Excited-state dynamics of the fluorescent probe Lucifer Yellow in liquid solutions and in heterogeneous media

Lipid peroxidation increases membrane tension, Piezo1 gating, and cation permeability to execute ferroptosis

The ongoing metabolic and microbicidal pathways that support and protect cellular life generate potentially damaging reactive oxygen species (ROS). To counteract damage, cells express peroxidases, which are antioxidant enzymes that catalyze the reduction of oxidized biomolecules. Glutathione peroxidase 4 (GPX4) is the major hydroperoxidase specifically responsible for reducing lipid peroxides; this homeostatic mechanism is essential, and its inhibition causes a unique type of lytic cell death, ferroptosis. The mechanism(s) that lead to cell lysis in ferroptosis, however, are unclear. We report that the lipid peroxides formed during ferroptosis accumulate preferentially at the plasma membrane. Oxidation of surface membrane lipids increased tension on the plasma membrane and led to the activation of Piezo1 and TRP channels. Oxidized membranes thus became permeable to cations, ultimately leading to the gain of cellular Na⁺ and Ca²⁺ concomitant with loss of K⁺. These effects were reduced by deletion of Piezo1 and completely inhibited by blocking cation channel conductance with ruthenium red or 2-aminoethoxydiphenyl borate (2-APB). We also found that the oxidation of lipids depressed the activity of the Na⁺/K⁺-ATPase, exacerbating the dissipation of monovalent cation gradients. Preventing the changes in cation content attenuated ferroptosis. Altogether, our study establishes that increased membrane permeability to cations is a critical step in the execution of ferroptosis and identifies Piezo1, TRP channels, and the Na⁺/K⁺-ATPase as targets/effectors of this type of cell death.

Conformational Consequences for Compatible Osmolytes on Thermal Denaturation

Compatible osmolytes are a broad class of small organic molecules employed by living systems to combat environmental stress by enhancing the native protein structure. The molecular features that make for a superior biopreservation remain elusive. Through the use of time-resolved and steady-state spectroscopic techniques, in combination with molecular simulation, insight into what makes one molecule a more effective compatible osmolyte can be gained. Disaccharides differing only in their glycosidic bonds can exhibit different degrees of stabilization against thermal denaturation. The degree to which each sugar is preferentially excluded may explain these differences. The present work examines the biopreservation and hydration of trehalose, maltose, and gentiobiose.

Universal quenching of common fluorescent probes by water and alcohols

Although biological imaging is mostly performed in aqueous media, it is hardly ever considered that water acts as a classic fluorescence quencher for organic fluorophores. By investigating the fluorescence properties of 42 common organic fluorophores recommended for biological labelling, we demonstrate that H2O reduces their fluorescence quantum yield and lifetime by up to threefold and uncover the underlying fluorescence quenching mechanism. We show that the quenching efficiency is significantly larger for red-emitting probes and follows an energy gap law. The fluorescence quenching finds its origin in high-energy vibrations of the solvent (OH groups), as methanol and other linear alcohols are also found to quench the emission, whereas it is restored in deuterated solvents. Our observations are consistent with a mechanism by which the electronic excitation of the fluorophore is resonantly transferred to overtones and combination transitions of high-frequency vibrational stretching modes of the solvent through space and not through hydrogen bonds. Insight into this solvent-assisted quenching mechanism opens the door to the rational design of brighter fluorescent probes by offering a justification for protecting organic fluorophores from the solvent via encapsulation.

Hydration Dynamics in Solutions of Cyclic Polyhydroxyl Osmolytes

Simple sugars are remarkably effective at preserving protein and enzymatic structures against thermal and hydrostatic stress. Here, we investigate the hydrodynamic and biopreservative properties of three small cyclic molecules: glucose, myo-inositol, and methyl-α-d-glucopyranoside using circular dichroism spectroscopy and isothermal calorimetry. Using ultrafast fluorescence frequency upconversion spectroscopy, we measure the dynamical retardation of hydration dynamics in cosolute solutions. We find that all three molecules are effective modifiers of hydration dynamics in solution and all are also effective at protecting model protein systems against thermal denaturation. Methyl-α-d-glucopyranoside is found to be the most effective dynamic reducer displaying an approximately 30% increase in solvation relaxation time as compared to water in a cosolute free solution. myo-Inositol and glucose both exhibit a smaller reduction in dynamics with similar magnitudes of concentration dependence. Using these cosolute models, we demonstrate that the thermal enhancement of protein structure does not correlate strongly with either the dynamical reduction of the bulk solution nor with the number of hydrogen bonds a cosolute makes with the solvent. Furthermore, solutions of glucose at twice the concentration of trehalose are shown to have similar magnitudes of dynamical impact. This implies that regulation of hydration dynamics is not a distinguishing characteristic of successful osmolytes. This work highlights the need for further studies and computational analysis to understand the phenomena of preferential exclusion and the contribution of hydration dynamics to protein structural stability.

Synthesis and properties of the derivatives of triphenylamine and 1,8-naphthalimide with the olefinic linkages between chromophores

Two donor–acceptor type molecules consisting of triphenylamine and 1,8-naphthalimide moieties with the olefinic linkages between chromophores were synthesized by Heck reaction.

A new substrate for alkaline phosphatase based on quercetin pentaphosphate

We describe the characterization and application of quercetin pentaphosphate (QPP), a new fluorimetric substrate for the detection of alkaline phosphatase (ALP) activity. QPP exhibits major absorbance peaks at 260/410 nm and a strong fluorescence at λex/λem = 425/510 nm at alkaline pH. The product of enzymatic reaction between QPP and ALP has a strong absorbance peak at 324 nm with no fluorescence at the investigated wavelengths. The product generated from the enzymatic reaction was found to be proportional to ALP activity, and the ALP activity was monitored by the absorbance difference at 310 nm and 410 nm. The change in absorbance was found to be proportional to the ALP concentration with a linear detection range and a limit of detection of 0.01-16 U L(-1) and 0.766 U L(-1), respectively. The enzyme activity was also monitored by evaluating the change in fluorescence emission at 530 nm with a linear range of 0.01-8 U L(-1) and a detection limit of 0.062 U L(-1). Further, the validity of the new substrate for ALP in conjugated form was tested using Bacillus globigii spores as the model sample. A detection limit of 5998 spores per mL was obtained using QPP as the substrate. Unlike the parent compound, QPP substrate exhibits stability in solution for over three and half months and was stable under storage for over 12 months. The results obtained demonstrate the effectiveness of QPP for ALP and compare well with other fluorescent substrates, such as Fluorescein, Alexa Fluor and Cy5.

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption

The dynamics of a moderately exergonic photoinduced charge separation has been investigated by ultrafast time-resolved infrared absorption with the dimethylanthracene/phthalonitrile donor/acceptor pair in solvents covering a broad range of polarity. A distinct spectral signature of an exciplex could be identified in the -C≡N stretching region. On the basis of quantum chemistry calculations, the 4-5 times larger width of this band compared to those of the ions and of the locally excited donor bands is explained by a dynamic distribution of exciplex geometry with different mutual orientations and distances of the constituents and, thus, with varying charge-transfer character. Although spectrally similar, two types of exciplexes could be distinguished by their dynamics: short-lived, "tight", exciplexes generated upon static quenching and longer-lived, "loose", exciplexes formed upon dynamic quenching in parallel with ion pairs. Tight exciplexes were observed in all solvents, except in the least polar diethyl ether where quenching is slower than diffusion. The product distribution of the dynamic quenching depends strongly on the solvent polarity: whereas no significant loose exciplex population could be detected in acetonitrile, both exciplex and ion pair are generated in less polar solvents, with the relative population of exciplex increasing with decreasing solvent polarity. These results are compared with those reported previously with donor/acceptor pairs in different driving force regimes to obtain a comprehensive picture of the role of the exciplexes in bimolecular photoinduced charge separation.

Time-resolved spectroscopy of the singlet excited state of betanin in aqueous and alcoholic solutions

Transient UV-vis-NIR absorption of betanin in water.

Nanocapsules for 5-fluorouracil delivery decorated with a poly(2-ethylhexyl methacrylate-co-7-(4-trifluoromethyl)coumarin acrylamide) cross-linked wall

Nanocapsules with reverse cross-linked polymer walls containing coumarin moieties are capable of encapsulating 5-fluorouracil and accomplishing a comprehensive strategy in a drug delivery system.

Testing fluorescence lifetime standards using two-photon excitation and time-domain instrumentation: rhodamine B, coumarin 6 and lucifer yellow

Having good information about fluorescence lifetime standards is essential for anyone performing lifetime experiments. Using lifetime standards in fluorescence spectroscopy is often regarded as a straightforward process, however, many earlier reports are limited in terms of lifetime concentration dependency, solvents and other technical aspects. We have investigated the suitability of the fluorescent dyes rhodamine B, coumarin 6, and lucifer yellow as lifetime standards, especially to be used with two-photon excitation measurements in the time-domain. We measured absorption and emission spectra for the fluorophores to determine which wavelengths we should use for the excitation and an appropriate detector range. We also measured lifetimes for different concentrations, ranging from 10(-2)- 10(-6) M, in both water, ethanol and methanol solutions. We observed that rhodamine B lifetimes depend strongly on concentration. Coumarin 6 provided the most stable lifetimes, with a negligible dependency on concentration and solvent. Lucifer yellow lifetimes were also found to depend little with concentration. Finally, we found that a mix of two fluorophores (rhodamine B/coumarin 6, rhodamine B/lucifer yellow, and coumarin 6/lucifer yellow) all yielded very similar lifetimes from a double-exponential decay as the separate lifetimes measured from a single-exponential decay. All lifetime measurements were made using two-photon excitation and obtaining lifetime data in the time-domain using time-correlated single-photon counting.

Increasing the brightness of cyanine fluorophores for single-molecule and superresolution imaging

In spite of their relatively low fluorescence quantum yield, cyanine dyes such as Cy3, Cy5, or Cy7 are widely used in single-molecule fluorescence applications due to their high extinction coefficients and excellent photon yields. We show that the fluorescence quantum yield and lifetime of red-emitting cyanine dyes can be substantially increased in heavy water (D2 O) compared with water (H2 O). We find that the magnitude of the quantum yield increase in D2 O scales with the emission wavelength, reaching a particularly high value of 2.6-fold for the most red-emitting dye investigated, Cy7. We further demonstrate a higher photon yield in single-molecule superresolution experiments in D2 O compared to H2 O, which leads to an improved localization precision and hence better spatial resolution. This finding is especially beneficial for biological applications of fluorescence microscopy, which are typically carried out in aqueous media and which greatly profit from the red spectral range due to reduced cellular auto-fluorescence.

Fluorescent dye conjugates of rabbit arylsulfatase A as a biological tracer for protein endocytosis

Fluorescent dye conjugates of arylsulfatase A (ASA) from rabbit liver were prepared at pH 9.0 in 0.1 M sodium bicarbonate buffer. The modification of amino or sulphadryl groups by dichlorotriazinylamino-fluorescein or Lucifer yellow fluorescent dyes did not alter the characteristic features of the enzyme molecule such as enzyme activity, dimerization of the protein molecule at pH 4.5 and anomalous kinetics of the native enzyme. The fluorescence intensity of the Lucifer yellow enzyme conjugates were quenched when the pH of the protein solution was changed from pH 7.5 to 4.5. Therefore, the Lucifer yellow enzyme conjugate can be used to study the kinetics of pH-dependent association and dissociation of the ASA. Availability of such fluorescent dyes conjugates of ASA or other lysosomal enzyme may be used as a biological tracer to study the receptor dependent endocytosis of enzyme molecules.

Excited-state dynamics of organic dyes at liquid/liquid interfaces

Liquid/liquid interfaces play a crucial role in numerous areas of science. However, direct spectroscopic access to this thin (~1 nm) region is not possible with conventional optical methods. After a brief review of the most used techniques to perform interfacial optical spectroscopy, we will focus on time-resolved surface second harmonic generation, which allows the measurement of the excited-state dynamics of probe molecules at interfaces. By comparing these dynamics with those measured in bulk solutions, precious information on the properties of the interfacial region can be obtained. To illustrate this, several studies performed in our group will be presented.

Highly Solvatochromic 7-Aryl-3-hydroxychromones

Introduction of the dialkylaminophenyl group in position 7 of 3-hydroxychromone changes the orientation of the excited-state dipole moment and leads to superior solvatochromic properties (>170 nm emission shift in aprotic media). The excited-state intramolecular proton-transfer (ESIPT) reaction of 7-aryl-3-hydroxychromones is almost completely inhibited in most solvents. Methylation of the 3-OH abolishes ESIPT completely and also leads to improved photostability. The probes exhibit a ∼100-fold increase in fluorescence intensity and large Stokes shifts upon binding to membranes, reflecting differences in membrane phase and charge by a >40 nm spread in the emission band position.

Lifetimes of triplet dissolved natural organic matter (DOM) and the effect of NaBH₄ reduction on singlet oxygen quantum yields: implications for DOM photophysics

The natural lifetimes of triplet dissolved organic matter ((3)DOM) were determined by an O(2) saturation kinetics study of singlet oxygen quantum yields (Φ(1O2)) in buffered D(2)O. At least two distinct (3)DOM pools are present, and the observed lifetime range (∼20 to 80 μs) leads to a dependence of Φ(1O2) on O(2) concentrations between 29 and 290 μM. Thus, steady-state (1)O(2) concentrations will depend on [O(2)] in natural waters. The lifetimes are essentially identical for DOM samples of different origins and do not vary with excitation wavelength. However, Φ(1O2) varies greatly between samples and decreases with excitation wavelength. These data strongly suggest that (3)DOM quantum yields decrease with excitation wavelength, which gives rise to the Φ(1O2) variation. Borohydride reduction of several samples in both D(2)O and H(2)O lowers the absorbance and (1)O(2) production rates, but it does not alter Φ(1O2). This is consistent with a model in which (1)O(2) sensitizing chromophores are borohydride reducible groups in DOM, such as aromatic ketones. Interpreted in the framework of a charge transfer (CT) model for DOM optical properties, the collective data suggest a model in which electron acceptor moieties are important (1)O(2) sensitizers and where CT interactions of these moieties disrupt their ability to produce (1)O(2).

Excited state relaxation dynamics of model green fluorescent protein chromophore analogs: evidence for cis-trans isomerism

Two green fluorescent protein (GFP) chromophore analogs (4Z)-4-(N,N-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DMPI) and (4Z)-4-(N,N-diphenylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DPMPI) were investigated using femtosecond fluorescence up-conversion spectroscopy and quantum chemical calculations with the results being substantiated by HPLC and NMR measurements. The femtosecond fluorescence transients are found to be biexponential in nature and the time constants exhibit a significant dependence on solvent viscosity and polarity. A multicoordinate relaxation mechanism is proposed for the excited state relaxation behavior of the model GFP analogs. The first time component (τ(1)) was assigned to the formation of twisted intramolecular charge transfer (TICT) state along the rotational coordinate of N-substituted amine group. Time resolved intensity normalized and area normalized emission spectra (TRES and TRANES) were constructed to authenticate the occurrence of TICT state in subpicosecond time scale. Another picosecond time component (τ(2)) was attributed to internal conversion via large amplitude motion along the exomethylenic double bond which has been enunciated by quantum chemical calculations. Quantum chemical calculation also forbids the involvement of hula-twist because of high activation barrier of twisting. HPLC profiles and proton-NMR measurements of the irradiated analogs confirm the presence of Z and E isomers, whose possibility of formation can be accomplished only by the rotation along the exomethylenic double bond. The present observations can be extended to p-HBDI in order to understand the role of protein scaffold in reducing the nonradiative pathways, leading to highly luminescent nature of GFP.

Hydrogen-bond-assisted excited-state deactivation at liquid/water interfaces

The excited-state dynamics of eosin B (EB) at dodecane/water and decanol/water interfaces has been investigated with polarization-dependent and time-resolved surface second harmonic generation. The results of the polarization-dependent measurements vary substantially with (1) the EB concentration, (2) the age of the sample, and (3) the nature of the organic phase. All of these effects are ascribed to the formation of EB aggregates at the interface. Aggregation also manifests itself in the time-resolved measurements as a substantial shortening of the excited-state lifetime of EB. However, independently of the dye concentration used, the excited-state lifetime of EB at both dodecane/water and decanol/water interfaces is much longer than in bulk water, where the excited-state population undergoes hydrogen-bond-assisted non-radiative deactivation in a few picoseconds. These results indicate that hydrogen bonding between EB and water molecules at liquid/water interfaces is either much less efficient than in bulk water or does not enhance non-radiative deactivation. This strong increase of the excited-state lifetime of EB at liquid/water interfaces opens promising avenues of applying this molecule as a fluorescent interfacial probe.

Ultrafast excited-state dynamics of eosin B: a potential probe of the hydrogen-bonding properties of the environment

The photophysics of two dyes from the xanthene family, eosin B (EB), and eosin Y (EY) has been investigated in various solvents by femtosecond transient absorption spectroscopy, first, to clarify the huge disparity of the EB fluorescence lifetimes reported in literature, and, second, to understand the mechanism responsible for the ultrafast excited-state deactivation of EB in water. The excited-state lifetime of EB was found to be much shorter in water and in other protic solvents, due to the occurrence of hydrogen-bond assisted nonradiative deactivation. This mechanism is associated with the hydrogen bonds between the solvent molecules and the nitro groups of EB, which become stronger upon optical excitation due to the charge-transfer character of the excited-state. This process is not operative with EY, where the nitro groups are replaced by bromine atoms. Therefore, the excited-state lifetime of EB in solution is directly related to the strength of the solvent as a hydrogen-bond donor, offering the possibility to build a corresponding scale based on the fluorescence quantum yield or lifetime of EB. This scale of hydrogen-bonding strength could be especially useful for studies of liquid interfaces by time-resolved surface second harmonic generation.

What is solvatochromism?

Solvatochromism is commonly used in many fields of chemical and biological research to study bulk and local polarity in macrosystems (membranes, etc.), or even the conformation and binding of proteins. Despite its wide use, solvatochromism still remains a largely unknown phenomenon due to the extremely complex coupling of many different interactions and dynamical processes which characterize it. In this study we analyze the influence of different solvents on the photophysical properties of selected charge-transfer probes (4-AP, PRODAN, and FR0). The purpose is to achieve a microscopic understanding of the intermolecular effects which govern the absorption and fluorescence properties of solvated molecular probes, such as solvent-induced structural modifications, polarization effects, solubility, solute-solvent hydrogen-bonding interactions, and solute aggregation. To this aim we have exploited a time dependent density functional theory (TDDFT) approach coupled to complementary solvation approaches (continuum, discrete and mixed discrete and continuum). Such an integration has allowed us to clearly disentangle the complex interplay between specific and nonspecific interactions of the solvent with the probes and show that strong H-bonding effects not only can lead to large solvatochromic shifts but also can affect the nature of the emitting species with resulting reduction of the quantum yield.

Exciton formation, relaxation, and decay in PCDTBT

The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband π-π* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes lead to a progressive relaxation of the inhomogeneously broadened emission spectrum with time constants ranging from about 500 fs to tens of picoseconds. The time scales found here for the relaxation processes in pristine PCDTBT are compared to the time scale (<0.2 ps) previously reported for photoinduced charge transfer in phase-separated PCDTBT:fullerene blends (Phys. Rev. B 2010, 81, 125210). We point out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PCDTBT:fullerene blends. This disparity in time scales is not consistent with the commonly proposed idea that photoinduced charge separation occurs after diffusion of the polymer exciton to a fullerene interface. We therefore discuss alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton.

Distance mapping in proteins using fluorescence spectroscopy: the tryptophan-induced quenching (TrIQ) method

Studying the interplay between protein structure and function remains a daunting task. Especially lacking are methods for measuring structural changes in real time. Here we report our most recent improvements to a method that can be used to address such challenges. This method, which we now call tryptophan-induced quenching (TrIQ), provides a straightforward, sensitive, and inexpensive way to address questions of conformational dynamics and short-range protein interactions. Importantly, TrIQ only occurs over relatively short distances (∼5-15 Å), making it complementary to traditional fluorescence resonance energy transfer (FRET) methods that occur over distances too large for precise studies of protein structure. As implied in the name, TrIQ measures the efficient quenching induced in some fluorophores by tryptophan (Trp). We present here our analysis of the TrIQ effect for five different fluorophores that span a range of sizes and spectral properties. Each probe was attached to four different cysteine residues on T4 lysozyme, and the extent of TrIQ caused by a nearby Trp was measured. Our results show that, at least for smaller probes, the extent of TrIQ is distance dependent. Moreover, we also demonstrate how TrIQ data can be analyzed to determine the fraction of fluorophores involved in a static, nonfluorescent complex with Trp. Based on this analysis, our study shows that each fluorophore has a different TrIQ profile, or "sphere of quenching", which correlates with its size, rotational flexibility, and the length of attachment linker. This TrIQ-based "sphere of quenching" is unique to every Trp-probe pair and reflects the distance within which one can expect to see the TrIQ effect. Thus,TrIQ provides a straightforward, readily accessible approach for mapping distances within proteins and monitoring conformational changes using fluorescence spectroscopy.

Site-dependent excited-state dynamics of a fluorescent probe bound to avidin and streptavidin

The excited-state dynamics of biotin-spacer-Lucifer-Yellow (LY) constructs bound to avidin (Avi) and streptavidin (Sav) was investigated using femtosecond spectroscopy. Two different locations in the proteins, identified by molecular dynamics simulations of Sav, namely the entrance of the binding pocket and the protein surface, were probed by varying the length of the spacer. A reduction of the excited-state lifetime, stronger in Sav than in Avi, was observed with the long spacer construct. Transient absorption measurements show that this effect originates from an electron transfer quenching of LY, most probably by a nearby tryptophan residue. The local environment of the LY chromophore could be probed by measuring the time-dependent polarisation anisotropy and Stokes shift of the fluorescence. Substantial differences in both dynamics were observed. The fluorescence anisotropy decays analysed by using the wobbling-in-a-cone model reveal a much more constrained environment of the chromophore with the short spacer. Moreover, the dynamic Stokes shift is multiphasic in all cases, with a approximately 1 ps component that can be ascribed to diffusive motion of bulk-like water molecules, and with slower components with time constants varying not only with the spacer, but with the protein as well. These slow components, which depend strongly on the local environment of the probe, are ascribed to the motion of the hydration layer coupled to the conformational dynamics of the protein.

Anomalous Excited-State Dynamics of Lucifer Yellow CH in Solvents of High Polarity: Evidence for an Intramolecular Proton Transfer

The photophysics of the fluorescent probe Lucifer yellow CH has been investigated using fluorescence spectroscopic and computational techniques. The nonradiative rate is found to pass through a minimum in solvents of intermediate empirical polarity. This apparently anomalous behavior is rationalized by considering the possibility of predominance of different kinds of nonradiative processes, viz. intersystem crossing (ISC) and excited-state proton transfer (ESPT), in solvents of low and high empirical polarity, respectively. The feasibility of the proton transfer is examined by the structure determined by the density functional theory (DFT) calculations. The predicted energy levels based on the time-dependent density functional theory (TD-DFT) method in the gas phase identifies the energy gap between the S(1) and nearest triplet state to be close enough to facilitate ISC. Photophysical investigation in solvent mixtures and in deuterated solvents clearly indicates the predominance of the solvent-mediated intramolecular proton transfer in the excited state of the fluorophore in protic solvents.