Protonation state of radiation-produced cytosine anions and cations in the solid state: EPR/ENDOR of cytosine monohydrate single crystals x-irradiated at 10 K

Abstract

8269where ISC, CC, and TTA respectively represent intersystem crossing, conformational change, and triplet-triplet annihilation, and Ep and DEF denote the exciplex phosphorescence and delayed exciplex fluorescence, respectively. This scheme is expected to apply to other molecules that exhibit highly structured delayed excimer (or exciplex) fluorescence and relatively diffuse excimer (or exciplex) phosphorescence.'S2 ConclusionIn this paper the results of time-resolved emission studies of PNA in fluid solutions, as well as the steady-state emission studies of the same molecule in rigid matrices, have been presented to demonstrate the conformational change associated with the intramolecular triplet exciplex formation. The formation of the triplet exciplex was confirmed by the observation of the exciplex phosphorescence and by the appearance of the delayed exciplex fluorescence, which has been demonstrated to arise from the bimolecular annihilation of the triplex exciplex. Dependence of the formation rate of the exciplex on solvent viscosity indicates that the exciplex formation involves a large-amplitude rotation of the phenyl group in PNA with respect to the N p N H moiety, which is strongly dependent on medium friction (and hence is greatly hindered in viscous solvents). Comparison of the spectral features of the delayed exciplex fluorescence in fluid solutions with those of the prompt fluorescence of the ground-state van der Waals complex in rigid glasses demonstrates that the excited singlet state from which the delayed fluorescence originates is the lowest excited singlet state of the van der Waals complex. These results lead to a kinetic scheme for the triple exciplex formation and disap p r a n c e , which is consistent with all available experimental data.(DE-FG02-89ER14024), for support of this work.RMstry NO. PNA, 135-88-6. References and Notes1) Modiano, S. H.; Dresner, J.; Lim, E. C. J. Phys. Chem. 1991,95,9144. (2) Cai, J.; Lim, E. C. J. Phys. Chem. 1992, 96, 2935. (3) For low-temperature emission attributed to the triplet exciplex of tenzophenone/aniline, see: Masuhara, J.; Maeda, Y.; Nakajo, H.; Mataga, N.; Tomita, K.; Tatemitsu, H.; Sakata, Y.; Mitsumi, S. J. Am. Chem. SOC. 1981, 203, 634. Hoshino, J.; Kogure, M. J. Phys. Chem. 1989, 93, 728. (4) Dresner, J.; Modiano, S. H.; Lim, E. C. To be submitted to J. Phys.Single crystals of cytosine monohydrate were X-irradiated at 10 K and examined using K-band EPR, ENDOR, and FSE spectroscopy at temperatures between 10 K and room temperature. Three radicals dominated the low-temperature spectra. Radical RI is the cytosine anion protonated at the N3-position. A large coupling to HC6 and smaller couplings to HN3 and to the two amino protons were detected and their characteristics unambiguously established the structure of this radical. Radical RII exhibits major couplings to HCS, N1, and to the nitrogen and the protons of the amino group. This radical is shown to be the N1-deprotonated cation of cytosine. No evidence for the trapping of the charged cytosine c...

Mechanisms of Direct Radiation Damage to DNA: The Effect of Base Sequence on Base End Products

“…In crystalline derivatives of Cyt and duplex DNA, Cyt •− has not been observed. Even at 10K it is protonated at N3, 47 , shown by Reaction 6.1b in Scheme 6. The resulting neutral radical Cyt(N3+H) • , as best explained by Steenken, 22 is the most favorable electron trapping site in DNA because of proton transfer from N1 of Gua to N3 Cyt.…”

Radical Formation in Pyrimidine Bases after X, Proton and α-Particle Irradiation

“…Both the Cm5-and Cm6-yl radicals are present after irradiation and measurement at 10 K, but as minority species in comparison with the reduction and oxidation products (42). As the anion is stabilized through protonation (at N 3 ), this pathway dominates at low temperatures, and the formation of the Cm5-and Cm6-yl species at this temperature could be mostly due to hydrogen addition.…”

DFT study on addition reaction mechanism of guanine-cytosine base pair with OH radical

“…Genetic information of the living organisms was recorded in DNA, which can be damaged by normal metabolic activities and environmental affects, in particular the effects of radiation . Most of the effects have belonged to the role of free radicals .…”