Semi-empirical study of two-color fluorescent dyes based on 3-hydroxychromone

Solvation Dynamics and Proton Transfer in Diethylaminohydroxyflavone

4'-N,N-Diethylamino-3-hydroxyflavone (DEAHF) exhibits dual fluorescence in most solvents as a result of a rapid excited-state intramolecular proton transfer reaction. The high sensitivity of its dual emission to solvent polarity and hydrogen bonding make DEAHF of interest as a ratiometric fluorescence sensor. In addition, prior work has suggested that the rate of this proton transfer should depend on solvent relaxation in an unusual manner. It has been proposed that rapid solvation of the initially excited reactant should retard reaction. The present work tests this idea by using femtosecond Kerr-gated emission spectroscopy to measure the reaction kinetics of DEAHF in mixtures of propylene carbonate (PC) + acetonitrile (ACN). This mixture was chosen to maintain constant solvent polarity and thereby constant reaction energies while varying solvation times ∼10-fold with composition. The reaction kinetics observed in these mixtures are multiexponential, consisting of resolvable components of ∼2 and ∼30 ps and a small fraction of reaction faster than detectable by the 400 fs resolution of the experiment. Average reaction times increase by approximately a factor of 2 as a function of ACN mole fraction, primarily as a result of changes to the slower kinetic component. This trend is opposite to the composition dependence of solvation times, thereby supporting the unusual role of polar solvation dynamics in this proton transfer. In n-alkane solvents, where electrostatic coupling is minimized, frictional properties of the solvent do not influence reaction rates.

Excited-state proton coupled charge transfer modulated by molecular structure and media polarization

Charge and proton transfer reactions in the excited states of organic dyes can be coupled in many different ways. Despite the complementarity of charges, they can occur on different time scales and in different directions of the molecular framework. In certain cases, excited-state equilibrium can be established between the charge-transfer and proton-transfer species. The interplay of these reactions can be modulated and even reversed by variations in dye molecular structures and changes of the surrounding media. With knowledge of the mechanisms of these processes, desired rates and directions can be achieved, and thus the multiple emission spectral features can be harnessed. These features have found versatile applications in a number of cutting-edge technological areas, particularly in fluorescence sensing and imaging.

Any colour you like. Excited state and ground state proton transfer in flavonols and applications

The photoinduced and ground state proton transfer processes occurring in flavonols are responsible for their multi-wavelength emission. This peculiar behavior has touched on a wide range of research areas, ranging from biology to chemistry of materials leading, among others, to the development of fluorescent probes for physical and biophysical parameters, laser dyes, and wavelentgh shifting devices. This account aims to be a brief introduction to the multi-faceted applications of flavonols.

Ab initio study of the solvent H-bonding effect on ESIPT reaction and electronic transitions of 3-hydroxychromone derivatives

The electronic transitions occurring in 4-(N,N-dimethylamino)-3-hydroxyflavone (DMAF) and 2-furanyl-3-hydroxychromone (FHC) were investigated using the TDDFT method in aprotic and protic solvents. The solvent effect was incorporated into the calculations via the PCM formalism. The H-bonding between solute and protic solvent was taken into account by considering a molecular complex between these molecules. To examine the effect of the H-bond on the ESIPT reaction, the absorption and emission wavelengths as well as the energies of the different states that intervene during these electronic transitions were calculated in acetonitrile, ethanol and methanol. The calculated positions of the absorption and emission wavelengths in various solvents were in excellent agreement with the experimental spectra, validating our approach. We found that in DMAF, the hydrogen bonding with protic solvents makes the ESIPT reaction energetically unfavourable, which explains the absence of the ESIPT tautomer emission in protic solvents. In contrast, the excited tautomer state of FHC remains energetically favourable in both aprotic and protic solvents. Comparing our calculations with the previously reported time-resolved fluorescence data, the ESIPT reaction of DMAF in aprotic solvents is reversible because the emitting states are energetically close, whereas in FHC, ESIPT is irreversible because the tautomer state is below the corresponding normal state. Therefore, the ESIPT reaction in DMAF is controlled by the relative energies of the excited states (thermodynamic control), while in FHC the ESIPT is controlled probably by the energetic barrier (kinetic control).

Measurements and implications of the membrane dipole potential

There are three kinds of membrane potentials: the surface potentials, resulting from the accumulation of charges at the membrane surfaces; the transmembrane potential, determined by imbalance of charge in the aqueous solutions; and the dipole potential, a membrane-internal potential from the dipolar components of the phospholipids and interface water. The absolute value of the dipole potential has been very difficult to measure, although its value has been estimated to be in the range of 200-1,000 mV from ion translocation rates (determined by the planar lipid bilayer method), the surface potential of lipid monolayers (determined by the lipid monolayer method), molecular-dynamics calculations, and electron scattering using cryoelectron microscopy (cryo-EM). Spectroscopy methods have also been used to monitor the dipole potential changes on the basis of the observed fluorescence changes of voltage-sensitive probes. The dipole potential accounts for the much larger permeability of a bare phospholipid membrane to anions than cations and affects the conformation and function of membrane proteins.

Monitoring biophysical properties of lipid membranes by environment-sensitive fluorescent probes

We review the main trends in the development of fluorescence probes to obtain information about the structure, dynamics, and interactions in biomembranes. These probes are efficient for studying the microscopic analogs of viscosity, polarity, and hydration, as well as the molecular order, environment relaxation, and electrostatic potentials at the sites of their location. Progress is being made in increasing the information content and spatial resolution of the probe responses. Multichannel environment-sensitive probes that can distinguish between different membrane physicochemical properties through multiple spectroscopic parameters show considerable promise.

Nanoscopic description of biomembrane electrostatics: results of molecular dynamics simulations and fluorescence probing

Electrostatic fields generated on and inside biological membranes are recognized to play a fundamental role in key processes of cell functioning. Their understanding requires an adequate description on the level of elementary charges and the reconstruction of electrostatic potentials by integration over all elementary interactions. Out of all the available research tools, only molecular dynamics simulations are capable of this, extending from the atomic to the mesoscopic level of description on the required time and space scale. A complementary approach is that offered by molecular probe methods, with the application of electrochromic dyes. Highly sensitive to intermolecular interactions, they generate integrated signals arising from electric fields produced by elementary charges at the sites of their location. This review is an attempt to provide a critical analysis of these two approaches and their present and potential applications. The results obtained by both methods are consistent in that they both show an extremely complex profile of the electric field in the membrane. The nanoscopic view, with two-dimensional averaging over the bilayer plane and formal separation of the electrostatic potential into surface (Psi(s)), dipole (Psi(d)) and transmembrane (Psi(t)) potentials, is constructive in the analysis of different functional properties of membranes.

Visualization of lipid domains in giant unilamellar vesicles using an environment-sensitive membrane probe based on 3-hydroxyflavone

We characterized the recently introduced environment-sensitive fluorescent membrane probe based on 3-hydroxyflavone, F2N12S, in model lipid membranes displaying liquid disordered (Ld) phase, liquid ordered (Lo) phase, or their coexistence. Steady-state fluorescence studies in large unilamellar vesicles show that the probe dual emission drastically changes with the lipid bilayer phase, which can be correlated with the difference in their hydration. Using two-photon excitation microscopy on giant unilamellar vesicles, the F2N12S probe was found to bind both Ld and Lo phases, allowing visualization of the individual phases from the fluorescence intensity ratio of its two emission bands. By using a linearly polarized excitation light, a strong photoselection was observed for F2N12S in the Lo phase, indicating that its fluorophore is nearly parallel to the lipid chains of the bilayer. In contrast, the absence of the photoselection with the Ld phase indicated no predominant orientation of the probe in the Ld phase. Comparison of the present results with those reported previously for F2N12S in living cells suggests a high content of the Lo phase in the outer leaflet of the cell plasma membranes. Taking into account the high selectivity of F2N12S for the cell plasma membranes and its suitability for both single- and two-photon excitation, applications of this probe to study membrane lateral heterogeneity in biological membranes are foreseen.

Fluorescent dyes undergoing intramolecular proton transfer with improved sensitivity to surface charge in lipid bilayers

4'-(Dialkylamino)-3-hydroxyflavones are characterized by an excited-state proton transfer reaction between two tautomeric excited states, which results in two emission bands well separated on the wavelength scale. Due to the high sensitivity of the relative intensities of the two emission bands to solvent polarity, hydrogen bonding and local electric fields, these dyes found numerous applications in biomembrane studies. In order to further improve their fluorescence characteristics, we have synthesized new dyes where the 2-phenyl group is substituted with a 2-thienyl group. In organic solvents, the new dyes exhibit red shifted absorption and dual fluorescence. Although they show lower sensitivity to solvent polarity and H-bond donor ability (acidicity) than their parent 3-hydroxyflavone dyes, they exhibit a much higher sensitivity to solvent H-bond acceptor ability (basicity). Moreover, when tested in lipid vesicles of different surface charge, the new dyes show much better resolved dual emission and higher sensitivity to the surface charge of lipid bilayers than the parent dyes. The response of the new dyes to surface charge is probably connected with the H-bond basicity of the membrane surface, which is the highest for negatively charged surfaces. As a consequence, the new dyes appear as prospective fluorophores for the development of new fluorescent probes for biomembranes.

Fluorescent Probe Based on Intramolecular Proton Transfer for Fast Ratiometric Measurement of Cellular Transmembrane Potential

Development of fast-response potentiometric probes for measuring the transmembrane potential Vm in cell plasma membranes remains a challenge. To overcome the limitations of the classical charge-shift potentiometric probes, we selected a 3-hydroxychromone fluorophore undergoing an excited-state intramolecular proton transfer (ESIPT) reaction that generates a dual emission highly sensitive to electric fields. To achieve the highest sensitivity to the electric field associated to Vm, we modified the fluorophore by adding two rigid legs containing terminal polar sulfonate groups to allow a deep vertical insertion of the fluorophore into the membrane. Fluorescence spectra of the new dye in lipid vesicles and cell membranes confirm the fluorophore location in the hydrophobic region of the membranes. Variation of Vm in lipid vesicles and cell plasma membranes results in a change of the intensity ratio of the two emission bands of the probe. The ratiometric response of the dye in cells is approximately 15% per 100 mV, and is thus quite large in comparison with most single-fluorophore, fast-response probes reported to date. Combined patch-clamp/fluorescence data further show that the ratiometric response of the dye in cells is faster than 1 ms. Analysis of the excitation and emission shifts further suggests that the probe responds to changes in Vm by a mechanism based on electrochromic modulation of its ESIPT reaction. Thus, for the first time, the ESIPT reaction has been successfully applied as a sensing principle for detection of transmembrane potential, allowing to couple classical electrochromic band shifts with changes in the relative intensities of the two well-separated emission bands. The fast two-band ratiometric response as well as the relatively high sensitivity of the new probe are the key features that make it useful for rapid detection of Vm changes in cell suspensions and single cells. Moreover, the new design principles proposed in the present work should allow further improvement of the probe sensitivity.

Visualization and sensing of intermolecular interactions with two-color fluorescent probes

We developed a new generic fluorescence sensing technology based on the change of relative intensities between two well-separated emission bands of the novel functional 3-hydroxychromone (3HC) dyes. A greatly enhanced self-calibrating wavelength-ratiometric response is obtained to all major types of non-covalent interactions that can be used in sensing--to polarity, hydrogen bonding ability and to local electrostatic fields. This technology may find a broad range of applications--from homogeneous assays in solutions to microarrays, microfluidic devices, nanosensors and whole cell imaging systems. It allows transforming micelles or phospholipid vesicles into nanosensor devices. In cellular research a high sensitivity to membrane potentials can be obtained and the membrane changes during apoptosis detected.

Modulation of dual fluorescence in a 3-hydroxyquinolone dye by perturbation of its intramolecular proton transfer with solvent polarity and basicity

A representative of a new class of dyes with dual fluorescence due to an excited state intramolecular proton transfer (ESIPT) reaction, namely 1-methyl-2-(4-methoxy)phenyl-3-hydroxy-4(1H)-quinolone (QMOM), has been studied in a series of solvents covering a large range of polarity and basicity. A linear dependence of the logarithm of its two bands intensity ratio, log(I(N*)/I(T*)), upon the solvent polarity expressed as a function of the dielectric constant, (epsilon- 1)/(2epsilon + 1), is observed for a series of protic solvents. A linear dependence for log(I(N*)/I(T*)) is also found in aprotic solvents after taking into account the solvent basicity. In contrast, the positions of the absorption and the two emission bands of QMOM do not noticeably depend on the solvent polarity and basicity, indicating relatively small changes in the transition moment of QMOM upon excitation and emission. Time-resolved experiments in acetonitrile, ethyl acetate and dimethylformamide suggest an irreversible ESIPT reaction for this dye. According to the time-resolved data, an increase of solvent basicity results in a dramatic decrease of the ESIPT rate constant, probably due to the disruption of the intramolecular H-bond of the dye by the basic solvent. Due to this new sensor property, 3-hydroxyquinolones are promising candidates for the development of a new generation of environment-sensitive fluorescence dyes for probing interactions of biomolecules.