Photoexcitation of adenine cation radical [A*+] in the near UV-vis region produces sugar radicals in adenosine and in its nucleotides.

In this study, we report the formation of ribose sugar radicals in high yields (85-100%) via photoexcitation of adenine cation radical (A*+) in Ado and its ribonucleotides. Photoexcitation of A*+ at low temperatures in homogeneous aqueous glassy samples of Ado, 2'-AMP, 3'-AMP, and 5'-AMP forms sugar radicals predominantly at C5'- and also at C3'-sites. The C5'* and C3'* sugar radicals were identified employing Ado deuterated at specific carbon sites: C1', C2', and C5'. Phosphate substitution is found to deactivate sugar radical formation at the site of substitution. Thus, in 5'-AMP, C3'* is observed to be the main radical formed via photoexcitation at ca. 143 K, whereas, in 3'-AMP, C5'* is the only species found. These results were supported by results obtained employing 5'-AMP with specific deuteration at the C5'-site (i.e., 5',5'-D,D-5'-AMP). Moreover, contrary to the C5'* observed in 3'-dAMP, we find that C5'* in 3'-AMP shows a clear pH-dependent conformational change as evidenced by a large increase in the C4' beta-hyperfine coupling on increasing the pH from 6 to 9. Calculations performed employing DFT (B3LYP/6-31G*) for C5'* in 3'-AMP show that the two conformations of C5'* result from strong hydrogen bond formation between the O5'-H and the 3'-phosphate dianion at higher pHs. Employing time-dependent density functional theory [TD-DFT, B3LYP/6-31G(d)], we show that, in the excited state, the hole transfers to the sugar moiety and has significant hole localization at the C5'-site in a number of allowed transitions. This hole localization is proposed to lead to the formation of the neutral C5'-radical (C5'*) via deprotonation.