posted on 2022-12-28, 20:07authored byEvangelos Balanikas, Thomas Gustavsson, Dimitra Markovitsi
The paper deals with the fluorescence of guanine quadruplexes
(G4)
formed by association of two DNA strands d(GGGGTTTTGGGG) in
the presence of K+ cations, noted as OXY/K+ in
reference to the protozoon Oxytricha nova, whose telomere contains TTTTGGGG repeats. They were studied by
steady-state and time-resolved techniques, time-correlated single
photon counting, and fluorescence upconversion. The maximum of the
OXY/K+ fluorescence spectrum is located at 334 nm, and
the quantum yield is 5.8 × 10–4. About 75%
of the photons are emitted before 100 ps and stem from ππ*
states, possibly with a small contribution of charge transfer. Time-resolved
fluorescence anisotropy measurements indicate that ultrafast (<330
fs) excitation transfer, due to internal conversion among exciton
states, is more efficient in OXY/K+ compared to previously
studied G4 structures. This is attributed to the arrangement of the
peripheral thymines in two diagonal loops with restricted mobility,
facilitating the interaction among them and with guanines. Thymines
should also be responsible for a weak intensity excimer/exciplex emission
band, peaking at 445 nm. Finally, the longest living fluorescence
component (∼2.1 ns) is observed at the blue side of the spectrum.
So far, high-energy long-lived emitting states had been reported only
for double-stranded structures but not for G4.