Electronic structures and spectra of 6-mercaptopurine and 6-thioguanine

Ultraviolet absorption, steady-state fluorescence and excitation spectra of 6-mercaptopurine (6MP) and 6-(thioguanine) (6TG) have been studied, and excited state life-time measurements on their fluorescence performed. The experimental results have been interpreted with the help of molecular orbital calculations. The study shows that these molecules predominantly exist in thione forms. The properties of the molecules have been compared with those of guanine and the possible significance of the results is discussed.