Physical Chemistry

Anomalous excited state behaviour of purine tautomers -- an excited dynamics study



The remarkable photostability of canonical nucleobases makes them ideal building blocks for DNA and RNA. Even minor structural changes are expected to lead to drastic alteration of their sub-picosecond excited state lifetimes. However, it is interesting to note that while the 9H- and 7H-amino tautomers of adenine possess drastically different lifetimes, 9H- and 7H-keto guanine possess similar excited state lifetimes. With an aim to explain this anomalous difference in sensitivity of lifetimes to tautomerization, we have investigated the excited state relaxation mechanism of UV-excited adenine and guanine tautomers using surface hopping based nonadiabatic molecular dynamics. We find that internal conversion in both guanine tautomers is almost barrierless while both adenine tautomers encounter significant barriers before they can deactivate. Moreover, the major deactivation channel (C2-puckering) in 9H-amino adenine is overall more efficient than the one (C6-puckering) in the 7H-amino form. We trace this difference to the frequent rotation of the amino group which disrupts its conjugation with the heterocyclic ring thereby reducing the strength of non-adiabatic coupling and, hence, delaying internal conversion.


Thumbnail image of purine_manuscripts.pdf

Supplementary material

Thumbnail image of Supplementary.pdf
Supplementary information for Anomalous excited state behaviour of purine tautomers -an excited dynamics study
This file contains supplementary information, including the tables for comparison of vertical excitation energies at a different levels of theory, plots for KS-orbitals involved in the lowest-lying singlet excited state for all purine tautomers, plots for critical structures of all tautomers, LIIC paths C6-puckered CI channels for 9HA and 7HA, and KDE plots for structural parameters for all tautomers.