Sugar radicals formed by photo-excitation of guanine cation radical in oligonucleotides

• Published in: The journal of physical chemistry. B Vol. 111; no. 25; pp. 7415 - 7421
• Main Authors: Adhikary, Amitava, Collins, Sean, Khanduri, Deepti, Sevilla, Michael D.
• Format: Journal Article
• Language: English
• Published: 05-06-2007
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp071107c

This work presents evidence that photo-excitation of guanine cation radical (G•
+
) 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•
+
on photo-excitation, at 143 K leads to the formation of predominantly C5′
•
and C1′
•
with small amounts of C3′
•
. Photo-conversion yields of G•
+
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•
+
to C1′• (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′• with length of the oligonucleotide along with decreases in C3′• and C5′• For dsDNA in frozen solutions, only the formation of C1′• is found via photo-excitation of G•
+
without a significant temperature dependence (77 K to 180 K). Long wavelength visible light (>540 nm) is observed to be about as effective as light under 540 nm for photoconversion of G•
+
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.