Photostability and Photoreactivity in Biomolecules: Quantum Chemistry of Nucleic Acid Base Monomers and Dimers

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The great potentials of high-level ab initio methods, in particular, the CASPT2//CASSCF protocol, are fully illustrated through: (i) the study of ultrafast energy relaxation in DNA/RNA base monomers, (ii) the intrinsic population mechanism of the lowest triplet state, and (iii) how bioexcimers can be considered as precursors of charge transfer and photoinduced reactivity. In order to describe these processes properly, the presence of conical intersections (CIs) and the topology of the involved pathways have to be determined correctly. Thus, in theoretical calculations the dynamic electronic correlation has to be considered. The accessibility of the CIs (or the seam of CIs) becomes crucial to understand the theoretical foundations of the overall photochemistry of the system. It is shown that from the minimum energy path (MEP) computed for the spectroscopic ΠΠ* excited state for the five natural nucleic acid bases, i.e., uracil, thymine, cytosine, adenine, and guanine, the system ultimately reaches in a direct fashion a CI connecting the initially excited state and the respective ground state, where a distorted out-of-plane ethene-like structure is obtained. Such CI can be seen as the basic feature responsible for the known photostability of the genetic material. Along the internal conversion processes, efficient singlet-triplet crossings are made apparent by the favorable intersystem crossing (ISC) mechanism. Similarly, it is concluded that the ultrafast electron transfer taking place in photosynthetic reaction centers can essentially be understood as a radiationless transition mediated by a CI. In addition, the photodimerization reaction of cytosine along the triplet manifold is mediated by a triplet-singlet crossing and it is revealed to be barrierless