Protonation of cytosine: Cytosinium vs hemicytosinium duplexes

Cytosine, a nucleobase, can exhibit two protonated states, cytosinium and hemicytosinium. The controlled synthesis of structures containing these ions is highly desired but not yet achieved. Herein, we report strategies for robust synthesis of both structures by controlling the strength of an acid used for protonation and its concentration. The duplex structure is always obtained by using an acid with a pK_a > 4.2, which is incapable of disrupting the relatively stable duplex structure. When stronger acids (pK_a < 4.19) are used, the duplex structure is obtained by controlling acid concentration to protonate a half of cytosine in solution, and the cytosinium structure is obtained with excess acid. These strategies are successfully applied to synthesize both forms of 5-fluorocytosine, an antifungal drug. The hemicytosinium structure exhibits superior physicochemical properties than the parent drug and the cytosinium salt. These strategies may be useful to prepare materials important to various branches of science, ranging from biology to nanodevice fabrication and to pharmaceuticals.