Triplet Excimer Formation in a DNA Duplex with Silver Ion-Mediated Base Pairs

Abstract

The dynamics of excited electronic states in selfassembled structures formed between silver(I) ions and cytosinecontaining DNA strands or monomeric cytosine derivatives were investigated by time-resolved infrared (TRIR) spectroscopy and quantum mechanical calculations. The steady-state and timeresolved 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.

The photoactivated dynamics of dGpdC and dCpdG sequences in DNA: a comprehensive quantum mechanical study

“…59,60 This value, which is consistent with those suggested on the basis of extensive comparison between experimental and computed harmonic spectra, considering the functional and the basis set adopted, 59,60 has been already profitably adopted in previous studies of oligonucleotides. 16,21,61,62 Each stick transition has been broadened with a Lorentzian with half width at half maximum of 10 cm −1 . Difference IR spectra associated with the different excited states are then computed by subtracting the GS IR spectrum to the IR spectrum of each ES.…”

Mentha suaveolens Leaves Extract‐Mediated Synthesis of CoO@TiO2 and Fe2O3@TiO2 Core/Shell Nanoparticles with Synergistic Antimicrobial and Antioxidant Activities

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