The formation of H-addition radicals in monocrystals of adenosine, adenosine ·HCl, adenine ·HCl· ½H2O, and adenine·2 HCl by X-irradiation has been studied by using electron spin resonance spectroscopy at 9.5 GHz and at 35 GHz. In all crystals, both H-addition radicals at position C2 and at position C8 were observed. The coupling constants of these two H-addition radicals are different and depend strongly on the protonation state of the adenine base. INDO calculations reproduce well the observed trends of the coupling constants. It is shown that the C2- addition radical is transformed into the C8-addition radical by heat and vice versa the C8-addition into the C2-addition by light of λ>360 nm.
After x irradiation of single crystals of 1-CH3-uracil, 1-CH3-uracil⋅HBr, cytosine⋅H2O, and deoxycytidine⋅HCl at room temperature C5-addition radicals, which result from H addition at carbon 5 of the unsaturated C5–C6 bond of the pyrimidine ring, are formed. It is shown that, upon irradiation with light of wavelength λ≳400 nm, these radicals transform into C6-addition radicals where the added hydrogen atom is localized on carbon 6. INDO calculations show that, whereas C5-addition radicals are neutral, the C6-addition radicals must be positively charged in order to reproduce the experimental couplings.
Single crystals of anhydrous thymine (T), thymine monohydrate (T · H2O), 5,6-dihydrothymine (TH2), 1-methylthymine (mT) and thymidine (dT) were irradiated with X-rays and UV between 77 K and 300 K. Six types of radicals were analyzed by ESR-spectroscopy at 9.5 GHz and 35 GHz after exposure to X-rays. The anion radical occurred only in T · H2O at 77 K, the 4-yl radical only in TH2 at 77 K and the 1-yl radical only in T between 77 K and 300 K. The 6-yl radical was found in T, TH2 and mT. It was converted into the 5-yl radical irreversibly by heat or white light (λ < 600 nm). The 5-yl radical appeared in all compounds at room temperature. The highest thermal stability was found for the 7-yl radical which was present at room temperature in all compounds except TH2.UV-irradiation (λ = 320 nm) produced radicals only in three crystals (T, TH2, dT). In T the 5-yl radical was found after exposure at 300 K, and two other radicals, 1-yl and 7-yl, at 77 K. Also at 77 K, the 7-yl radical was present in dT and the 5-yl radical in TH2 and dT.
In pyrimidines with an unsubstituted C5 = C6 bond, H-addition cia an anionic stage, the " ionization path ", occurs preferentially at position C6 and direct hydrogen addition, the " excitation path ", at position C5. In adenine derivatives, the " excitation path " yields C8-addition radicals and the "ionization path " CZaddition radicals. However, when the molecule is doubly protonated, C8-addition radicals are preferentially formed by protonation of the electron adduct. In pyrimidine crystals, the CS-addition radicals transform into C6-addition radicals upon irradiation with light of i , 3 400 nm. In purine crystals, the C8-addition radicals can also be transformed into CZaddition radicals in this way. In "van der Waals crystals ", this transformation is reversible upon storage at room temperature. In " polar crystals ", the trailsformation is irreversible. This indicates that the C6-addition radicals of pyrimidines and the C2-addition radicals of adenine derivatives need a polar environment to be stabilized. INDO calculations support this conclusion. While the " ionization path " could not be detected in " van der Waals crystals " both paths have been observed in " polar crystals ". These observations bear out the view that, in nonpolar environments, ionization is followed by geminate ion recombination with eventual subsequent homolytic dissociation of atomic hydrogen.
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