UVA-visible photo-excitation of guanine radical cations produces sugar radicals in DNA and model structures

Abstract

This work presents evidence that photo-excitation of guanine radical cations results in high yields of deoxyribose sugar radicals in DNA, guanine deoxyribonucleosides and deoxyribonucleotides. In dsDNA at low temperatures, formation of C1′• is observed from photo-excitation of G•+ in the 310–480 nm range with no C1′• formation observed ≥520 nm. Illumination of guanine radical cations in 2′dG, 3′-dGMP and 5′-dGMP in aqueous LiCl glasses at 143 K is found to result in remarkably high yields (∼85–95%) of sugar radicals, namely C1′•, C3′• and C5′•. The amount of each of the sugar radicals formed varies dramatically with compound structure and temperature of illumination. Radical assignments were confirmed using selective deuteration at C5′ or C3′ in 2′-dG and at C8 in all the guanine nucleosides/tides. Studies of the effect of temperature, pH, and wavelength of excitation provide important information about the mechanism of formation of these sugar radicals. Time-dependent density functional theory calculations verify that specific excited states in G•+ show considerable hole delocalization into the sugar structure, in accord with our proposed mechanism of action, namely deprotonation from the sugar moiety of the excited molecular radical cation.

Theoretical Modeling of Radiation‐Induced DNA Damage

“…[24][25][26][27] It was found that excited DNA base cation radicals formed high yield of sugar radicals which confirmed the proposed hypothesis. [24][25][26][27] While strand breaks are biologically significant, it is combinations of DNA damages known as multiple damage sites (MDS) that are the most lethal type of DNA damage. Such combinations of single-and double-strand breaks and base damages with 10 base pairs are known to lead to irreparable damage because of the loss of local structural information.…”

“…þ to sugar radical formation (80-100%) was found, while with oligos of increasing length and DNA the yield was found to fall off to $50%. 24,26,27 This experimental observation was further supported by theoretical calculations using time-dependent density functional theory. [26][27][28] The excited states of G .…”

“…Ã method, which clearly shows the all the transitions in the near-UV-visible range originate from the inner shell (core) molecular orbitals (MOs), and many of them involved base-to-sugar hole transfer 26,27 as proposed in Scheme 1.1. The experimental studies were further extended to larger model systems such as dinucleoside phosphate TpdG cation radical, 28 and the photoexcitation of G .…”

“…In this context, an experimental wavelength-dependence study of sugar radical formation from G . þ in 2 0 -deoxyguanosine (dGuo) and in DNA was also carried out by Adhikary et al 26 by varying the photoexcitation wavelength from visible to UV range. Interestingly they found that while in dGuo the sugar radical formation was independent of wavelength of light, the formation of sugar radicals in dsDNA only occurred below 540 nm.…”

“…Ionization radiation also induces holes on the sugar-phosphate backbone site that lead to two competitive reactions: (i) deprotonation of the sugar cation radical at a carbon site resulting in the formation of neutral sugar radicals at carbon C 0 1 to C 0 5 sites [23][24][25][26][27][28] and (ii) transfer of hole to a nearby base in DNA. 28 Since most sugar radicals lead to DNA strand breaks, sugar radical formation in DNA becomes of crucial importance to the biological consequences of radiation.…”