Modelling Photoionisations in Tautomeric DNA Nucleobase Derivatives 7H-Adenine and 7H-Guanine: Ultrafast Decay and Photostability

The study of radiation effects in DNA is a multidisciplinary endeavour, connecting the physical, chemical and biological sciences. Despite being mostly filtered by ozone layer, sunlight still expected to (photo)ionise sizeable yields, triggering an electron removal process formation potentially reactive cationic species. In this manuscript, photoionisation decay channels important tautomeric derivatives, 7H-adenine 7H-guanine, are characterised with accurate CASSCF/XMS-CASPT2 theoretical methods. These simulation techniques place onset ionisation for 7H-guanine on average at 8.98 8.43 eV, line recorded experimental evidence when available. Cationic excited state decays analysed next, uncovering effective barrierless deactivation routes both species that their (cationic) ground ultrafast timescales. Conical intersection topographies reveal these processes facilitated sloped single-path crossings, known foster photostability, which predicted enable (VUV) photo-protection mechanisms present