Sugar radicals formed by photoexcitation of guanine cation radical in oligonucleotides

This work presents evidence that photoexcitation of guanine cation radical (G(+center dot)) in dGpdG and DNA-oligonucleotides TGT, TGGT, TGGGT, TTGTT, TTGGTT, TTGGTTGGTT, AGA, and AGGGA in frozen glassy aqueous solutions at low temperatures leads to hole transfer to the sugar phosphate backbone and results in high yields of deoxyribose radicals. In this series of oligonucleotides, we find that G(+center dot) on photoexcitation at 143 K leads to the formation of predominantly C5'center dot and C1'center dot with small amounts of C3'center dot. Photoconversion yields of G(+center dot) to sugar radicals in oligonucleotides decreased as the overall chain length increased. However, for high molecular weight dsDNA (salmon testes) in frozen aqueous solutions, substantial conversion of G(+center dot) to C1'center dot (only) sugar radical is still found (ca. 50%). Within the cohort of sugar radicals formed, we find a relative increase in the formation of C1'center dot with length of the oligonucleotide, along with decreases in C3'center dot and C5'center dot. For dsDNA in frozen solutions, only the formation of C1'center dot is found via photoexcitation of G(+center dot), without a significant temperature dependence (77-180 K). Long wavelength visible light (> 540 nm) is observed to be about as effective as light under 540 nm for photoconversion of G(+center dot) to sugar radicals for short oligonucleotides but gradually loses effectiveness with chain length. This wavelength dependence is attributed to base-to-base hole transfer for wavelengths > 540 nm. Base-to-sugar hole transfer is suggested to dominate under 540 nm. These results may have implications for a number of investigations of hole transfer through DNA in which DNA holes are subjected to continuous visible illumination.