J. Phys. Chem. 1982,86,1751-1755
1751
ARTICLES Electron Spin Resonance Study of Radicals Produced by One-Electron Loss from 6-Azauraci1, 6-Azathymine, and 6-Azacytosine. Evidence for both CT and 7r Radicals Michael D. Sevllla' and Steven Swarts Depertment of Chemistry, Oakkrnd University, Rochester, Mlchigan 48063 (Received July 30, 1981; I n Final Form: Januafy 8, 1982)
The radicals produced by loss of one electron from 6-azauracil, B-azathymine,and 6-azacytosine at low temperatures (77 K) have been investigated by ESR spectroscopy. The radicals are produced by Clz- attack in basic 12 M LiCl glaases. The azathymine radical has also been produced by a second technique, photoionization in 8 M NaC104. The azauracil radical is found to be a u radical whose ESR spectrum shows large couplings (ca. 30 G) to the nitrogens at positions 1and 6. The magnitude and anisotropic nature of the nitrogen couplings are characteristic of such a u radical. The Gazathymineradical gives an ESR spectrum characteristic of a *-cation radical in both matrices studied. This spectrum shows couplings to the 5-methylgroup (14.7 G) and the nitrogen at position 1 (ca. 9 G). Clz- attack on 6-azacytosineresults in a u radical with nitrogen couplings within 1 G of those found for 6-azauracil; however, this radical converts to a new radical species whose ESR spectrum is suggestive of a A radical. It is suggested that a change in the state of protonation (deuteration)in the Gazacytoeine radical results in the change from a u to A radical. Anisotropic computer simulations are used to verify the analysis of spectra. INDO-MO calculations were performed to aid in the assignment of radicals to structures. A comparison of the INDO calculations of the nitrogen hyperfine splittings with experimental values found for the azauracil u radical suggests a new three-electron N-N bond forms along with concomitant bond shortening after electron loss.
Introduction *-cation radicals' of a number of pyrimidine DNA bases have been found to be produced by biphotonic photoionization in aqueous matrices: y-irradiation of neat solid^,^ as well as by attack of OH. and C1f radicals.4~~Although one u reduction radical, the 5-bromouracil u* anion, has been reported! no a-cation radicals of DNA bases have yet been reported.' However, Kat0 and Shida have recently shown that a number of nitrogen heterocycles including pyrimidine formed u cation radicals in y-irradiated freon matrices through ionization of the freon matrix followed by intermolecular charge migrationa8 In our recent work eight pyrimidine DNA base *-cation radicals were produced by C1,- attack in slightly basic aqueous g l a ~ s e s .Reaction ~ 1 shows the reaction of Clzwith uracil.
AXH - Hxx; 0-
0
I
II
t
0
H
H
c1,-
t 2cI-
(1)
0
In none of the eight pyrimidines studies was another radical other than the ?r cation detected. Thus Clf appears (1)The term r cation (or u cation) refers to the fact that a species has lost one electron from the r-electron (or a-electron) system. It does not refer to the total charge on the radical. (2) M. D. Sevilla, J. Phys. Chem., 80, 353 (1976). (3)H.C.Box, "Radiation Effecta: ESR and ENDOR Analysis", Academic Press, New York, 1977. (4)M. D.Sevilla, D. Suryanarayana, and K. M. Morehouae, J. Phys. Chem., 85, 1027 (1981). (5)M. Bansal and R. W. Feseenden, Radiat. Res., 75, 497 (1978). (6)H. Riederer, J. Hottermann, and M. C. R. Symons,Chem. Commun., 313 (1978). (7)W.A. Bemhard, Adu. Radiat. Bioi., 9,200 (1981). (8)(a)T.Kat0 and T. Shida,J.Am. Chena. SOC.,101,6869(1979);(b) T.Shida and T. Kato, Chem. Phys. Lett., 68,106 (1979). 0022-3654/82/2086-1751$01.25/0
to be an excellent one-electron oxidizing agent for the production of DNA base ?r cations. In this work we report the first production of a-cation radicals in the nucleic acid base analogues 6-azauracil and 6-azacytosine. These radicals are again produced by the relatively mild oxidizing agent C12-.5 Surprisingly, the structurally similar compound 5-methyl-6-azauracil (6azathymine) is shown to form a a-cation radical under the same conditions.
Experimental Section The techniques used in this work, Clz- attack on DNA bases in 12 M LiCl and photoionization of DNA bases in 8 M NaC104,have been described in detail in our previous w01-k.~ The ~ ~ first employs y irradiation of 12 M LiCl at 77 K which produces mainly Clz- and electrons. The electrons are scavenged by M K,Fe(CN),. The second employs biphotonic ionization at 77 K with 254-nm light from low-pressure mercury vapor lamp. Concentrations of the azapyrimidines were 1.0 mg/mL in both techniques. 6-Azaurac4 6-azaWine, and 6-azathymine were obtained from Sigma. A Varian Century Series EPR spectrometer with dual cavity was employed. Hyperfine splittings and g values were measured vs. Fremy's salt with A N = 13.09 G and g = 2.0056. Computer simulations of anisotropic ESR spectra were performed by use of the Lefebve-Maruani p r ~ g r a m . ~ Results and Discussion 6-Azauracil u Radical. y irradiation of basic (low2M LiOH) 12 M LiCl solutions containing M azauracil and low2M K,Fe(CN), as an electron scavenger results in a dominant signal due to the Clz- radicaL4J0 When the (9)R. Lefebve and J. Maruani, J. Chem. Phys., 42, 1480 (1965).
0 1982 American Chemical Society
1752
The Journal of phvslcal Chemistry, Vol. 86, No. 10, 1982
Sevilia and Swarts
TABLE I: Hyperfine Splittings (Gauss) and g Values of 6-Azauracil,g-Azathymine, and 6-Azacytosine0 - and n-Cation Radicals structure
aN(1), aN(6) aN(3) g valuesC
z
Y
X
6-Azauracil u Radicalagb 26.0 38.5
