Single crystals of the amino acid l-α-alanine have been X-irradiated at room temperature, and the free radical formation has been studied using X-band and K-band EPR, ENDOR, and EIE (ENDOR-induced EPR) spectroscopy in the temperature interval 220−295 K. Aided by the results from EIE, as well as ENDOR from selected magnetic field positions, nine hyperfine coupling tensors were obtained and assigned to three different radicals. Room-temperature relaxation behaviors characterized by efficient W1 x and W1e and by slow W1 n relaxation rates allowed for determination of the signs of the various hyperfine couplings from the ENDOR spectra obtained at room temperature. The temperature dependence of the W1 x relaxation is qualitatively discussed. The EPR spectra from alanine are dominated by the well-known resonance of the “stable alanine radical”, SAR, formed by a net deamination of the protonated alanine anion. Precise hyperfine coupling tensors due to the α-proton coupling, the methyl group coupling, and a dipolar coupling to a methyl group of a neighboring molecule, as well as the g tensor, are given for this radical. Spectral simulations show that these parameters in a satisfactory manner reproduce all observable features of the resonance from this radical. Radical R2, apparently formed in roughly the same amounts as SAR, exhibits the structure H3N+−•C(CH3)C(O)O-. It is formed from alanine by a net H-abstraction from the Cα position. The hyperfine coupling tensor to the freely rotating methyl group was obtained from both X-band and K-band data. K-band spectra obtained at several temperatures between 220 and 290 K revealed that the amino group is not freely rotating; that is, the three protons of the amino group are locked in their hydrogen bonds also after radical formation. A significant increase in ENDOR line widths upon increasing temperature made the ENDOR lines due to the amino protons practically nonobservable at 295 K. However, the three corresponding hyperfine coupling tensors were easily obtained from K-band ENDOR data at 220 K. The B 0 and B 2 values for β-coupling to N+−H fragments were determined to be −4.3 and 117.6 MHz, respectively. Due to partly unresolved nitrogen hyperfine interaction leading to larger line widths, the individual EPR lines from radical R2 are of far less intensity as compared to those of the SAR. However, simulations strongly indicate that there is an almost equal relative distribution (60%:40%) of the two radicals. Two hyperfine coupling tensors were assigned to two conformations of a third minority radical species (radical R3) which tentatively is suggested to be the species H2N−•C(CH3)C(OH)O. Possible mechanisms for the formation of the radicals are discussed in light of the basic radiation chemistry of the amino acids. The simultaneous presence of two stable radicals of similar relative amounts in alanine may have consequences for the use of alanine as a radiation dosimeter.

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.

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...

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