THE POLYMORPHISM OF A CYTOSINE ANION STUDIED BY ELECTRON-PARAMAGNETIC RESONANCE SPECTROSCOPY

Electron addition to the nucleic acid base cytosine and several derivatives was studied in frozen aqueous solution and in aqueous glasses containing 10 M glucose, 7 M BeF2, 10 M LiCl and 8 N NaOH. Electron paramagnetic resonance spectroscopy revealed that three major types of spectra are obtained which can be classified into a group D giving a doublet (ca. 1.6 mT) and a group of triplet patterns T and T1 (ca. 3.0 mT total spread) which differ slightly in coupling values but which are both distinct from D by showing the loss of one sizeable proton coupling (ca. 1.0 mT) in deuterated solvents. Varying the type of glasses and other parameters such as pH, together with modification of the cytosine exocyclic groups, indicated that the triplet patterns represent electron adducts protonated at the amino group, whereas the doublets D should probably involve N3-protonated species. The equilibrium between doublets and triplets was found to depend strongly on the presence of Li+ and Be2+ and less strongly on pH or the presence of Na+, the former group of ions inducing the triplets. It is suggested that the influence of the ions derives mostly from their strength in stabilizing the amino-protonated tautomer of the electron adducts.