Free Radical Formation in Single Crystals of 2′-Deoxyguanosine 5′-Monophosphate Tetrahydrate Disodium Salt: An EPR/ENDOR Study

Abstract

Single crystals of 2'-deoxyguanosine 5'-monophosphate were X-irradiated at 10 K and at 65 K, receiving doses between 4.5 and 200 kGy, and studied using K-band EPR, ENDOR, and field-swept ENDOR (FSE) spectroscopy. Evidence for five base-centered and more than nine sugar-centered radicals was found at 10 K following high radiation doses. The base-centered radicals were the charged anion, the N10-deprotonated cation, the C8 H-addition radical, a C5 H-addition radical, and finally a stable radical so far unidentified but with parameters similar to those expected for the charged cation. The sugar-centered radicals were the H-abstraction radicals centered at C1', C2', C3', and C5', an alkoxy radical centered at O3', a C5'-centered radical in which the C5'-O5' phosphoester bond appears to be ruptured, a radical tentatively assigned to a C4'-centered radical involving a sugar-ring opening, as well as several additional unidentified sugar radicals. Most radicals were formed regardless of radiation doses. All radicals formed following low doses (4.5-9 kGy) were also observed subsequent to high doses (100-200 kGy). The relative amount of some of the radicals was dose dependent, with base radicals dominating at low doses, and a larger relative yield of sugar radicals at high doses. Above 200 K a transformation from a sugar radical into a base radical occurred. Few other radical transformations were observed. In the discussion of primary radicals fromed in DNA, the presence of sugar-centered radicals has been dismissed since they are not apparent in the EPR spectra. The present data illustrate how radicals barely traceable in the EPR spectra may be identified due to strong ENDOR resonances. Also, the observation of a stable radical with parameters similar to those expected for the charge guanine cation is interesting with regard to the nature of the primary radicals stabilized in X-irradiated DNA.

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

“…19 G doublet into a singlet on C5′ dideuteration. This is found both at room temperature in solution (45), and in a number of single crystal investigations (12,13,18,46) at a variety of temperatures. A single crystal investigation of C5′• in 2′-deoxyadenosine monohydrate reports a single anisotropic C5′ hydrogen hyperfine coupling with a iso = ca.…”

Effect of Base Stacking on the Acid−Base Properties of the Adenine Cation Radical [A•+] in Solution: ESR and DFT Studies

“…3,4,8 The site of deprotonation in G• + had been somewhat controversial as it is reported to be N1 in solution in pulse radiolysis studies and at the amine group (N2–H) in solid state. 9a Our recent ESR studies employing isotopically ( 15 N and deuterium) substituted dGuo confirm that the site of deprotonation in G• + is at N1 in aqueous solution, and theoretical calculations 9a confirm shifting of that locus of deprotonation of G• + from N2 in a low dielectric solid 9b to N1 in an aqueous environment. 9a …”

“…9a Our recent ESR studies employing isotopically ( 15 N and deuterium) substituted dGuo confirm that the site of deprotonation in G• + is at N1 in aqueous solution, and theoretical calculations 9a confirm shifting of that locus of deprotonation of G• + from N2 in a low dielectric solid 9b to N1 in an aqueous environment. 9a …”

“…magnet and with a 200 mW klystron. 9,12 ESR spectra were recorded at 77 K and at the microwave power range 25–40 dB. We have used Fremy’s salt (with g = 2.0056, A N = 13.09 G) for field calibration (shown as reference marks in all figures with ESR spectra).…”

Radicals Produced by γ-Irradiation of Hyperquenched Glassy Water Containing 2′-Deoxyguanosine-5′-monophosphate

“…Bottom spectrum, simulated signal of G -· anionradical; simulation parameters: A H(C8) [5.5; 3.0; 0.5] G, g [2.0028; 2.0037; 2.0026], line width 6 G 25,26. …”