Photoinduced Electron Transfer Mediated by π-Stacked Thymine-Hg2+-Thymine Base Pairs

Fluorescence spectra of thymine-containing oligonucleotides labeled with donor/acceptor in the presence of Hg2+ ions were investigated. Formation of T-Hg2+-T base pairs gives rise to a hairpin structure and makes both termini of the oligonucleotide close to each other. For fluorescein or tetramethylrhodarrine single-labeled oligonucleotide, fluorescence quenching was observed on addition of Hg2+ ions. For fluorescein and tetramethylrhodamine double-labeled oligonucleotide, the apparent FRET efficiencies decrease unexpectedly in the presence of Hg2+ ions. The unusual fluorescence quenching in the presence of Hg2+ ions was ascribed to formation of T-Hg2+-T base pairs, which can accept and mediate the electron transfer and provide an additional de-excitation process for the excited state of fluorophores via photoinduced electron transfer.