Fluorescence anisotropy of molecular rotors

Cyclopenta[b]naphthalene cyanoacrylate dyes: synthesis and evaluation as fluorescent molecular rotors

We describe the design, synthesis and fluorescent profile of a family of environment-sensitive dyes in which a dimethylamino (donor) group is conjugated to a cyanoacrylate (acceptor) unit via a cyclopenta[b]naphthalene ring system. This assembly satisfies the typical D-π-A motif of a fluorescent molecular rotor and exhibits solvatochromic and viscosity-sensitive fluorescence emission. The central naphthalene ring system of these dyes was synthesized via a novel intramolecular dehydrogenative dehydro-Diels-Alder (IDDDA) reaction that permits incorporation of the donor and acceptor groups in variable positions around the aromatic core. A bathochromic shift of excitation and emission peaks was observed with increasing solvent polarity but the dyes exhibited a complex emission pattern with a second red emission band when dissolved in nonpolar solvents. Consistent with other known molecular rotors, the emission intensity increased with increasing viscosity. Interestingly, closer spatial proximity between the donor and the acceptor groups led to decreased viscosity sensitivity combined with an increased quantum yield. This observation indicates that structural hindrance of intramolecular rotation dominates when the donor and acceptor groups are in close proximity. The examined compounds give insight into how excited state intramolecular rotation can be influenced by both the solvent and the chemical structure.

Photon Counting Imaging with an Electron-Bombarded Pixel Image Sensor

Electron-bombarded pixel image sensors, where a single photoelectron is accelerated directly into a CCD or CMOS sensor, allow wide-field imaging at extremely low light levels as they are sensitive enough to detect single photons. This technology allows the detection of up to hundreds or thousands of photon events per frame, depending on the sensor size, and photon event centroiding can be employed to recover resolution lost in the detection process. Unlike photon events from electron-multiplying sensors, the photon events from electron-bombarded sensors have a narrow, acceleration-voltage-dependent pulse height distribution. Thus a gain voltage sweep during exposure in an electron-bombarded sensor could allow photon arrival time determination from the pulse height with sub-frame exposure time resolution. We give a brief overview of our work with electron-bombarded pixel image sensor technology and recent developments in this field for single photon counting imaging, and examples of some applications.

Nanothermometry: From Microscopy to Thermal Treatments

Measuring temperature in cells and tissues remotely, with sufficient sensitivity, and in real time presents a new paradigm in engineering, chemistry and biology. Traditional sensors, such as contact thermometers, thermocouples, and electrodes, are too large to measure the temperature with subcellular resolution and are too invasive to measure the temperature in deep tissue. The new challenge requires novel approaches in designing biocompatible temperature sensors-nanothermometers-and innovative techniques for their measurements. In the last two decades, a variety of nanothermometers whose response reflected the thermal environment within a physiological temperature range have been identified as potential sensors. This review covers the principles and aspects of nanothermometer design driven by two emerging areas: single-cell thermogenesis and image guided thermal treatments. The review highlights the current trends in nanothermometry illustrated with recent representative examples.

High Fluorescence Anisotropy of Thioflavin T in Aqueous Solution Resulting from Its Molecular Rotor Nature

Thioflavin T (ThT) is widely used to study amyloid fibrils while its properties are still debated in the literature. By steady-state and femtosecond time-resolved fluorescence we showed that, unlike small sized rigid molecules, the fluorescence anisotropy value of the free ThT in aqueous solutions is very high, close to the limiting value. This is determined by the molecular rotor nature of ThT, where the direction of the ThT transition dipole moment S₀ → S₁* is not changed either by the internal rotation of the ThT benzothiazole and aminobenzene rings relative to each other in the excited state, because the axis of this rotation coincides with the direction of the transition dipole moment, or by the rotation of the ThT molecule as a whole, because the rate of this process is 3 orders of magnitude smaller than the rate of the internal rotation which leads to the fluorescence quenching. Consequently, ThT fluorescence anisotropy cannot be directly used to study amyloid fibrils formation, as it was proposed by some authors.

Fluorescence anisotropy (polarization): from drug screening to precision medicine

INTRODUCTION

Fluorescence anisotropy (FA) is one of the major established methods accepted by industry and regulatory agencies for understanding the mechanisms of drug action and selecting drug candidates utilizing a high-throughput format.

AREAS COVERED

This review covers the basics of FA and complementary methods, such as fluorescence lifetime anisotropy and their roles in the drug discovery process. The authors highlight the factors affecting FA readouts, fluorophore selection and instrumentation. Furthermore, the authors describe the recent development of a successful, commercially valuable FA assay for long QT syndrome drug toxicity to illustrate the role that FA can play in the early stages of drug discovery.

EXPERT OPINION

Despite the success in drug discovery, the FA-based technique experiences competitive pressure from other homogeneous assays. That being said, FA is an established yet rapidly developing technique, recognized by academic institutions, the pharmaceutical industry and regulatory agencies across the globe. The technical problems encountered in working with small molecules in homogeneous assays are largely solved, and new challenges come from more complex biological molecules and nanoparticles. With that, FA will remain one of the major work-horse techniques leading to precision (personalized) medicine.

Ultrafast dynamics of a molecular rotor in chemical and biological nano-cavities

Excited state dynamics of CCVJ are investigated inside the nano-cavities of CD and HSA using steady-state and femtosecond up-conversion techniques.

In situ viscosity measurement of confined liquids

Viscosity heterogeneity in a thin glycerol film (170 nm) at high pressure was observed with fluorescence lifetime measurements.

A homodimeric BODIPY rotor as a fluorescent viscosity sensor for membrane-mimicking and cellular environments

Fluorescence properties of a novel homodimeric BODIPY dye rotor for Fluorescence Lifetime Imaging Microscopy (FLIM) are reported. Steady state and time resolved fluorescence measurements established the viscosity dependent behaviour in vitro. Homodimeric BODIPY embedded in different membrane mimicking lipid vesicles (DPPC, POPC and POPC plus cholesterol) is demonstrated to be a viable sensor for fluorescence lifetime based viscosity measurements. Moreover, SKOV3 cells readily endocytosed the dye, which accumulated in membranous structures inside the cytoplasm thereby allowing viscosity mapping of internal cell components.

Photon counting imaging with an electron-bombarded CCD: towards a parallel-processing photoelectronic time-to-amplitude converter

We have used an electron-bombarded CCD for optical photon counting imaging. The photon event pulse height distribution was found to be linearly dependent on the gain voltage. We propose on this basis that a gain voltage sweep during exposure in an electron-bombarded sensor would allow photon arrival time determination with sub-frame exposure time resolution. This effectively uses an electron-bombarded sensor as a parallel-processing photoelectronic time-to-amplitude converter, or a two-dimensional photon counting streak camera. Several applications that require timing of photon arrival, including Fluorescence Lifetime Imaging Microscopy, may benefit from such an approach. A simulation of a voltage sweep performed with experimental data collected with different acceleration voltages validates the principle of this approach. Moreover, photon event centroiding was performed and a hybrid 50% Gaussian/Centre of Gravity + 50% Hyperbolic cosine centroiding algorithm was found to yield the lowest fixed pattern noise. Finally, the camera was mounted on a fluorescence microscope to image F-actin filaments stained with the fluorescent dye Alexa 488 in fixed cells.

Molecular dynamics simulation of configurational ensembles compatible with experimental FRET efficiency data through a restraint on instantaneous FRET efficiencies

Förster resonance energy transfer (FRET) measurements are widely used to investigate (bio)molecular interactions or/and association. FRET efficiencies, the primary data obtained from this method, give, in combination with the common assumption of isotropic chromophore orientation, detailed insight into the lengthscale of molecular phenomena. This study illustrates the application of a FRET efficiency restraint during classical atomistic molecular dynamics simulations of a mutant mastoparan X peptide in either water or 7 M aqueous urea. The restraint forces acting on the donor and acceptor chromophores ensure that the sampled peptide configurational ensemble satisfies the experimental primary data by modifying interchromophore separation and chromophore transition dipole moment orientations. By means of a conformational cluster analysis, it is seen that indeed different configurational ensembles may be sampled without and with application of the restraint. In particular, while the FRET efficiency and interchromophore distances monitored in an unrestrained simulation may differ from the experimentally-determined values, they can be brought in agreement with experimental data through usage of the FRET efficiency restraining potential. Furthermore, the present results suggest that the assumption of isotropic chromophore orientation is not always justified. The FRET efficiency restraint allows the generation of configurational ensembles that may not be accessible with unrestrained simulations, and thereby supports a meaningful interpretation of experimental FRET results in terms of the underlying molecular degrees of freedom. Thus, it offers an additional tool to connect the realms of computer and wet-lab experimentation.

Time-resolved fluorescence anisotropy imaging

Fluorescence can be characterized by its intensity, position, wavelength, lifetime, and polarization. The more of these features are acquired in a single measurement, the more can be learned about the sample, i.e., the microenvironment of the fluorescence probe. Polarization-resolved fluorescence lifetime imaging-time-resolved fluorescence anisotropy imaging, TR-FAIM-allows mapping of viscosity or binding or of homo-FRET which can indicate dimerization or generally oligomerization.

Viability of Saccharomyces cerevisiae incorporated within silica and polysaccharide hosts monitored via time-resolved fluorescence

The viability of Saccharomyces cerevisiae in biocompatible polymers under different growth conditions and studied using time-resolved fluorescence techniques is presented. Two fluorophores, the viscosity sensitive probe 4-(4-(dimethylamino)styryl)-N-methyl-pyridiniumiodine (DASPMI) and the yeast viability stain 2-chloro-4-(2,3-dihydro-3-methyl-(benzo-1,3-thiazol-2-yl)-methylidene)-1-phenylquinolinium iodide (FUN-1) are used to elucidate information on the incorporated yeast cell viability. Variations in cell viscosity, which are indicative of the cell state, were obtained using DASPMI. Prior to observing FUN-1 in yeast cells using fluorescence lifetime imaging, its photophysics in solution and heterogeneous media were investigated. Time-resolved emission spectra were measured and analysed to associate lifetimes to the spectral emission. Preliminary results show that monitoring the fluorescence lifetime of FUN-1 may give a useful insight into cellular metabolism. The results indicate that both fluorophores may be used to monitor the entrapped yeast cell viability, which is important for in vitro studies and applications, such as that in the biofuel industry, where Saccharomyces cerevisiae are required to remain active in high ethanol environments.

Long-term live cell microscopy studies of lipid droplet fusion dynamics in adipocytes

During the adipogenic differentiation process of mesenchymal stem cells, lipid droplets (LDs) grow slowly by transferring lipids between each other. Recent findings hint at the possibility that a fusion pore is involved. In this study, we analyze lipid transfer data obtained in long-term label-free microscopy studies in the framework of a Hagen-Poiseuille model. The data obtained show a LD fusion process in which the lipid transfer directionality depends on the size difference between LDs, whereas the respective rates depend on the size difference and additionally on the diameter of the smaller LDs. For the data analysis, the viscosity of the transferred material has to be known. We demonstrate that a viscosity-dependent molecular rotor dye can be used to measure LD viscosities in live cells. On this basis, we calculate the diameter of a putative lipid transfer channel which appears to have a direct dependence on the diameter of the smaller of the two participating LDs.

2-Cyano-3-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij]quinolizin-9-yl)prop-2-enoic acid dimethyl sulfoxide monosolvate

In dimethyl sulfoxide solvated 9-(2-carb-oxy-2-cyano-vin-yl)julolidine, C(16)H(16)N(2)O(2)·C(2)H(6)OS, the essentially planar -CH=(CN)-CO(2)H substituent (r.m.s. deviation = 0.014 Å) is almost coplanar with respect to the benzene ring, the dihedral angle between the two planes being 0.48 (2)°. The conformations of the fused, non-aromatic rings were found to be half-chair. In the crystal, the acid molecule forms a hydrogen bond to the O atom of the solvent mol-ecule. The acid mol-ecule is disordered over two positions with respect to the methyl-ene C atoms in a 1:1 ratio. The crystal studied was found to be a racemic twin.

Defining a polymethine dye for fluorescence anisotropy applications in the near-infrared spectral range

Fluorescence anisotropy in the near-infrared (NIR) spectral range is challenging because of the lack of appropriate NIR fluorescent labels. We have evaluated polymethine fluorescent dyes to identify a leading candidate for NIR anisotropy applications. The NIR dye LS601 demonstrated low fluorescence anisotropy values (r) as a result of its relatively long fluorescent lifetime 1.3 ns. The r value of LS601 unbound and coupled to biological macromolecules was found to have a sufficient dynamic range from 0.24 to 0.37, demonstrating the feasibility of fluorescence anisotropy in the NIR. The viability of fluorescence anisotropy using a NIR label was demonstrated by characterization of dye-protein conjugates. These results open the door to a number of applications in drug discovery, fluorescence anisotropy imaging and contrast agent development.