The formation of triplet states in DNA is important because these long-lived states can initiate damaging photoreactions such as the formation of cyclobutane pyrimidine dimers or radical oxygen species. Photodamage to natural DNAs is reduced through ultrafast excited state decay pathways, but silver ion (Ag+) binding to DNA strands enhances pyrimidine dimer formation in some cases. The mechanism behind the enhanced dimerization remains obscure because little is known about how silver ion binding affects DNA excited state dynamics. To investigate the effects of Ag+ binding on the excited state relaxation of DNA, we studied self-assembled structures formed between Ag+ and the dC20 DNA strand or cytosine or cytidine monomers. Using femtosecond time-resolved infrared spectroscopy and QM/MM calculations, we identified a long-lived triplet state in Ag+-bound dC20 and Ag+-bound cytidine, both of which feature silver ion-mediated CC base pairs with strong propeller twisting. In contrast, the triplet state does not form in the self-assembled structures formed between Ag+ and the cytosine monomer, which are believed to adopt planar, sheet-like structures. Calculations reveal a complex potential energy surface that implicates formation of a triplet excimer state from a singlet charge transfer state, which is more accessible in twisted than in planar CC base pairs.

Intersystem crossing in Ag+ bound DNA through propeller twisting

ACS Spring Meeting 2021

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