Thomas Gustavsson

Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds

Excited-state proton transfer has been hypothesized as a mechanism for UV energy dissipation in eumelanin skin pigments. By using time-resolved fluorescence spectroscopy, we show that the previously proposed, but unresolved, excited-state intramolecular proton transfer (ESIPT) of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) occurs with a time constant of 300 fs in aqueous solution but completely stops in methanol. The previously disputed excited-state proton transfer involving the 5- or 6-OH groups of the DHICA anion is now found to occur from the 6-OH group to aqueous solvent with a rate constant of 4.0 × 10(8) s(-1).

High-Energy Long-Lived Mixed Frenkel–Charge-Transfer Excitons: From Double Stranded (AT)n to Natural DNA

The electronic excited states populated upon absorption of UV photons by DNA are extensively studied in relation to the UV-induced damage to the genetic code. Here, we report a new unexpected relaxation pathway in adenine-thymine double-stranded structures (AT)n . Fluorescence measurements on (AT)n hairpins (six and ten base pairs) and duplexes (20 and 2000 base pairs) reveal the existence of an emission band peaking at approximately 320 nm and decaying on the nanosecond time scale. Time-dependent (TD)-DFT calculations, performed for two base pairs and exploring various relaxation pathways, allow the assignment of this emission band to excited states resulting from mixing between Frenkel excitons and adenine-to-thymine charge-transfer states. Emission from such high-energy long-lived mixed (HELM) states is in agreement with their fluorescence anisotropy (0.03), which is lower than that expected for π-π* states (≥0.1). An increase in the size of the system quenches π-π* fluorescence while enhancing HELM fluorescence. The latter process varies linearly with the hypochromism of the absorption spectra, both depending on the coupling between π-π* and charge-transfer states. Subsequently, we identify the common features between the HELM states of (AT)n structures with those reported previously for alternating (GC)n : high emission energy, low fluorescence anisotropy, nanosecond lifetimes, and sensitivity to conformational disorder. These features are also detected for calf thymus DNA in which HELM states could evolve toward reactive π-π* states, giving rise to delayed fluorescence.

Ultrafast Electron Transfer in Complexes of Doxorubicin with Human Telomeric G‐Quadruplexes and GC Duplexes Probed by Femtosecond Fluorescence Spectroscopy

Doxorubicin (DOX) is a natural anthracycline widely used in chemotherapy; its combined application as a chemotherapeutic and photodynamic agent has been recently proposed. In this context, understanding the photoinduced properties of DOX complexes with nucleic acids is crucial. Herein, the study of photoinduced electron transfer in DOX-DNA complexes by femtosecond fluorescence spectroscopy is reported. The behaviour of complexes with two model DNA structures, a G-quadruplex (G4) formed by the human telomeric sequence (Tel21) and a d(GC) duplex, is compared. The DOX affinity for these two sequences is similar. Although both 1:1 and 2:1 stoichiometries have been reported for DOX-G4 complexes, only 1:1 complexes form with the duplex. The steady-state absorption indicates a strong binding interaction with the duplex due to drug intercalation between the GC base pairs. In contrast, the interaction of DOX with Tel21 is much weaker and arises from drug binding on the G4 external faces at two independent binding sites. As observed for DOX-d(GC) complexes, fluorescence of the drug in the first binding site of Tel21 exhibits decays within a few picoseconds following a biphasic pattern; this is attributed to the existence of two drug conformations. The fluorescence of the drug in the second binding site of Tel21 shows slower decays within 150 ps. These timescales are consistent with electron transfer from the guanines to the excited drug, as favoured by the lower oxidation potential of the stacked guanines of G4 with respect to those in the duplex.

Xanthines studied via femtosecond fluorescence spectroscopy

Xanthines represent a wide class of compounds closely related to the DNA bases adenine and guanine. Ubiquitous in the human body, they are capable of replacing natural bases in double helices and give rise to four-stranded structures. Although the use of their fluorescence for analytical purposes was proposed, their fluorescence properties have not been properly characterized so far. The present paper reports the first fluorescence study of xanthine solutions relying on femtosecond spectroscopy. Initially, we focus on 3-methylxanthine, showing that this compound exhibits non-exponential fluorescence decays with no significant dependence on the emission wavelength. The fluorescence quantum yield (3 × 10−4) and average decay time (0.9 ps) are slightly larger than those found for the DNA bases. Subsequently, we compare the dynamical fluorescence properties of seven mono-, di- and tri-methylated derivatives. Both the fluorescence decays and fluorescence anisotropies vary only weakly with the site and the degree of methylation. These findings are in line with theoretical predictions suggesting the involvement of several conical intersections in the relaxation of the lowest singlet excited state.

A New Potent Inhibitor of Glycogen Phosphorylase Reveals the Basicity of the Catalytic Site

The design and synthesis of a glucose-based acridone derivative (GLAC), a potent inhibitor of glycogen phosphorylase (GP) are described. GLAC is the first inhibitor of glycogen phosphorylase, the electronic absorption properties of which are clearly distinguishable from those of the enzyme. This allows probing subtle interactions in the catalytic site. The GLAC absorption spectra, associated with X-ray crystallography and quantum chemistry calculations, reveal that part of the catalytic site of GP behaves as a highly basic environment in which GLAC exists as a bis-anion. This is explained by water-bridged hydrogen-bonding interactions with specific catalytic site residues.

Femtosecond Fluorescence Upconversion Study of a Naphthalimide–Bithiophene–Triphenylamine Push–Pull Dye in Solution

There is a high interest in the development of new push-pull dyes for the use in dye sensitized solar cells. The pronounced charge transfer character of the directly photoexcited state is in principle favorable for a charge injection. Here, we report a time-resolved fluorescence study of a triphenylamine-bithiophene-naphthalimide dye in four solvents of varying polarity using fluorescence upconversion. The recording of femtosecond time-resolved fluorescence spectra corrected for the group velocity dispersion allows for a detailed analysis discriminating between spectral shifts and total intensity decays. After photoexcitation, the directly populated state (S1/FC) evolves toward a relaxed charge transfer state (S1/CT). This S1/CT state is characterized by a lower radiative transition moment and a higher nonradiative quenching. The fast dynamic shift of the fluorescence band is well described by solvation dynamics in polar solvents, but less so in nonpolar solvents, hinting that the excited-state relaxation process occurs on a free energy surface whose topology is strongly governed by the solvent polarity. This study underlines the influence of the environment on the intramolecular charge transfer (ICT) process, and the necessity to analyze time-resolved data in detail when solvation and ICT occur simultaneously.

Drug/protein interactions studied by time-resolved fluorescence spectroscopy

We report here on a recent time-resolved fluorescence study [1] of the interaction between flurbiprofen (FBP), a chiral non-steroidal anti-inflammatory drug, and human serum albumin (HSA), the main transport protein in the human body. We compare the results obtained for the drug-protein complex with those of various covalently linked flurbiprofentryptophan dyads having well-defined geometries. In all cases stereoselective dynamic fluorescence quenching is observed, varying greatly from one system to another. In addition, the fluorescence anisotropy decays also display a clear stereoselectivity. For the drug-protein complexes, this can be interpreted in terms of the protein microenvironment playing a significant role in the conformational relaxation of FBP, which is more restricted in the case of the (R)- enantiomer.

A combined femtosecond fluorescence and TD-DFT study of uracyl derivatives in aqueous solution

This chapter provides several interesting indications on the excited-state behavior of uracil derivatives in aqueous solution. The excited-state lifetimes of all the compounds examined, except 5-fluoro uracil, are dominated by a component shorter than 250 femtosecond. Calculations indicate that the relevant state for discussing the absorption and fluorescence spectra of uracil derivatives in aqueous solution is the ππ*. The key steps in the ultrafast internal conversion are the pyramidalization of C5 and the out of plane motion of C5 substituent, which explains why the excited-state lifetime depends mainly on the nature of the C5 substituent. The shape of the potential energy surface of the S 1 electronic states seems consistent with a very fast nonradiative decay. The result strongly supports the reliability of the general picture of the ultrafast ground state recovery of excited uracils in aqueous solution.