Chemical photocatalysis with oligonucleotides and short peptides

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One of the major challenges for chemistry is the enhanced use of light as sustainable energy source for chemical transformations. Classic photosensitization and modern photoredox catalysis are nowadays combined under the roof of "chemical photocatalysis". For the first funding period of this project it was proposed to apply oligonucleotides and oligopeptides for chemical photocatalysis. Based on the published results from the first funding period, both photocatalytically active DNA (project part A) and photocatalytic nucleophilic additions and oligopeptides (project part B) will be further developed with respect to following main objectives:Project part A. The triplet energy transfer through the double-stranded DNA is the rate-limiting step if the triplet energy is kinetically trapped and analyzed by the yield of T-T dimers over the irradiation time. The exponential distance dependence that was determined for the T-T dimer yield in the first funding period of this project fits best with a Dexter-type triplet-triplet energy transfer mechanism, but is too shallow for a coherent, one-step process from the donor to the site of T-T dimerization. Hence, we propose a stepwise triplet energy hopping process over long ranges (more than 3-4 intervening base pairs) that consists of several short-range Dexter-type energy transfers from single base pair to single base pair, which is supported by theory. By our approach using benzophenones and xanthones as C-nucleosides in DNA, we are able to study this pathway of DNA-damaging energy dissipation for the first time in detail. The full understanding of the mechanism of triplet energy transfer through the DNA is an important task not only for the formation of photoinduced DNA damages, but also for the synthetic photoDNAzymes that were elucidated in the first funding period of this project. Project part B. For the photoredox catalytic nucleophilic addition of alcohols to styrene derivatives, we could show that the regioselectivity can be controlled by the type of photoinduced charge transfer that was initiated by the photoexcited catalyst. The photoreduction of styrene substrates yields the Markovnikov-type products, whereas the photooxidation yields the anti-Markovnikov-type products. The central objective of this project part is the broadening of this photoredox catalytic reactivity. The substrate scope will be extended to styrene derivatives and further to non-aromatic and alkylated olefins. This requires increased excited state redox potentials which will be achieved by extremely electron-rich N-phenyl phenothiazines and extremely electron-poor naphthalene bisimides. Two-photon excitation strategies will further shift the excited state potentials. Proline-induced enamines from aldehydes will be applied as C-nucleophiles for the nucleophilic additions, and stereoselectivity is achieved by proline-based short peptides.