Triplet Excimer Formation in a DNA Duplex with Silver Ion-Mediated Base Pairs
Lara Martínez‐Fernández, Forrest R. Kohl, Yuyuan Zhang, Supriya Ghosh, Andrew J. Saks, Bern Kohler
Abstract
The dynamics of excited electronic states in self-assembled structures formed between silver(I) ions and cytosine-containing DNA strands or monomeric cytosine derivatives were investigated by time-resolved infrared (TRIR) spectroscopy and quantum mechanical calculations. The steady-state and time-resolved spectra depend sensitively on the underlying structures, which change with pH and the nucleobase and silver ion concentrations. At pH ∼ 4 and low dC 20 strand concentration, an intramolecularly folded i-motif is observed, in which protons, and not silver ions, mediate C–C base pairing. However, at the higher strand concentrations used in the TRIR measurements, dC 20 strands associate pairwise to yield duplex structures containing C–Ag + –C base pairs with a high degree of propeller twisting. UV excitation of the silver ion-mediated duplex produces a long-lived excited state, which we assign to a triplet excimer state localized on a pair of stacked cytosines. The computational results indicate that the propeller-twisted motifs induced by metal-ion binding are responsible for the enhanced intersystem crossing that populates the triplet state and not a generic heavy atom effect. Although triplet excimer states have been discussed frequently as intermediates in the formation of cyclobutane pyrimidine dimers, we find neither computational nor experimental evidence for cytosine–cytosine photoproduct formation in the systems studied. These findings provide a rare demonstration of a long-lived triplet excited state that is formed in a significant yield in a DNA duplex, demonstrating that supramolecular structural changes induced by metal ion binding profoundly affect DNA photophysics.