Fluorescence anisotropy of molecular rotors

Molecular rotor as an in-situ fluorescent probe for the degree of polymerization of α-D-1,4-glucans

Various starch synthesis and tailoring processes involve prevailing adjustments in the degree of polymerization (DP) of linear α-D-1,4-glucan chains (LGCs) for the improved functional performances. Previous studies indicated that LGCs might hinder the twisted relaxation of 9-(2-carboxy-2-cyanovinyl)-julolidine (CCVJ, a hydrophilic molecular rotor), highlighting CCVJ as a potential in-situ structural probe for LGC. In this study, glucose and its α-D-1,4 oligomers and polymers with molecular weights ranging from 0.18 kDa to 70.00 kDa were prepared as the model molecules (MM). The fluorescent emission behavior of CCVJ in various concentrations (1-5 g/L) of MM solutions or dispersions were analyzed. Results showed that for the low-DP MMs (≤ 3.98 kDa) with good aqueous stability, CCVJ emission increased by about 20 times with the DP of MMs. In contrast, CCVJ generally emitted weak DP-relevant but glucan content-dependent fluorescence in response to the interaction with high-DP MMs (> 3.98 kDa). Furthermore, a double-logarithmic linear relationship was found between the emission intensity of CCVJ and the molar-based molecular weight of glucan. The result combined with the molecular dynamic simulation suggested that CCVJ underwent surface-to-surface interaction with MMs. This study may contribute to the real-time analysis of the DP of α-D-1,4 oligoglucosides in maltodextrin and starch syrup.

Viscosity-Induced Emission of 5-(Diarylmethylene)imidazolone with Extended Conjugation via Attachment of N-Methylpyrrole at the 2-Position

We have developed a series of 2-monoaryl-5-diarylmethylene analogs of the green fluorescent protein chromophore to study their viscosity-induced emission (VIE) properties. The analogs were synthesized by a condensation with methyl imidate and N-(diarylmethylene)glycinate. Among the analogs, the N-methylpyrrol-2-yl-substituted analog 1h induced the most remarkable VIE behavior in triglyceride and lipid bilayers probably due to the high π-electron-rich property of the pyrrole ring. The pyrrole substituent in imidazolone analogs can be expected to become a common template for introducing VIE behavior.

Thioflavin-T: A Quantum Yield-Based Molecular Viscometer for Glycerol-Monohydroxy Alcohol Mixtures

Molecular rotor dye thioflavin T (ThT) is almost nonfluorescent in low-viscosity solvents but highly fluorescent when bound to amyloid fibrils. This unique property arises from the rotation of the dimethylaniline moiety relative to the benzothiazole moiety in the excited state, which drives the dye from an emissive locally excited state to a twisted intramolecular charge-transfer state. This process is viscosity-controlled, and therefore, we can use the quantum yield of ThT to assess the viscosity of the environment. In this study, we have investigated the quantum yield of ThT (φThT) in various compositions of six alcoholic solvent mixtures of glycerol with methanol, ethanol, n-propanol, iso-propanol, n-butanol, and tert-butanol. We have proposed an empirical model using φThT as a function of the mole fraction of glycerol to estimate the interaction parameters between the components of the solvent mixtures. This analysis allowed us to predict the extent of nonideality of the solvent mixtures. The Förster-Hoffmann- and Loutfy-Arnold-type power law relationship was established between the quantum yield of ThT and bulk viscosity for solvent mixtures of methanol, ethanol, n-butanol, and tert-butanol with glycerol, and it was found to be similar in nature in all the four mixtures. Applying this knowledge, we proposed a methodology to quantify and predict the bulk viscosity coefficient values of several compositions of n-propanol-glycerol and iso-propanol-glycerol mixtures which have not been previously documented.

Insight on the Intracellular Supramolecular Assembly of DTTO: A Peculiar Example of Cell-Driven Polymorphism

The assembly of supramolecular structures within living systems is an innovative approach for introducing artificial constructs and developing biomaterials capable of influencing and/or regulating the biological responses of living organisms. By integrating chemical, photophysical, morphological, and structural characterizations, it is shown that the cell-driven assembly of 2,6-diphenyl-3,5-dimethyl-dithieno[3,2-b:2',3'-d]thiophene-4,4-dioxide (DTTO) molecules into fibers results in the formation of a "biologically assisted" polymorphic form, hence the term bio-polymorph. Indeed, X-ray diffraction reveals that cell-grown DTTO fibers present a unique molecular packing leading to specific morphological, optical, and electrical properties. Monitoring the process of fiber formation in cells with time-resolved photoluminescence, it is established that cellular machinery is necessary for fiber production and a non-classical nucleation mechanism for their growth is postulated. These biomaterials may have disruptive applications in the stimulation and sense of living cells, but more crucially, the study of their genesis and properties broadens the understanding of life beyond the native components of cells.

BODIPY-Ethynylestradiol molecular rotors as fluorescent viscosity probes in endoplasmic reticulum

Due to their capability for sensing changes in viscosity, fluorescent molecular rotors (FMRs) have emerged as potential tools to develop several promising viscosity probes; most of them, however, localize non-selectively within cells, precluding changes in the viscosity of specific cellular microdomains to be studied by these means. Following previous reports on enhanced fluorophore uptake efficiency and selectivity by incorporation of biological submolecular fragments, here we report two potential BODIPY FMRs based on an ethynylestradiol spindle, a non-cytotoxic semisynthetic estrogen well recognized by human cells. A critical evaluation of the potential of these fluorophores for being employed as FMRs is presented, including the photophysical characterization of the probes, SXRD studies and TD-DFT computations, as well as confocal microscopy imaging in MCF-7 (breast cancer) cells.

A palette of site-specific organelle fluorescent thermometers

Intracellular micro-temperature is closely related to cellular processes. Such local temperature inside cells can be measured by fluorescent thermometers, which are a series of fluorescent materials that convert the temperature information to detectable fluorescence signals. To investigate the intracellular temperature fluctuation in various organelles, it is essential to develop site-specific organelle thermometers. In this study, we develop a new series of fluorescent thermometers, Thermo Greens (TGs), to visualize the temperature change in almost all typical organelles. Through fluorescence lifetime-based cell imaging, it was proven that TGs allow the organelle-specific monitoring of temperature gradients created by external heating. The fluorescence lifetime-based thermometry shows that each organelle experiences a distinct temperature increment which depends on the distance away from the heat source. TGs are further demonstrated in the quantitative imaging of heat production at different organelles such as mitochondria and endoplasmic reticulum in brown adipocytes. To date, TGs are the first palette batch of small molecular fluorescent thermometers that can cover almost all typical organelles. These findings can inspire the development of new fluorescent thermometers and enhance the understanding of thermal biology in the future.

Red fluorescent BODIPY molecular rotor for high microviscosity environments

Microviscosity has a strong impact for diffusion-controlled processes in biological environments. BODIPY molecular rotors are viscosity-sensitive fluorophores that provide a simple and non-invasive way to visualise microviscosity. Although green fluorescent probes are already well developed for imaging, thick biological samples require longer wavelengths for investigation. This work focuses on the examination of novelβ-substitutedmeso-phenyl-BODIPYs possessing a red emission. We report a new red fluorescent BODIPY-based probe BP-Vinyl-NO₂suitable for sensing microviscosity in rigid environments of over 100 000 cP viscosities. Furthermore, we demonstrate that changing the methyl position fromorthotometaon theβ-phenyl-substituted conjugate BP-PH-m2M-NO₂redshifts absorbance and fluorescence spectra while maintaining viscosity sensitivity. Finally, we show that nitro-substitution ofmeso-phenyl is a versatile approach to improve the sensitivity to viscosity while suppressing sensitivity to polarity and temperature of such derivatives. In summary, we present two nitro-substituted red fluorescent probes that could be used as lifetime-based microviscosity sensors.

Transforming Dyes into Fluorophores: Exciton‐Induced Emission with Chain‐like Oligo‐BODIPY Superstructures

Herein we present a systematic study demonstrating to which extent exciton formation can amplify fluorescence based on a series of ethylene‐bridged oligo‐BODIPYs. A set of non‐ and weakly fluorescent BODIPY motifs was selected and transformed into discrete, chain‐like oligomers by linkage via a flexible ethylene tether. The prepared superstructures constitute excitonically active entities with non‐conjugated, Coulomb‐coupled oscillators. The non‐radiative deactivation channels of Internal Conversion (IC), also combined with an upstream reductive Photoelectron Transfer (rPET) and Intersystem Crossing (ISC) were addressed at the monomeric state and the evolution of fluorescence and (non‐)radiative decay rates studied along the oligomeric series. We demonstrate that a “masked” fluorescence can be fully reactivated irrespective of the imposed conformational rigidity. This work challenges the paradigm that a collective fluorescence enhancement is limited to sterically induced motional restrictions.

Azobenzene photoisomerization probes cell membrane viscosity

The viscosity of cell membranes is a crucial parameter that affects the diffusion of small molecules both across and within the lipid membrane and that is related to several diseases. Therefore, the possibility to measure quantitatively membrane viscosity on the nanoscale is of great interest. Here, we report a complete investigation of the photophysics of an amphiphilic membrane-targeted azobenzene (ZIAPIN2) and we propose its use as a viscosity probe for cell membranes. We exploit ZIAPIN2 trans-cis photoisomerization to develop a molecular viscometer and to assess the viscosity of Escherichia coli bacteria membranes employing time-resolved fluorescence spectroscopy. Fluorescence lifetime measurements of ZIAPIN2 in E. coli bacteria suspensions correctly indicate that the membrane viscosity decreases as the temperature of the sample increases. Given the non-homogeneity and the anisotropy of cell membranes, as supported by the photophysical characterization of the probe within the lipid bilayer, we shed new light on the intricate membrane rheology.

Design and synthesis of first environment-sensitive coumarin fluorescent agonists for MrgX2

Mas related G-protein-coupled receptor member X2 (MrgX2) has been identified as the crucial receptor in drug induced pseudo-allergic reactions and allergic diseases. In this research, the first type of fluorescent agonists (ZX1, ZX2 and ZX3) for MrgX2 were developed by conjugating environment-sensitive fluorophore coumarin to MrgX2 selective agonists (R)-ZINC-3573. Their environment-sensitive property was confirmed by the dramatically increase of fluorescent intensity after binding to the hydrophobic ligand binding domain MrgX2, which help to overcome the high background signal. Based on these characteristics, they can be used for selective visualization of MrgX2 in living cells even with their own background interference. Among these fluorescent agonists, compound ZX2 possessed splendid spectroscopic properties, outstanding pharmacological activities (EC₅₀ = 0.93 μM, K_D = 1.97 μM). And a competitive binding assay was established with ZX2 to analysis the binding affinity of MrgX2 agonists, which shown high coherence with the results of cell membrane chromatography. To our knowledge, these probes are the first fluorescent ligands of MrgX2 with agonistic activity and environment-sensitive property, which is expected to use for the development of MrgX2 molecular pharmacology and serve as a convenient high-throughput screening tool for the drug candidates targeting MrgX2.

A fluorene based probe: Synthesis and "turn-on" water sensitivity of the in-situ formed Cu2+ complex: Application in bio-imaging

A new fluorene based probe (FTH) has been evaluated for its photo-physical properties in solution as well as in the aggregated state/viscous environment. Addition of a poor solvent (water) to the solution of the probe in a good (acetonitrile) solvent significantly enhanced the otherwise weak emission due to aggregation induced emission (AIE). The emission enhancement is also related to the increase in viscosity of the solution, leading to the restricted intramolecular rotation of the peripheral (phenyl) groups. Interestingly, the emission behaviour of the non-emissive in-situ formed Cu²⁺ complex is drastically modulated in the presence of water. The solution of the putative Cu²⁺ complex of the probe turns highly emissive (yellow colour) upon addition of a small fraction of water (up to 7.6 wt %), but the yellow emission diminishes upon increasing higher water fraction. We propose that the initially formed Cu²⁺ complex undergoes hydrolysis in the presence of higher water content releasing the free amine possessing the diaryl amino rotors thus rendering the solution non-emissive. Thus the current probe being reported herein discloses its potential to generate trace water sensitive turn-on Cu²⁺ complex. Additionally, the bio-imaging potential of FTH for live cancer cells and its sensitivity towards intracellular presence of Cu²⁺ ions has been demonstrated.

Give or Take: Effects of Electron-Accepting/-Withdrawing Groups in Red-Fluorescent BODIPY Molecular Rotors

Mapping microviscosity, temperature, and polarity in biosystems is an important capability that can aid in disease detection. This can be achieved using fluorescent sensors based on a green-emitting BODIPY group. However, red fluorescent sensors are desired for convenient imaging of biological samples. It is known that phenyl substituents in the β position of the BODIPY core can shift the fluorescence spectra to longer wavelengths. In this research, we report how electron-withdrawing (EWG) and -donating (EDG) groups can change the spectral and sensory properties of β-phenyl-substituted BODIPYs. We present a trifluoromethyl-substituted (EWG) conjugate with moderate temperature sensing properties and a methoxy-substituted (EDG) molecule that could be used as a lifetime-based polarity probe. In this study, we utilise experimental results of steady-state and time-resolved fluorescence, as well as quantum chemical calculations using density functional theory (DFT). We also explain how the energy barrier height (Ea) for non-radiative relaxation affects the probe’s sensitivity to temperature and viscosity and provide appropriate Ea ranges for the best possible sensitivity to viscosity and temperature.

Design, Synthesis, and Evaluation of Near-Infrared Fluorescent Molecules Based on 4H-1-Benzopyran Core

A series of novel fluorescent 4H-1-benzopyrans was designed and developed as near-infrared fluorescent molecules with a compact donor–acceptor-donor architecture. Spectral intensity of the fluorescent molecules M-1, M-2, M-3 varied significantly with the increasing polarities of solvents, where M-3 showed high viscosity sensitivity in glycerol-ethanol system with a 3-fold increase in emission intensity. Increasing concentrations of compound M-3 to 5% BSA in PBS elicited a 4-fold increase in fluorescence intensity, exhibiting a superior environmental sensitivity. Furthermore, the in vitro cellular uptake behavior and CLSM assay of cancer cell lines demonstrated that M-3 could easily enter the cell nucleus and bind to proteins with low toxicity. Therefore, the synthesized near-infrared fluorescent molecules could provide a new direction for the development of optical imaging probes and potential further drugs.

Designing a Red-Emitting Viscosity-Sensitive BODIPY Fluorophore for Intracellular Viscosity Imaging

Viscosity imaging at a microscopic scale can provide important information about biosystems, including the development of serious illnesses. Microviscosity imaging is achievable with viscosity-sensitive fluorophores, the most popular of which are based on the BODIPY group. However, most of the BODIPY probes fluoresce green light, whereas the red luminescence is desired for the imaging of biological samples. Designing a new viscosity probe with suitable spectroscopic properties is a challenging task because it is difficult to preserve viscosity sensitivity after modifying the molecular structure. Here we describe how we developed a new red-emitting, viscosity-sensitive, BODIPY fluorophore BP-PH-2M-NO₂ that is suitable for reliable intracellular viscosity imaging of lipid droplets in MCF-7 breast cancer cells. The design of BP-PH-2M-NO₂ was aided by DFT calculations that allowed a successful prediction of the viscosity sensitivity of fluorophores before synthesis. In summary, we report a new red viscosity probe possessing monoexponential fluorescence decay that makes it attractive for lifetime-based viscosity imaging.

Pathway-Dependent Phase Transitions of Supramolecular Self-assemblies Containing Cationic Amphiphiles with Azobenzene and Disulfide Groups

There have been several attempts to construct supramolecular chemical systems that mimic the phase transitions in living systems. However, most of these phase transitions are one-to-one and induced by one stimulus or chemical; there have been few reports on the pathway-dependent phase transition of supramolecular self-assemblies in multi-step. To induce multistep phase transitions, molecular crystals were prepared that contained a cationic amphiphile bearing azobenzene and disulfide groups. A reducing agent caused the crystals to become vesicles, and adjacent, non-touching vesicles fused under UV and subsequent visible light. Adding a reducing agent to the worm-like aggregates that were generated after UV irradiation of the original crystals resulted in the growth of sheet-like aggregates. ¹ H NMR and fluorescence anisotropy measurements showed that a series of phase transitions was induced by changes in the phase structures from molecular conversions of the reactive amphiphiles. The multiple pathway-dependent phase transitions of supramolecular self-assemblies can provide a methodology for developing new stimuli-responsive materials that exhibit the desirable properties under specific circumstances from a systems chemistry viewpoint.

Time-Resolved Fluorescence Anisotropy and Molecular Dynamics Analysis of a Novel GFP Homo-FRET Dimer

Förster resonance energy transfer (FRET) is a powerful tool to investigate the interaction between proteins in living cells. Fluorescence proteins, such as the green fluorescent protein (GFP) and its derivatives, are coexpressed in cells linked to proteins of interest. Time-resolved fluorescence anisotropy is a popular tool to study homo-FRET of fluorescent proteins as an indicator of dimerization, in which its signature consists of a very short component at the beginning of the anisotropy decay. In this work, we present an approach to study GFP homo-FRET via a combination of time-resolved fluorescence anisotropy, the stretched exponential decay model, and molecular dynamics simulations. We characterize a new, to our knowledge, FRET standard formed by two enhanced GFPs (eGFPs) and a flexible linker of 15 aminoacids (eGFP15eGFP) with this protocol, which is validated by using an eGFP monomer as a reference. An excellent agreement is found between the FRET efficiency calculated from the fit of the eGFP15eGFP fluorescence anisotropy decays with a stretched exponential decay model (〈EFRETexp〉 = 0.25 ± 0.05) and those calculated from the molecular dynamics simulations (〈EFRETMD〉 = 0.18 ± 0.14). The relative dipole orientation between the GFPs is best described by the orientation factors 〈κ2〉 = 0.17 ± 0.16 and 〈|κ|〉 = 0.35 ± 0.20, contextualized within a static framework in which the linker hinders the free rotation of the fluorophores and excludes certain configurations. The combination of time- and polarization-resolved fluorescence spectroscopy with molecular dynamics simulations is shown to be a powerful tool for the study and interpretation of homo-FRET.

Photophysical Properties of Some Fluorescent Dyes in SDS Micellar Solutions

Photophysical properties of fluorescent dyes such as Safranin T, Acridine Orange, Pyronin B and Pyronin Y in SDS micelles were examined by using spectroscopic techniques. Firstly, spherical micelles in deionized water were prepared with Sodium Dodecyl Sulfate (SDS) surfactants and they were transformed into their layered structures (lamellar micelles) by the aid of NaCl (sodium chloride). SEM studies confirmed the transformation of SDS micelles from the spherical structures to the lamellar structures. Secondly, absorption and fluorescence characteristics of the dyes in deionized water and the SDS micelles aqueous solutions were characterized in the presence of various NaCl concentrations at above the critical micelle concentration (CMC). Moreover, the photophysical properties of the dyes in various media were discussed by fluorescence quantum yield and fluorescence lifetime data. The micellar structures called a mimetic membrane system changed the photophysical properties of the dyes compared to those in deionized water.

Electrochemical Switching of a Fluorescent Molecular Rotor Embedded within a Bistable Rotaxane

We report how the nanoconfined environment, introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent molecular rotor, namely, an 8-phenyl-substituted boron dipyrromethene (BODIPY). The electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state coconformation and in the corresponding excited-state properties of the BODIPY rotor. In the starting redox state, when no external potential is applied, the cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component, leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the TTF unit to a TTF²⁺ dication, the CBPQT⁴⁺ ring is forced toward the molecular rotor, leading to an increased energy barrier for the excited state to rotate the rotor into the state with a high nonradiative rate constant, resulting in an overall 3.4-fold fluorescence enhancement. On the other hand, when the solvent polarity is high enough to stabilize the excited charge-transfer state between the BODIPY rotor and the CBPQT⁴⁺ ring, movement of the ring toward the BODIPY rotor produces an unexpectedly strong fluorescence signal decrease as the result of photoinduced electron transfer from the BODIPY rotor to the CBPQT⁴⁺ ring. The nanoconfinement effect introduced by mechanical bonding can effectively lead to modulation of the physicochemical properties as observed in this bistable [2]rotaxane. On account of the straightforward synthetic strategy and the facile modulation of switchable electrochromic behavior, our approach could pave the way for the development of new stimuli-responsive materials based on mechanically interlocked molecules for future electro-optical applications, such as sensors, molecular memories, and molecular logic gates.

Time-Resolved Fluorescence Anisotropy of a Molecular Rotor Resolves Microscopic Viscosity Parameters in Complex Environments

Understanding viscosity in complex environments remains a largely unanswered question despite its importance in determining reaction rates in vivo. Here, time-resolved fluorescence anisotropy imaging (TR-FAIM) is combined with fluorescent molecular rotors (FMRs) to simultaneously determine two non-equivalent viscosity-related parameters in complex heterogeneous environments. The parameters, FMR rotational correlation time and lifetime, are extracted from fluorescence anisotropy decays, which in heterogeneous environments show dip-and-rise behavior due to multiple dye populations. Decays of this kind are found both in artificially constructed adiposomes and in live cell lipid droplet organelles. Molecular dynamics simulations are used to assign each population to nano-environments within the lipid systems. The less viscous population corresponds to the state showing an average 25° tilt to the lipid membrane normal, and the more viscous population to the state showing an average 55° tilt. This combined experimental and simulation approach enables a comprehensive description of the FMR probe behavior within viscous nano-environments in complex, biological systems.

A Method for High-Throughput Measurements of Viscosity in Sub-micrometer-Sized Membrane Systems

To unravel the underlying principles of membrane adaptation in small systems like bacterial cells, robust approaches to characterize membrane fluidity are needed. Currently available relevant methods require advanced instrumentation and are not suitable for high-throughput settings needed to elucidate the biochemical pathways involved in adaptation. We developed a fast, robust, and financially accessible quantitative method to measure the microviscosity of lipid membranes in bulk suspension using a commercially available plate reader. Our approach, which is suitable for high-throughput screening, is based on the simultaneous measurements of absorbance and fluorescence emission of a viscosity-sensitive fluorescent dye, 9-(2,2-dicyanovinyl)julolidine (DCVJ), incorporated into a lipid membrane. We validated our method using artificial membranes with various lipid compositions over a range of temperatures and observed values that were in good agreement with previously published results. Using our approach, we were able to detect a lipid phase transition in the ruminant pathogen Mycoplasma mycoides.

Fluorogenic Pt complexes distinguish the quantity and folding behavior of RNA G-quadruplexes between live cancerous and healthy cells

Fluorogenic Pt complexes were developed to track and compare the intracellular un/folding behaviors of RNA G-quadruplexes among different cell lines.

The effect of solvent polarity and macromolecular crowding on the viscosity sensitivity of a molecular rotor BODIPY-C10

Viscosity is the key parameter of many biological systems such as live cells. It can be conveniently measured with ‘molecular rotors’ – fluorescent sensors of microviscosity. Here, we investigate one of the most applied molecular rotors BODIPY-C₁₀.

Enhancing the Viscosity-Sensitive Range of a BODIPY Molecular Rotor by Two Orders of Magnitude

Molecular rotors are a class of fluorophores that enable convenient imaging of viscosity inside microscopic samples such as lipid vesicles or live cells. Currently, rotor compounds containing a boron-dipyrromethene (BODIPY) group are among the most promising viscosity probes. In this work, it is reported that by adding heavy-electron-withdrawing -NO₂ groups, the viscosity-sensitive range of a BODIPY probe is drastically expanded from 5-1500 cP to 0.5-50 000 cP. The improved range makes it, to our knowledge, the first hydrophobic molecular rotor applicable not only at moderate viscosities but also for viscosity measurements in highly viscous samples. Furthermore, the photophysical mechanism of the BODIPY molecular rotors under study has been determined by performing quantum chemical calculations and transient absorption experiments. This mechanism demonstrates how BODIPY molecular rotors work in general, why the -NO₂ group causes such an improvement, and why BODIPY molecular rotors suffer from undesirable sensitivity to temperature. Overall, besides reporting a viscosity probe with remarkable properties, the results obtained expand the general understanding of molecular rotors and show a way to use the knowledge of their molecular action mechanism for augmenting their viscosity-sensing properties.

Printing Life-Inspired Subcellular Scale Compartments with Autonomous Molecularly Crowded Confinement

A simple, rapid, and highly controlled platform to prepare life-inspired subcellular scale compartments by inkjet printing has been developed. These compartments consist of fL-scale aqueous droplets (few µm in diameter) incorporating biologically relevant molecular entities with programmed composition and concentration. These droplets are ink-jetted in nL mineral oil drop arrays allowing for lab-on-chip studies by fluorescence microscopy and fluorescence life time imaging. Once formed, fL-droplets are stable for several hours, thus giving the possibility of readily analyze molecular reactions and their kinetics and to verify molecular behavior and intermolecular interactions. Here, this platform is exploited to unravel the behavior of different molecular probes and biomolecular systems (DNA hairpins, enzymatic cascades, protein-ligand couples) within the compartments. The fL-scale size induces the formation of molecularly crowded confined shell structures (hundreds of nanometers in thickness) at the droplet surface, allowing discovery of specific features (e.g., heterogeneity, responsivity to molecular triggers) that are mediated by the intermolecular interactions in these peculiar environments. The presented results indicate the possibility of using this platform for designing nature-inspired confined reactors allowing for a deepened understanding of molecular confinement effects in living subcellular compartments.

Targeted fluorescence lifetime probes reveal responsive organelle viscosity and membrane fluidity

The only way to visually observe cellular viscosity, which can greatly influence biological reactions and has been linked to several human diseases, is through viscosity imaging. Imaging cellular viscosity has allowed the mapping of viscosity in cells, and the next frontier is targeted viscosity imaging of organelles and their microenvironments. Here we present a fluorescent molecular rotor/FLIM framework to image both organellar viscosity and membrane fluidity, using a combination of chemical targeting and organelle extraction. For demonstration, we image matrix viscosity and membrane fluidity of mitochondria, which have been linked to human diseases, including Alzheimer's Disease and Leigh's syndrome. We find that both are highly dynamic and responsive to small environmental and physiological changes, even under non-pathological conditions. This shows that neither viscosity nor fluidity can be assumed to be fixed and underlines the need for single-cell, and now even single-organelle, imaging.

Comparing the molecular and global rheology of a fluid under high pressures

The viscosity of liquids is a strong function of pressure. While viscosity is relatively easy to measure at low pressure, high-pressure rheology presents significant experimental challenges. As a result, rheological models are often used to extrapolate viscosity from low pressure measurements to higher pressures. Techniques to obtain data over a wide range of pressures and shear rates, as well as understanding the validity and limitations of methods to fill the gaps in the available data, are therefore of crucial practical and theoretical importance. This work examines the viscosity of polyalphaolefin (PAO) by combining average global area averaged measurements at high pressure and local molecular viscosity measurements at moderate pressures. Viscosities spanning five orders of magnitude are examined at pressures up to 720 MPa. High pressure results were obtained with friction measurements where the fluid is sheared between two surfaces in a loaded point contact. The local molecular microviscosity at medium and low pressures was measured by applying a technique based on fluorescence anisotropy, which probes the rotational motion of dye molecules in a nanoscale film under shear. Both sets of measurements are taken in the same configuration, an elastohydrodynamic (EHD) contact. This is the first set of quantitative local viscosity measurements that have been verified against both friction and high pressure rheometry measurements. Commonly used rheological models were compared to experimental results. Our work shows that fluorescence anisotropy and friction measurements can be used to determine the viscosity of liquids over a wide range of conditions from a single experimental setup. The results obtained match results from low- and high-pressure rheometry for PAO. The importance of correcting friction data for pressure non-uniformity, temperature and shear thinning is also highlighted.

Compositional adaptability in NPM1-SURF6 scaffolding networks enabled by dynamic switching of phase separation mechanisms

The nucleolus, the site for ribosome biogenesis contains hundreds of proteins and several types of RNA. The functions of many non-ribosomal nucleolar proteins are poorly understood, including Surfeit locus protein 6 (SURF6), an essential disordered protein with roles in ribosome biogenesis and cell proliferation. SURF6 co-localizes with Nucleophosmin (NPM1), a highly abundant protein that mediates the liquid-like features of the granular component region of the nucleolus through phase separation. Here, we show that electrostatically-driven interactions between disordered regions of NPM1 and SURF6 drive liquid-liquid phase separation. We demonstrate that co-existing heterotypic (NPM1-SURF6) and homotypic (NPM1-NPM1) scaffolding interactions within NPM1-SURF6 liquid-phase droplets dynamically and seamlessly interconvert in response to variations in molecular crowding and protein concentrations. We propose a mechanism wherein NPM1-dependent nucleolar scaffolds are modulated by non-ribosomal proteins through active rearrangements of interaction networks that can possibly contribute to the directionality of ribosomal biogenesis within the liquid-like nucleolus.

The nucleolus is a membrane-less organelle and both Nucleophosmin (NPM1) and Surfeit locus protein 6 (SURF6) are abundant proteins within the nucleolus. Here the authors employ biophysical methods to study the properties of NPM1-S6N droplets and provide insights into the role of SURF6 in maintaining and modulating the liquid-like structure of the nucleolus.

Excited state intramolecular proton transfer in julolidine derivatives: an ab initio study

We have studied, using ab initio tools, a series of recently prepared fluorescent julolidine derivatives, undergoing Excited State Intramolecular Proton Transfer (ESIPT). We show that the computed free energy change in the excited state (ΔGES) can be used to predict the preference for enol, keto, or dual emission. Indeed, two julolidine molecules experimentally show dual emission, consistent with our finding of a small ΔGES. In agreement with experimental outcomes the complexation between the ESIPT centre and BF2 increases the rigidity of the fluorophore and greatly facilitates emission at energies close to the original enol (E*) fluorescence band. The protonation of the imino group also suppresses ESIPT and sole E* emission is obtained. We disclose that chemical substitution can significantly tune the radiationless deactivation of the enol related to the C[double bond, length as m-dash]N bond rotation of the ESIPT centre. While there is a significant barrier for the experimentally studied compounds we have found a strong correlation between the barrier height and the electron donating strength of the phenyl substituent. Strong donors such as amines facilitate the barrierless non-radiative decay from E* back to the ground state, while weak electron donors make the barrier sufficiently high to allow ESIPT. Strong electron accepting groups such as -NO2 further increase this barrier. This work therefore illustrates the fine interplay necessary to design dual emitters.

Exploring viscosity, polarity and temperature sensitivity of BODIPY-based molecular rotors

Microviscosity is a key parameter controlling the rate of diffusion and reactions on the microscale. One of the most convenient tools for measuring microviscosity is by fluorescent viscosity sensors termed 'molecular rotors'. BODIPY-based molecular rotors in particular proved extremely useful in combination with fluorescence lifetime imaging microscopy, for providing quantitative viscosity maps of living cells as well as measuring dynamic changes in viscosity over time. In this work, we investigate several new BODIPY-based molecular rotors with the aim of improving on the current viscosity sensing capabilities and understanding how the structure of the fluorophore is related to its function. We demonstrate that due to subtle structural changes, BODIPY-based molecular rotors may become sensitive to temperature and polarity of their environment, as well as to viscosity, and provide a photophysical model explaining the nature of this sensitivity. Our data suggests that a thorough understanding of the photophysics of any new molecular rotor, in environments of different viscosity, temperature and polarity, is a must before moving on to applications in viscosity sensing.

Flow-dependent fluorescence of CCVJ

Background

The molecular rotor 9-(2-Carboxy-2-cyanovinyl)julolidine (CCVJ) is presumed to have a sensitivity towards velocity or shear which is supposed to result in a change in fluorescence quantum yield. Furthermore, a previously reported photoisomeric behavior may contribute to the measured fluorescence intensity changes. The goal of this research was to examine the hypothesized behavior theoretically and experimentally from the perspective of fluid dynamics.

Results

A correlation between stirring rate and intensity could not be established in the present experiments with a completely illuminated sample in contrast to previously reported experiments in spectrofluorometers. Experiments and theoretical models of a Poiseuille flow were in good agreement with the photoisomeric behavior but excluded the influence of shear. Further experiments in a flow chamber supported the photoisomery hypothesis as well.

Conclusion

No experimental evidence for the influence of velocity on the fluorescence intensity of CCVJ was found. The hypothesis of shear sensitivity was excluded as well. The results are consistent with the photoisomeric behavior of CCVJ.

Thermally-Activated, Delayed Fluorescence in O,B,O- and N,B,O-Strapped Boron Dipyrromethene Derivatives

A small series of boron dipyrromethene (BODIPY) dyes has been synthesized whereby the boron atom is constrained in a five-membered ring formed from either o-dihydroxypyridine or o-aminophenol. In the latter case, the amino group has been converted into the corresponding amide derivative so as to curtail the possibility for light-induced charge transfer from strap to BODIPY. These compounds are weakly emissive in fluid solution but cleavage of the strap, by treatment with a photoacid generator, restores strong fluorescence. Surprisingly, the same compounds remain weakly fluorescent in a rigid glass at 80 K where light-induced charge transfer is most unlikely. In fluid solution, the fluorescence quantum yield increases with increasing temperature due to a thermally activated step but does not correlate with the thermodynamics for intramolecular charge transfer. It is proposed that the strap causes rupture of the potential energy surface for the excited state, creating traps that provide new routes by which the wave packet can return to the ground state. Access to the trap from the excited state is reversible, leading to the delayed emission. Analysis of the temperature dependent emission intensities allows estimation of the kinetic parameters associated with entering and leaving the trap.

Ultrafast dynamics and solvent-dependent deactivation kinetics of BODIPY molecular rotors

The fluorescent excited state of a molecular rotor based on the meso-substituted boron-dipyrromethane (BODIPY) core decays rapidly to the ground state via a conical intersection. The fluorescence is strongly increased in viscous solvents, but solvent polarity has only a small effect.

A Molecular Rotor that Measures Dynamic Changes of Lipid Bilayer Viscosity Caused by Oxidative Stress

Oxidation of cellular structures is typically an undesirable process that can be a hallmark of certain diseases. On the other hand, photooxidation is a necessary step of photodynamic therapy (PDT), a cancer treatment causing cell death upon light irradiation. Here, the effect of photooxidation on the microscopic viscosity of model lipid bilayers constructed of 1,2-dioleoyl-sn-glycero-3-phosphocholine has been studied. A molecular rotor has been employed that displays a viscosity-dependent fluorescence lifetime as a quantitative probe of the bilayer's viscosity. Thus, spatially-resolved viscosity maps of lipid photooxidation in giant unilamellar vesicles (GUVs) were obtained, testing the effect of the positioning of the oxidant relative to the rotor in the bilayer. It was found that PDT has a strong impact on viscoelastic properties of lipid bilayers, which 'travels' through the bilayer to areas that have not been irradiated directly. A dramatic difference in viscoelastic properties of oxidized GUVs by Type I (electron transfer) and Type II (singlet oxygen-based) photosensitisers was also detected.

A Simple BODIPY-Based Viscosity Probe for Imaging of Cellular Viscosity in Live Cells

Intracellular viscosity is a fundamental physical parameter that indicates the functioning of cells. In this work, we developed a simple boron-dipyrromethene (BODIPY)-based probe, BTV, for cellular mitochondria viscosity imaging by coupling a simple BODIPY rotor with a mitochondria-targeting unit. The BTV exhibited a significant fluorescence intensity enhancement of more than 100-fold as the solvent viscosity increased. Also, the probe showed a direct linear relationship between the fluorescence lifetime and the media viscosity, which makes it possible to trace the change of the medium viscosity. Furthermore, it was demonstrated that BTV could achieve practical applicability in the monitoring of mitochondrial viscosity changes in live cells through fluorescence lifetime imaging microscopy (FLIM).

A Deoxyuridine-Based Far-Red Emitting Viscosity Sensor

A novel deoxyuridine (dU) benzothiazolium (BZ) derivative, referred to as dU-BZ, is reported that was synthesized via Sonogashira coupling reaction methodology. The deoxyuridine building block was introduced to enhance hydrophilicity, while an alkynylated benzothiazolium dye was incorporated for long wavelength absorption to reduce potential phototoxicity that is characteristic of using UV light to excite common fluorphores, better discriminate from native autofluorescence, and potentially facilitate deep tissue imaging. An impressive 30-fold enhancement of fluorescence intensity of dU-BZ was achieved upon increasing viscosity. Fluorescence quantum yields in 99% glycerol/1% methanol (v/v) solution as a function of temperature (293–343 K), together with viscosity-dependent fluorescence lifetimes and radiative and non-radiative rate constants in glycerol/methanol solutions (ranging from 4.8 to 950 cP) were determined. Both fluorescence quantum yields and lifetimes increased with increased viscosity, consistent with results predicted by theory. This suggests that the newly-designed compound, dU-BZ, is capable of functioning as a probe of local microviscosity, an aspect examined by in vitro bioimaging experiments.

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.