EXCITED-STATE TWO-PROTON TAUTOMERISM IN HYDROGEN-BONDED N-HETEROCYCLIC BASE PAIRS*

  1. Carl Al Taylor,
  2. M. Ashraf El-Bayoumi†, and
  3. Michael Kasha
  1. INSTITUTE OF MOLECULAR BIOPHYSICS, DEPARTMENT OF CHEMISTRY, FLORIDA STATE UNIVERSITY, TALLAHASSEE

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.

Footnotes

  • ↵ † Present address: Department of Biophysics, Michigan State University, East Lansing, Michigan.

  • ↵ * This work was supported by a contract between the Division of Biology and Medicine, U.S. Atomic Energy Commission, and the Florida State University.

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  1. PNAS 1969-06 vol. 63 no. 2 253-260
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