Excited-state two-proton tautomerism in hydrogen-bonded n-heterocyclic base pairs.
The optical absorption and luminescence spectra of 7-azaindole and its doubly hydrogen-bonded dimer were investigated as a model for the study of electronic interactions in DNA base pairs. It is demonstrated that a biprotonic phototautomerism occurs in the dimer and in suitable ethanol solvates in fluid solvents but that the phenomenon is not observed in a rigid solvent matrix. The normal violet structured fluorescence of 7-azaindole monomer becomes a broad green fluorescence in the tautomer. It is shown that spectral band interchanges, excimer formation, excited-state single-proton transfer, and proton tunneling cannot account for the luminescence change, but that the molecular exciton effect facilitates the cooperative two-proton reversible transfer. It is proposed that biprotonic phototautomerism with molecular environmental sensitivity could provide a mechanism for the initial step in ultraviolet mutagenic effects in DNA.
D004247:DNA; D006571:Heterocyclic Compounds; D006859:Hydrogen; D008956:Models, Chemical; D011522:Protons; D013057:Spectrum Analysis; D014466:Ultraviolet Rays
D006571:Heterocyclic Compounds; D011522:Protons; D006859:Hydrogen; D004247:DNA