Electron Spin Resonance of the Excited Triplet States of Pyrimidines and Purines

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Abstract
The electron spin resonances of the metastable triplet states of various pyrimidines and purines have been observed in ethylene glycol: H2O glasses at 77°K. Generally the Δm=2 transitions were observed and the parameter of spin—spin interaction determined was (D2+3E2)½. However for orotic acid, guanylic acid, and adenylic acid it was possible to observe the Δm=1 transitions and to determine D and E separately. From the values of D and E and the decay times, which were all >0.2 sec, it was possible to characterize the lowest triplet states as π—π* states in all cases. For thymine, 5‐methylcytosine, and orotic acid the values of D (and E) agreed quite well with those calculated approximately for π—π* states from Hückel molecular orbitals. In addition to resonances attributed to neutral molecules, additional resonances were observed for the ionized states of azathymine, thymine, 5‐methylcytosine, and 5‐hydroxymethylcytosine. From the pH dependence of azathymine and thymine it was possible to show that one resonance appeared when the molecule lost its first proton (T−) and the second resonance replaced it when the molecules had lost two protons. The T− resonance was also observed in TMP and thymidine at pH values above 9.8 where they each lose the N3 proton. However, these compounds did not show the resonance characteristic of the doubly ionized state, presumably because they contain only one ionizable proton. In 5‐methylcytosine and 5‐hydroxymethylcytosine, one resonance was observed in the neutral molecules. Above pH 12.4, which corresponded to the pK value for the loss of a proton, a different resonance was observed. In 5‐methylcytidine, in which the ionizable proton has been replaced by a ribose, only one resonance, characteristic of the neutral molecule, is observed. It is suggested that these results on the lowest triplet state can all be explained by dominant resonance structures in which double bonds switch from the carbonyl group to the ring.