-3‘ sequences toward photoinduced one-electron oxidation was examined experimentally and by ab initio molecular orbital (MO) calculations. It was confirmed experimentally that G 2 of 5‘-TG 1 G 2 G 3 T-3‘ is more reactive than G 1, while for 5‘-CG 1 G 2 G 3 C-3‘ the selectivity is reversed, that is, G 1 > G 2 . The ab initio MO analyses were performed to elucidate the difference of the selectivities between 5‘-TGGGT-3‘ and 5‘-CGGGC-3‘ sequences. For the 5‘-TGGG-3‘ sequence, the spin densities of G 1 • and G 2 • in neutral radical (5‘-TG 1 G 2 G 3 -3‘) • have a similar pattern, and the shapes of the corresponding radical orbitals are also very similar. It was concluded that the selectivity is due to the stability of the (5‘-TG 1 G 2 G 3 -3‘) • neutral radicals; that is, 5‘-TG 1 G 2 • G 3 -3‘ is more stable in energy than 5‘-TG 1 • G 2 G 3 -3‘. For the 5‘-CGGG-3‘ sequence, it was found that the spin density on N1 of G 1 • in neutral radical (5‘-CG 1 G 2 G 3 -3‘) • is distinguishably different from the corresponding spin density of G 2 •, which has a pattern similar to those of G 1 • and G 2 • in 5‘-TG 1 G 2 G 3 -3‘. The radical orbital (SOMO) of G 1 • is delocalized on guanine base and up to the paired cytosine base, while the radical orbital of G 2 • is essentially localized on guanine base. This drastic difference of the electron population in the radical orbitals, caused by the stacking interaction with the 5‘-side G of the opposite strand, can explain why G 1 is more reactive than G 2 in the 5‘-CG 1 G 2 G 3 -3‘ sequence.