Fluorescence of Bimolecular Guanine Quadruplexes: From Femtoseconds to Nanoseconds

Abstract

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 timeresolved 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.

The Ubiquity of High-Energy Nanosecond Fluorescence in DNA Duplexes

“…This means that a helical geometry alone is not sufficient to give rise to HENE. In contrast, it was detected for a four-stranded structure (G-Quadruplex) formed by association of two identical DNA strands d­(G 4 T 4 G 4 ) in the presence of K + cations . For this system, a long-lived fluorescence with a lifetime of 2.1 ns was observed at 315 nm, representing 10.8% of the photons emitted at this wavelength.…”

“…A short description of HENE was reported recently in our account of intrinsic DNA fluorescence . In the meantime, new studies ,, rendered the assessment of the excited states responsible for this intriguing emission much more complex than that proposed by the first computational approaches. , Therefore, we have decided to revisit the current state of knowledge of this poorly understood, yet ubiquitous, fluorescence emission in the present Perspective. After presenting an overview of the most striking results, we highlight the importance of HENE relative to LENE in the steady-state fluorescence spectra of duplexes, discuss potential connections with structural and electronic issues, stress the need for further computational studies, and provide experimental tips that are crucial for more refined studies.…”

“…Not only is it formed by two identical DNA strands, but also the bases are arranged in a symmetric way. , First, the guanines adopt an antiparallel arrangement. Second, we have two identical diagonal thymine loops allowing for orbital overlap between a guanine and a thymine . In contrast, no HENE was found for its tetramolecular analogue, formed by association of four identical DNA strands d­(TG 4 T).…”

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

“…Note that previous research, based mainly on the steady-state measurements, has observed substantial influences of the loop and/or dangling bases on the yield and maximum intensity wavelength of GQ fluorescence. ,, Recent studies on BGQ/Na + reported a strong CT state between a thymine in a loop and a guanine in the outer tetrad; , whether this state may contribute to the fluorescence of BGQ/Na + has however remained unresolved. On the other hand, it was observed in a very recent time-resolved study that thymines in loops are responsible for a weak low-energy excimer emission (at ∼445 nm) derived from comparison of steady-state fluorescence for a bimolecular DNA GQ formed by dG 4 T 4 G 4 . In light of this, the capture and identification of the ∼475 nm transient fluorescence provide the first time-resolved evidence for participation of loop nucleotides through CT in the deactivation and intrinsic fluorescence of GQ.…”

“…On the other hand, it was observed in a very recent time-resolved study that thymines in loops are responsible for a weak low-energy excimer emission (at ∼445 nm) derived from comparison of steady-state fluorescence for a bimolecular DNA GQ formed by dG 4 T 4 G 4 . 53 In light of this, the capture and identification of the ∼475 nm transient fluorescence provide the first time-resolved evidence for participation of loop nucleotides through CT in the deactivation and intrinsic fluorescence of GQ. The result therefore improves understanding of the origin of the complex fluorescence from GQs.…”