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

Area of Science:

• Photochemistry
• Biophysical Chemistry
• Supramolecular Chemistry

Background:

• Excited electronic states in DNA are crucial for understanding photophysical processes.
• Metal ion interactions can significantly alter DNA structure and properties.
• Cytosine-metal ion complexes are of interest due to their unique structural and electronic features.

Purpose of the Study:

• Investigate the photodynamics of excited electronic states in silver(I)-cytosine DNA structures.
• Elucidate the role of DNA structure and silver ion binding on excited state properties.
• Determine the formation and nature of excited states in these self-assembled systems.

Main Methods:

• Time-resolved infrared (TRIR) spectroscopy to probe excited state dynamics.
• Quantum mechanical calculations to understand electronic structure and energetics.
• Varying pH, DNA strand, and silver ion concentrations to control self-assembly.

Main Results:

• DNA-silver complexes form propeller-twisted duplexes with C-Ag+-C base pairs at higher concentrations.
• UV excitation of these duplexes yields a long-lived triplet excimer state localized on stacked cytosines.
• Computational and experimental data show no evidence of cytosine-cytosine photoproduct formation.
• Intersystem crossing to the triplet state is facilitated by propeller-twisted motifs, not a heavy atom effect.

Conclusions:

• Supramolecular structural changes induced by silver ion binding profoundly impact DNA photophysics.
• A long-lived triplet excited state can form in significant yield within DNA duplexes.
• Metal-ion mediated DNA structures exhibit unique photophysical behavior distinct from standard DNA photochemistry.