Photoinduced phenomena in DNA is of interest to the scientific community due to its relation with growing healthcare concerns such as skin cancer melanoma. Most studies so far have focused on the effects of low-energy UV-A/B laser pulses, as these are akin to the type of incident Sunlight radiation we are exposed to on a daily basis, which is not fully filtered by the ozone layer and that is believed to trigger a number of photochemically productive reactions (mutations) in our genomic material. Much less attention has been paid however to the effects of photoionisation, which refers to the process of photoinduced electron removal in chromophoric species in DNA, i.e. the nucleobases, producing their cationic forms, and that is featured in the presence of higher-energy (UV-C and Alpha-particle) light sources or in multimeric DNA/RNA species upon UV-A/B exposure. Albeit scarcely studied, cations are significantly produced in DNA and are considered to be responsible of damaging instances such as DNA cross links, base releases and single/double strand breaks known to occur upon radiation exposure.<br/> <br/>In this contribution we thoroughly analyse a range of photoionisation phenomena in DNA/RNA nucleobases (uracil, thymine, cytosine and the epigenetic 5-methyl-cytosine), assessing the differences observed by initially accessing diverse cationic states, and predict spectral fingerprints to identify the specific molecular state and unique molecular motions driving these ultrafast photo-reactions. To do so we simulate the deactivation channels of all low-lying accessible cationic states with the strongly correlated complete active-space self-consistent field (CASSCF) technique and its second-order perturbation theory extension (XMS-CASPT2). We employ two separate computational approaches; one based on static calculations where we characterise the critical molecular structures modulating the photo-process such as ground, excited state minima and conical intersections connecting the different electronic states, identifying the most energetically favourable decay channel; and another one where we use non-adiabatic molecular dynamics schemes to obtain estimates of the lifetimes and time-resolved spectral fingerprints unique to the differently populated electronic cationic states. We then combine the results obtained for all systems, connecting the similarities among them and uncovering the role played by 5-methylation in this ultrafast photo-process.

Simulating photoionisation phenomena in DNA/RNA pyrimidine nucleobases

2019 Fall National Meeting

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