On the use of the spectral luminescent method for studying acid-base equilibria of uracil derivatives in aqueous solutions.

The effect of pH on fluorescence parameters (spectral composition, intensity I, and quantum yield φ) of thymine (T), 5-fluorouracil (FU) and 6-aminouracil (AU) have been investigated in aqueous solutions. As found, there is the regularity of the changes in the spectral composition, intensity, and quantum yields of uracil fluorescence depending on pH. The constants of the acid-base equilibria of uracils (pK) determined by the fluorescence titration method are: 9.9 (T), 8.0 (FU), and 9.1 (AU) at 298 K. These pK values satisfactorily agree with the previous estimates obtained by independent experimental or quantum chemical methods. The possibilities of the spectral fluorescence method for the quantitative description of the acid-base equilibria of uracils depending on the fluorescent features of the pyrimidinedione and its conjugate base are discussed.