The relationship between interfacial bonding and radiation damage in adsorbed DNA

We have performed a comparison of the radiation damage occurring in DNA adsorbed on gold in two different configurations, when the DNA is thiolated and bound covalently to the substrate and when it is unthiolated and interacts with the substrate through the bases. Both molecules were found to organize so as to protrude from the surface at ~45 degrees. Changes in the time-dependent C 1s and O 1s X-ray photoelectron (XP) spectra resulting from irradiation were interpreted to arise from cleavage of the phosphodiester bond and possibly COH desorption. By fitting the time-dependent XP spectra to a simple kinetic model, time constants were extracted, which were converted to cross sections and quantum yields for the damage reaction. The radiation induced damage is significantly higher for the thiolated DNA. N 1s X-ray absorption spectrum revealed the N-C=N LUMO is more populated in the unthiolated molecule, which is due to a higher degree of charge transfer from the substrate to this LUMO in the unthiolated case. Since the N-C=N LUMO of the thiolated molecule is comparatively less populated, it is more effective in capturing low energy electrons resulting in a higher degree of damage.