Abstract
The study of radiation effects in DNA is a multidisciplinary endeavour, connecting the physical, chemical and biological sciences. Despite being mostly filtered by the ozone layer, sunlight radiation is still expected to (photo)ionise DNA in sizeable yields, triggering an electron removal process and the formation of potentially reactive cationic species. In this manuscript, photoionisation decay channels of important DNA tautomeric derivatives, 7H-adenine and 7H-guanine, are characterised with accurate CASSCF/XMS-CASPT2 theoretical methods. These simulation techniques place the onset of ionisation for 7H-adenine and 7H-guanine on average at 8.98 and 8.43 eV, in line with recorded experimental evidence when available. Cationic excited state decays are analysed next, uncovering effective barrierless deactivation routes for both species that are expected to decay to their (cationic) ground state on ultrafast timescales. Conical intersection topographies reveal that these photoionisation processes are facilitated by sloped single-path crossings, known to foster photostability, and which are predicted to enable the (VUV) photo-protection mechanisms present in these DNA tautomeric species.
Highlights
The study of radiation induced DNA damage is a topic of utmost interest to a broad range of disciplines as it is known to mediate in the formation of lesions [1] that lead to mutations and to healthcare concerns such as skin cancer melanoma [2]
The first cationic state in 7H-purine derivatives is characterised by the 2π+H state, which corresponds to an unpaired electron in the singly occupied molecular orbital (SOMO)
This is in line with what has been observed for other DNA/RNA nucleobase systems, where the SOMO is always the one embodying the first electron removal upon ionising radiation exposure [24,27,28,29,30]
Summary
The study of radiation induced DNA damage is a topic of utmost interest to a broad range of disciplines as it is known to mediate in the formation of lesions [1] that lead to mutations and to healthcare concerns such as skin cancer melanoma [2]. We focus on studying the 7H tautomers of both adenine and guanine, as these feature several (accessible) cationic electronic excited states upon ionisation, making the comparison between analogous tautomers featuring different purine-substituted frames more interesting in terms of understanding how chemical substitution impacts their photophysics These systems are of interest due to their strong resemblance with our current genetic lexicon, and studying their photo-protection mechanisms may eventually help us understand how our current nucleobases were chosen during prebiotic times and whether photostability played a critical role in it [13,14]. The characterised sloped and single-path conical intersection topographies support ultrafast deactivation channels present for both systems, which are expected to be central in fostering photostability, and that suggest (VUV) photo-protection mechanisms in DNA nucleobases might be available to (at least some of) their tautomeric derivatives
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