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Excited-State Two-Proton Tautomerism in Hydrogen-Bonded N-Heterocyclic Base Pairs

Carl Al Taylor, M. Ashraf El-Bayoumi and Michael Kasha
Proceedings of the National Academy of Sciences of the United States of America
Vol. 63, No. 2 (Jun. 15, 1969), pp. 253-260
Stable URL: http://www.jstor.org/stable/59250
Page Count: 8
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Excited-State Two-Proton Tautomerism in Hydrogen-Bonded N-Heterocyclic Base Pairs
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Abstract

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.

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