Excited-State Charge Transfer, Proton Transfer, and Exciplex Formation Revealed by Ultrafast Time-Resolved Spectroscopy in Human Telomeric Ribonucleic Acid Quadruplex

Guanine quadruplexes (GQs), important for genome stability and biotechnology application, can form from both DNA and RNA. However, unlike the study of DNA GQs, little research has been conducted on excited states of GQs from RNA, which due to the ribose 2'-hydroxy group have structures distinct from their DNA counterparts. Combining ultrafast broadband time-resolved fluorescence and transient absorption measurements, we report the first direct probe of excitation dynamics for a bimolecular GQ from human telomeric repeat-containing RNA taking the typical highly compacted parallel folding with a propeller-like loop structure. The result revealed a multichannel decay containing an unusual high-energy excimer featuring charge transfer deactivated by rapid proton transfer in the tetrad core region. It also identified an unprecedented exciplex displaying massively red-shifted fluorescence produced from charge transfer in the loop region. The findings underscore the role of structural conformation and base content in determining the energy, electronic attribution, and decay dynamics of GQ excited states.