EPR and ENDOR Studies of X-Irradiated Single Crystals of Deoxycytidine 5‘-Phosphate Monohydrate at 10 and 77 K

Abstract

The EPR spectra of single crystals of deoxycytidine 5‘-phosphate monohydrate (5‘dCMP), X-irradiated at 10 K, exhibit signals from several distinct radical species. Analysis of the ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The reduced species exhibits hyperfine coupling to the C6−Hα proton, and an additional small exchangeable hyperfine coupling from the N3−H proton. No additional couplings that may be associated with protonation of the amino group have been observed. Since the native molecule is protonated at N3, it appears that reduction of the cytosine base does not result in any further protonation. The oxidized species exhibits hyperfine couplings to C5−Hα and C1‘−Hβ, and two small exchangeable couplings from the C4−NH2 protons. Since no hyperfine coupling to N3−H was observed, the oxidized species is believed to be the N3−H deprotonated cation. At high X-ray dose there is also evidence for both C5 and C6 H-addition radicals at 10 K. The fate of these radicals has been studied under controlled warming conditions. Attempts have been made to relate the fate of the low-temperature radicals with several radicals that have been detected previously at 77 K and at room temperature.

Reductive Damage in Directly Ionized DNA: Saturation of the C5 = C6 Bond of Cytosine in d(CGCG)2Crystals

“…This is expected to be 1.6-1.8 mT (44,45,(48)(49)(50), and 1.7 mT is observed. When B 0 is parallel to the base planes, the six magnetically distinct sites will give couplings that vary between the maximum principal value of 2.3-2.8 mT and the minimum of 0.6-0.9 mT (46,47,50,51). This is consistent with broadening of these features as the crystal is rotated from B 0 ⊥ base planes to // base planes (i.e., from 90° to 0°).…”

“…The Cyt(C6+H,N3+H + ) +• cation has been detected in irradiated 5′dCMP (46) and dGMP:dCMP, but not in polyG:polyC or poly[dGdC]:poly[dGdC] (12). It has been tentatively identified in DNA (52).…”

A theoretical study of structures and electron affinities of radical anions of guanine‐cytosine, adenine‐thymine, and hypoxanthine‐cytosine base pairs

“…It is clear that all the methods except HF/6-31G(d) 44 predict the base pair anions to be more stable than the neutral ones (Tables 6 and 7). Hobza and Š poner 95 used the HF/6-31G** method to study thermodynamic characteristics for the formation of several hydrogen bonded DNA base pairs from their respective fragments considering reaction (6). They calculated ⌬E, ⌬H, Gibbs free energy ⌬G, entropy change ⌬S, and the reaction equilibrium constant K. They also considered the effect of temperature on ⌬H and found it to be negligible.…”

Adiabatic Electron Affinities of the Polyhydrated Adenine−Thymine Base Pair: A Density Functional Study

“…6 Location of preferred hydration sites around DNA bases and base pairs has been the subject of a number of experimental and theoretical studies. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] In recent years, the study of charge transport through DNA has become a highly important subject from the biological [21][22][23][24][25][26][27][28][29][30] and technological [31][32][33][34][35][36][37][38][39][40][41] points of view. It is found that the easily oxidized purine bases, especially guanine, play a crucial role in charge transport in DNA.…”