6MP consists of a purine core chromophore thionated at the C6 position (Scheme 1). Aza has the same thiopurine chromophore but is functionalized with an electron-withdrawing nitroimidazole group at the sulfur atom (Scheme 1). The pKa corresponding to the purine core for these prodrugs lies between 7.7-7.87, which results in both 6MP and Aza existing as a mixture of the neutral and the conjugate base (deprotonated) species at physiological pH. Using the Henderson-Hasselbalch relationship, we estimate that about 67% to 75% of the prodrugs exist in its neutral form at pH 7.4, depending on the pKa value used, while the remaining 25% to 33% exist in the deprotonated form. Hence, to isolate the neutral and deprotonated species of 6MP and Aza, experiments were also performed in PBS at pH 5.5 (> 99% neutral) and in borate buffer at pH 9.8 (>99% deprotonated).