Thymine dimer repair by electron transfer from photo-excited 2′,3′,5′-tri-O-acetyl-8-oxo-7,8-dihydroguanosine or 2′,3′,5′-tri-O-acetyl-ribosyluric acid – a theoretical study

Abstract

Electronic structure calculations are combined with published experimental data from another laboratory to interpret trends in the rates of thymine dimer repair induced by photo-exciting the title molecules or their deprotonated derivatives. Opening of the thymine dimer's cyclobutane ring is believed to be initiated by electron transfer from the photo-excited molecule and to then pass over thermally accessible energy barriers. Therefore, the repair rates are determined by rates of accessing activation barriers connecting the photo-excited state to the electron-transferred state. These barriers are shown to depend on the electronic excitation energy and electron-binding energy of the donor and the electron affinity of the thymine dimer acceptor. For neutral donors, the barriers also depend on the distance between the donor and the thymine dimer through a screened Coulomb interaction between the donor cation and acceptor anion. For the deprotonated (anionic) donors, this Coulomb-derived distance dependence is absent. For both neutral and anionic donors, the range for electron transfer is spatially limited by the strength of the electronic couplings. The model put forth here rationalizes why anionic donors can be expected to perform better than neutrals and offers a framework for designing electron transfer agents optimal for a given electron acceptor.

Keywords:

• thymine dimer repair
• electron transfer

Acknowledgements

We thank the Polish National Science Center (NSC) for grant no. 530-8371-D191-12 to P.S. Access to computer resources from the Center for High Performance Computing at the University of Utah is also acknowledged. Fruitful instruction on Marcus theory from Prof. R.J. Cave is much appreciated.

Notes

^aAs explained earlier, these DE data reflect solvent entropic contributions as well. In each case, the first number is the adiabatic DE and the second is the vertical DE.

^aAs explained earlier, these IP data reflect solvent entropic contributions as well. In each case, the first number is the adiabatic IP and the second is the vertical IP.

As discussed in ref. [4] fluctuations in orientation and donor-acceptor distance would be expected to alter the strength of electronic couplings. This would lead us to expect that the intrinsic coupling strengths in the solution-phase experiments could be less than in the DNA experiments where the OGH and T=T distances are orientations are relatively fixed. However, the order of magnitude estimate this analysis provides suggests that the solution-phase coupling strengths may not be attenuated much if at all.

This assumes that the electronic couplings strengths can extend the range over which electron transfer is facile a bit beyond 6 Å. In the DNA oligomers, the base spacing of ca. 3.5 Å per helical turn limits the OG-to-T=T distances to ca. 3.5, 7, 10.5, etc. Å. Using the exponential decay exp(−βR) fit inferred in ref. 1 with β = 0.6 Å⁻¹, one predicts a falloff in electron transfer rates of when the OG-to-T=T distance increases from 7 Å to 10.5 Å. This order of magnitude falloff is consistent with the decay of T=T repair yields seen in the data of ref. [2]. On the other hand, for the solution-phase experiments, a decay governed by exp(−0.6R) could generate T=T repair for R-values between R = 6 Å and 8 Å at rates within the same order of magnitude as for R = 6 Å.