Prototropic Interactions of Pyrimidine Nucleic Acid Bases with Acridine: A Spectroscopic Investigation

In this article, we have investigated the interactions of three pyrimidine nucleic acid bases, cytosine (C), thymine (T), and uracil (U) with acridine (Acr), an N-heterocyclic DNA intercalator, through the changes in photophysics of Acr inside SDS micelles. Fluorescence of AcrH⁺* at 478 nm and its lifetime are quenched on addition of C, T, and U, while a concomitant increment of Acr* is observed only with C. However, the relative amplitude of Acr* increases with a simultaneous decrease in AcrH⁺* only with C. The fluorescence quenching of AcrH⁺* is explained by photoinduced electron transfer (PET), while changes in the relative contributions of Acr* and AcrH⁺* with C are due to associated excited-state proton transfer (ESPT). The rate of electron transfer (<i>k</i>_ET) is maximum for T, followed by U and C. The associated ESPT from AcrH⁺* is the reason behind the reduced efficiency of PET with C. The lack of proton transfer with T and U as well as the higher <i>k</i>_ET for T compared to U are explained by keto–enol tautomerization and subtle changes in the structure and geometry of the pyrimidine bases