(Specially Arranged Invited Paper) THE COMPETITION BETWEEN BIPROTONIC PHOTOTAUTOMERISM, EXIMER FORMATION, AND PROTON TUNNELING IN DNA BASE-PAIRS
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Publisher:Ohio State University
The competition between the rapid excitation phenomena proton-tunneling, molecular excimer formation, and biprotonic phototautomerism is discussed with special reference to the radiation chemistry and photochemistry of DNA and its component bases. Watson and Crick in their earliest consideration pointed out the possible genetic effect of tautomerization of the bases in the primary structure of DNA. Recent discussions by Lowdin, followed by electronic structure calculations on GC base pairs by Rein and Harris, Rein and Ladik, et al., have emphasized proton-tunneling as a quantum-mechanical mechanism involved in tautomerization. In contrast, studies of luminescence of DNA and polynucleotide analogs have suggested excimer formation as a preliminary to photochemical change. New experimental studies in our laboratory, carried out by C. A. Taylor, M. Ashraf El-Bayoumi and M. Kasha, have established the existence of a new phenomenon in N-heterocyclic base-pairs under excitation. It was shown that a cooperative transfer of two protons involved in hydrogen-bonding of the base-pair occurs in 7-azaindole as a model compound. Tautomenzation of the bases simultaneously occurred. Excimer formation was excluded. A temperature-dependence study in a continuation of the research by El-Bayoumi excludes proton-tunneling as a significant phenomenon in this case. The existence of four typical potential models covering possible tautomerization cases in natural base-pairs is discussed in relation to an extension of these results to the DNA photomutation problem. Theoretical calculations recently carried out by Horowitz, Evleth, and Kasha (Santa Cruz) on the variation of excited state energies of the normal and tautomerized N-perturbed indoles from 7-azaindole to purine to adenine support the biprotonic phototautomerization as a relevant excitation step in DNA base-pair excited state chemistry. Recent ab initio theoretical work by Clementi, et al., confirms the double-minimum potential model for cooperative transfer of two protons in doubly-hydrogen bonded systems, as envisaged in the biprotonic phototautomerism.
Author Institution: Institute of Molecular Biophysics, Florida State University