The stability of 1 and 2 was analyzed at 80 °C in Pyrex® glass tubes with screw caps. Therefore, the nucleobases were dissolved to a concentration of 5 mM in the following four buffers: (a) 50 mM glycine/NaOH buffer pH 9, (b) 50 mM glycine/NaOH buffer pH 9 with 5 mM 1,4dithioreitol (DTT), (c) 50 mM glycine/NaOH buffer pH 9, saturated with nitrogen and (d) 50 mM gylcine/NaOH buffer pH 9 with 5 mM DTT, saturated with nitrogen. Samples were taken after 0 h, 2 h, 4 h and 24 h, and diluted to a final concentration of 1 mM nucleobase with MeOH. After centrifugation (4 °C and 21,500 g for 20 min), samples were analyzed by analytical HPLC as described below.