Ionization of adenine derivatives: EPR and ENDOR studies of X-irradiated adenine.HCl.1/2H2O and adenosine.HCl.

Following X irradiation of adenine.HCl.H2O at 10 K, evidence for five distinct radical products was present in the EPR/ENDOR. (In both adenine.HCl.1/2H2O and adenosine.HCl, the adenine base is present in a cationic form as it is protonated at N1). From ENDOR data, radical R1, stable at temperatures up to 250 K, was identified as the product of net hydrogen loss from N1. This product, evidently formed by electron loss followed by proton loss, is equivalent to the radical cation of the neutral adenine base. Radical R2, unstable at temperatures above 60 K, was identified as the product of net hydrogen addition to N3, and evidently formed by electron addition followed by proton addition. Radicals R3-R5 could not be identified with certainty. Similar treatment of adenosine.HCl provided evidence for six identifiable radical products. Radical R6, stable to ca. 150 K, was identified as the result of net hydrogen loss from the amino group, and evidently was the product of electron loss followed by proton loss. Radical R7 was tentatively identified as the product of net hydrogen addition to C4 of the adenine base. Radical R8 was found to be the product of net hydrogen addition to C2 of the adenine base, and R9 was the product of net hydrogen addition to C8. Radical R10 was identified as the product of net hydrogen abstraction from C1' of the ribose, and R11 was an alkoxy radical formed from the ribose. With the exception of R11, all products were also found following irradiation at 65 K. Only radical R8 and R9 were stable at room temperature. Most notable is the different deprotonation behavior of the primary electron-loss products (radical R1 vs. R6) and the different protonation behavior of the primary electron-gain products (radical R2 vs. no similar product in adenosine.HCl). The major structural difference in the two crystals is the electrostatic environment of the adenine base. Therefore, this study provides further evidence that environmental influences are important in determining proton transfer processes.