H-Bonded N-Heterocyclic Base-Pair Phototautomerizational Potential Barrier and Mechanism: The 7-Azaindole Dimer

• Published in: Proceedings of the National Academy of Sciences - PNAS 2004-01, Vol.101 (2), p.419-422
• Main Authors: Catalán, J., Pérez, P., del Valle, J. C., de Paz, J. L. G., Kasha, M.
• Format: Article
• Language: English
• Subjects: Activation energy; Chemistry; Creeks; Deoxyribonucleic acid; Dimers; DNA; Emission spectra; Energy value; Fluorescence; Geometry; Kinetics; Physical Sciences; Protons; Tautomers
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0306721101

A theoretical analysis of the double proton transfer (PT) in a hydrogen-bonded N-heterocyclic base pair is presented. The calculated (time-dependent density functional theory) double PT barrier calculated for the concerted process of the 7-azaindole C2hdimer in the first excited singlet electronic state S1conforms well to the kinetic data and the photophysical evidence reported in this article. The calculated PT energy barrier of 4.8 kcal/mol height, and the corresponding zero point energy value, yield for the S1state an activation energy barrier of 0.3 kcal/mol. This finding implies that the double PT concerted process is almost barrierless, confirming previous experiments. Upon N-H deuteration of the 7-azaindole dimer, the theoretical excited-state activation energy for the double deuterium transfer is determined to be 1.4 kcal/mol, in agreement with experiment, which in low-temperature spectroscopy is shown to negate excited-state double-deuteron transfer.