Neutral fluorescence probe with strong ratiometric response to surface charge of phospholipid membranes

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.

Multi-parametric sensing by multi-channel molecular fluorescent probes based on excited state intramolecular proton transfer and charge transfer processes

Considering the applications of fluorescent probes and the information they provide, their brightness of fluorescence and photostability are of paramount importance. However, in the case of steady-state fluorescence spectroscopy and fluorescence microscopy, the amount of information can be increased by the application of multi-channel probes, via a multi-band fluorophore introduced in the probe molecule. In most cases, the use of such a multi-band (or multi-channel) fluorophore can also be combined with the concomitant introduction of one or several analyte receptors. Most often, the design of ratiometric probes with multi-band fluorescence emission are based on phenomena such as photoinduced intramolecular charge transfer (ICT) or excited state intramolecular proton transfer (ESIPT). Although ICT probes were up to recently the most popular, ESIPT probes and among them 3-hydroxyflavone derivatives, were shown to be the most productive. Several general problems were resolved by this family of probes, as for example the measurement of local dielectric constant, local H-bond accepting ability, water local concentration and ATP concentration in small volumes. Incorporation of such multi-channel probes into lipid membranes allowed to measure the different membrane potentials and to detect cell apoptosis. Also, it enabled to recognize and characterize the rafts formation in different lipid bilayers and peculiar features of the charged membrane interface. Such probes are also able to provide a concentration-dependent fluorescence signals upon binding of H+, Mg2+and Ba2+ions, and thus to recognize these different cations. The multi-channel probes are effective tools in the study of interactions of macromolecules such as peptides, proteins and nucleic acids. The most useful feature is that they inform simultaneously about several physical parameters, in this way giving a better insight in the investigated system. Thus, by comparing the reviewed probes with other modern fluorescent approaches, it can be concluded they are more informative and accurate tools.

Tools shaping drug discovery and development

Spectroscopic, scattering, and imaging methods play an important role in advancing the study of pharmaceutical and biopharmaceutical therapies. The tools more familiar to scientists within industry and beyond, such as nuclear magnetic resonance and fluorescence spectroscopy, serve two functions: as simple high-throughput techniques for identification and purity analysis, and as potential tools for measuring dynamics and structures of complex biological systems, from proteins and nucleic acids to membranes and nanoparticle delivery systems. With the expansion of commercial small-angle x-ray scattering instruments into the laboratory setting and the accessibility of industrial researchers to small-angle neutron scattering facilities, scattering methods are now used more frequently in the industrial research setting, and probe-less time-resolved small-angle scattering experiments are now able to be conducted to truly probe the mechanism of reactions and the location of individual components in complex model or biological systems. The availability of atomic force microscopes in the past several decades enables measurements that are, in some ways, complementary to the spectroscopic techniques, and wholly orthogonal in others, such as those related to nanomechanics. As therapies have advanced from small molecules to protein biologics and now messenger RNA vaccines, the depth of biophysical knowledge must continue to serve in drug discovery and development to ensure quality of the drug, and the characterization toolbox must be opened up to adapt traditional spectroscopic methods and adopt new techniques for unraveling the complexities of the new modalities. The overview of the biophysical methods in this review is meant to showcase the uses of multiple techniques for different modalities and present recent applications for tackling particularly challenging situations in drug development that can be solved with the aid of fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, atomic force microscopy, and small-angle scattering.

Spatially Resolved Analytical Chemistry in Intact, Living Tissues

Tissues are an exciting frontier for bioanalytical chemistry, one in which spatial distribution is just as important as total content. Intact tissue preserves the native cellular and molecular organization and the cell-cell contacts found in vivo. Live tissue, in particular, offers the potential to analyze dynamic events in a spatially resolved manner, leading to fundamental biological insights and translational discoveries. In this Perspective, we provide a tutorial on the four fundamental challenges for the bioanalytical chemist working in living tissue samples as well as best practices for mitigating them. The challenges include (i) the complexity of the sample matrix, which contributes myriad interfering species and causes nonspecific binding of reagents; (ii) hindered delivery and mixing; (iii) the need to maintain physiological conditions; and (iv) tissue reactivity. This framework is relevant to a variety of methods for spatially resolved chemical analysis, including optical imaging, inserted sensors and probes such as electrodes, and surface analyses such as sensing arrays. The discussion focuses primarily on ex vivo tissues, though many considerations are relevant in vivo as well. Our goal is to convey the exciting potential of analytical chemistry to contribute to understanding the functions of live, intact tissues.

Probing the self-assembly dynamics and internal structure of amphiphilic hyaluronic acid conjugates by fluorescence spectroscopy and molecular dynamics simulations

Polymeric nanoparticles are increasingly used as biocompatible carriers for drugs and imaging agents. Understanding their self-assembly dynamics and morphology is of ultimate importance to develop nanoformulations with optimal characteristics. To achieve better performance, it is vital to account for cargo-carrier interactions at the molecular level. The self-assembly dynamics were studied and the internal structure of nanoparticles derived from a series of hydrophobically modified hyaluronic acid was revealed. Environment-sensitive ratiometric fluorescent probes provide valuable information about the nanoparticle's interior morphology, and molecular dynamics simulations complement the overall picture with insights into intramolecular and intermolecular interactions of the polymer, as well as its interactions with the small-molecule load. van der Waals and π-π interactions of the hydrophobic side fragments play a leading role in self-assembly and loading of hydrophobic small molecules. Aliphatic substituents form more extensive hydrophobic domains, while aromatic moieties allow more interaction of the loaded small molecules with the surrounding solvent.

Ratiometric Fluorescent Sensor Array as a Versatile Tool for Bacterial Pathogen Identification and Analysis

Rapid and reliable identification of pathogenic microorganisms is of great importance for human and animal health. Most conventional approaches are time-consuming and require expensive reagents, sophisticated equipment, trained personnel, and special storage and handling conditions. Sensor arrays based on small molecules offer a chemically stable and cost-effective alternative. Here we present a ratiometric fluorescent sensor array based on the derivatives of 2-(4'- N, N-dimethylamino)-3-hydroxyflavone and investigate its ability to provide a dual-channel ratiometric response. We demonstrate that, by using discriminant analysis of the sensor array responses, it is possible to effectively distinguish between eight bacterial species and recognize their Gram status. Thus, multiple parameters can be derived from the same data set. Moreover, the predictive potential of this sensor array is discussed, and its ability to analyze unknown samples beyond the list of species used for the training matrix is demonstrated. The proposed sensor array and analysis strategies open new avenues for the development of advanced ratiometric sensors for multiparametric analysis.

Fast and slow excited-state intramolecular proton transfer in 3-hydroxychromone: a two-state story?

The photodynamics of 3-hydroxychromone in its first-excited singlet electronic state (bright state of ππ* character) is investigated with special emphasis given to two types of reaction pathways: the excited-state intramolecular-proton-transfer coordinate and the hydrogen-torsion coordinate linking the excited cis and trans isomers. A newly-found conical intersection with the second-excited singlet electronic state (dark state of nπ* character) is suspected to be, to some extent, the reason for the slower rate constant. This hypothesis based on quantum-chemistry calculations is supported by quantum-dynamics simulations in full dimensionality. They show significant transfer of electronic population and provide consistently a vibronic interpretation for the forbidden band in the UV absorption spectrum.

Using membrane composition to fine-tune the pKa of an optical liposome pH sensor

Liposomes containing membrane-anchored pH-sensitive optical probes are valuable sensors for monitoring pH in various biomedical samples. The dynamic range of the sensor is maximized when the probe pK_a is close to the expected sample pH. While some biomedical samples are close to neutral pH there are several circumstances where the pH is 1 or 2 units lower. Thus, there is a need to fine-tune the probe pK_a in a predictable way. This investigation examined two lipid-conjugated optical probes, each with appended deep-red cyanine dyes containing indoline nitrogen atoms that are protonated in acid. The presence of anionic phospholipids in the liposomes stabilized the protonated probes and increased the probe pK_a values by < 1 unit. The results show that rational modification of the membrane composition is a general non-covalent way to fine-tune the pK_a of an optical liposome sensor for optimal pH sensing performance.

Synthesis, screening and sensing applications of a novel fluorescent probe based on C-glycosides

A novel water soluble fluorescent probe based on C-glycoside with an aromatic aldehyde unit has been synthesized and its UV/Vis and fluorescence spectra, aggregation and disaggregation with bovine serum albumin were studied.

A Structurally-Tunable 3-Hydroxyflavone Motif for Visible Light-Induced Carbon Monoxide-Releasing Molecules (CORMs)

Molecules that can be used to deliver a controlled amount of carbon monoxide (CO) have the potential to facilitate investigations into the roles of this gaseous molecule in biology and advance therapeutic treatments. This has led to the development of light-induced CO-releasing molecules (photoCORMs). A goal in this field of research is the development of molecules that exhibit a combination of controlled CO release, favorable biological properties (e.g., low toxicity and trackability in cells), and structural tunability to affect CO release. Herein, we report a new biologically-inspired organic photoCORM motif that exhibits several features that are desirable in a next-generation photoCORM. We show that 3-hydroxyflavone-based compounds are easily synthesized and modified to impart changes in absorption features and quantum yield for CO release, exhibit low toxicity, are trackable in cells, and can exhibit both O2-dependent and -independent CO release reactivity.

Facile ultrasensitive monitoring of mercury ions in water by fluorescent ratiometric detection

Prospects for analytical application of 2,5-diphenyloxazole-substituted 3-hydroxychromone for detection of mercury ions are presented. Sensitivity and selectivity for a number of metal ions both in methanol solution and in the plasticized polymer are outlined. Ultrasensitive and highly selective fluorescent ratiometric response of the polymer film containing the title compound for mercury ions in water media is revealed. Reversibility of ratiometric response to mercury ions and influence of plasticizer’s content in the polymer film on the optical feedback are also discussed.

Identifying apoptotic cells with the 3-hydroxyflavone derivative F2N12S, a ratiometric fluorescent small molecule probe selective for plasma membranes: a possible general mechanism for selective uptake into apoptotic cells

The mechanism of selective targeting of the plasma membrane of apoptotic cells by F2N12S, a recently reported ratiometric, fluorescent small molecule probe, was analyzed using decision-rule QSAR models. Selectivity was determined by a combination of the probe's weak amphiphilicity and slow flip-flop with the increased plasma membrane fluidity of apoptotic cells. The probable chemical features required for such probes may be defined in terms of numerical structural parameters as: 3.5 < AI < ∼ 5.5; log P < 5.0; HGS > 400 (where AI, log P and HGS parameters model amphiphilicity, lipophilicity and headgroup size, respectively). When HGS is <400, compounds are initially membrane selective, but subsequently are internalized.

Liquid ordered and gel phases of lipid bilayers: fluorescent probes reveal close fluidity but different hydration

Hydration and fluidity of lipid bilayers in different phase states were studied using fluorescent probes selectively located at the interface. The probe of hydration was a recently developed 3-hydroxyflavone derivative, which is highly sensitive to the environment, whereas the probe of fluidity was the diphenylhexatriene derivative, 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene. By variation of the cholesterol content and temperature in large unilamellar vesicles composed of sphingomyelin or dipalmitoylphosphatidlycholine, we generated different phases: gel, liquid ordered (raft), liquid crystalline, and liquid disordered (considered as liquid crystalline phase with cholesterol). For these four phases, the hydration increases in the following order: liquid ordered << gel approximately liquid disordered < liquid crystalline. The membrane fluidity shows a somewhat different trend, namely liquid ordered approximately gel < liquid disordered < liquid crystalline. Thus, gel and liquid ordered phases exhibit similar fluidity, whereas the last phase is significantly less hydrated. We expect that cholesterol due to its specific H-bonding interactions with lipids and its ability to fill the voids in lipid bilayers expels efficiently water molecules from the highly ordered gel phase to form the liquid ordered phase. In this study, the liquid ordered (raft) and gel phases are for the first time clearly distinguished by their strong difference in hydration.

Sensing of adenosine-5'-triphosphate anion in aqueous solutions and mitochondria by a fluorescent 3-hydroxyflavone dye

The current work demonstrates the formation of complexes between the tetraanion adenosine-5'-triphosphate (ATP) and the flavone derivative 3-hydroxy-4'-(dimethylamino)flavone (FME). Two kinds of complexes are evidenced. The higher affinity ATP-FME complex corresponds to a stacking of the two aromatic molecules and leads to a strong hypochromicity of the absorption spectrum of the dye. The lower affinity (ATP)(2)-FME complex results in a strong increase of the fluorescence intensity ( approximately 20-fold), due mainly to the appearance of the anionic form of FME, as shown by the important red shift (60 nm) of both excitation and emission spectra. Molecular modeling indicates that this anionic form results from the deprotonation induced by the influence of the tetra-charged triphosphate group of the ATP molecules. Using its strong enhancement of fluorescence intensity in the presence of ATP, the dye was used successfully to monitor the succinate-induced production of endogenous ATP in mitochondria. As a consequence, FME can be considered as a starting point to design efficient ATP sensors.

Fluorescent Biomembrane Probe for Ratiometric Detection of Apoptosis

Herein, we developed the first ratiometric fluorescent probe for apoptosis detection. This probe incorporates selectively into the outer leaflet of the cell plasma membrane and senses the loss of the plasma membrane asymmetry occurring during the early steps of apoptosis. The high specificity to the plasma membranes was achieved by introduction into the probe of a membrane anchor, composed of a zwitterionic group and a long (dodecyl) hydrophobic tail. The fluorescence reporter of this probe is 4'-(diethylamino)-3-hydroxyflavone, which exhibits excited-state intramolecular proton transfer (ESIPT), resulting in two-band emission highly sensitive to the lipid composition of the biomembranes. Fluorescence spectroscopy, flow cytometry, and microscopy measurements show that the ratio of the two emission bands of the probe changes dramatically in response to apoptosis. This response reflects the changes in the lipid composition of the outer leaflet of the cell plasma membrane because of the exposure of the anionic phospholipids from the inner leaflet at the early steps of apoptosis. Being ratiometric, the response of the new probe can be easily quantified on an absolute scale. This allows monitoring by laser scanning confocal microscopy the degree and spatial distribution of the apoptotic changes at the cell plasma membranes, a feature that can be hardly achieved with the commonly used fluorescently labeled annexin V assay.

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.

Dramatic Pressure-Dependent Quenching Effects in Supercritical CO2 Assessed by the Fluorescence of 4‘-Dimethylamino-3-hydroxyflavone. Thermodynamic versus Kinetics Control of Excited-State Intramolecular Proton Transfer

Steady-state fluorescence of 4'-dimethylamino-3-hydroxyflavone (DMA3HF) was observed in supercritical carbon dioxide (scCO(2)). Excited-state intramolecular proton transfer (ESIPT) occurs resulting in two well-separated emission bands corresponding to the normal and tautomer forms. As the scCO(2) density exceeds 0.7 g/mL, the relative intensity of the two bands tends to a constant value, comparable to that observed for organic solvents with ET(30) = 33.0 +/- 0.5 kcal/mol, such as toluene and di-n-butyl ether. At lower densities, the substantial decrease of the total fluorescence intensity (a 600-fold decrease as the pressure decreases from 100 to 80 bar) is accompanied by an even more accentuated decrease of the tautomer fluorescence. This can be explained by a shift in the equilibrium between normal and tautomer forms, concomitant with a more efficient quenching of the less solvated fluorophore, that may change the thermodynamic control of the relative population of the two emissive species to a kinetic control.

Influence of freezing and low molecular weight cryoprotectants on microsomal membrane structure: a study by multiparametric fluorescent probe

The influence of low molecular weight cryoprotectants (CPs) such as glycerol (GL), 1,2-propanediol (PD) and dimethylsulfoxide (DMSO) on the structure of rat liver microsomal membranes on the stages of equilibration and upon freezing up to -196 degrees C was studied using a multiparametric fluorescent probe of flavonol nature. It was estimated that the studied CPs have individual concentration ranges defining low amplitude of their action on biomembranes. An exceeding of these ranges strongly increases the violation of membrane native structure already at the stage of incubation with CPs, strengthening it during the freezing procedure. According to the perturbation effect on microsomal membranes the studied CPs can be arranged in a sequence: DMSO>PD>GL.

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.

Fluorometric detection of adenosine triphosphate with 3-hydroxy-4'-(dimethylamino)flavone in aqueous solutions

An effect of appearance of new band in the excitation spectra of 3-hydroxy-4'-(dimethylamino)flavone (FME probe) in presence of adenosine triphosphate (ATP) is described. Considerable shift of new band up to the red and increase of fluorescence intensity points to the formation of FME-ATP associate, in which FME molecule undergoes to a strong electrostatic stabilization by tetra-charged ATP anion. It is shown the FME anion formation is possible under influence of ATP in the studied conditions. The dynamics of the observed effect is studied in mitochondria. The registered phenomenon allows the quantitative evaluation of ATP concentration in the range of 10(-3)-10(-5) M. In contrast to ATP, other nucleoside phosphates do not give a new band in the excitation spectra of FME probe. This implies the possibility of the in vivo determination of the ATP concentration.

Fluorescence sensing of intermolecular interactions and development of direct molecular biosensors

Molecular biosensors are devices of molecular size that are designed for sensing different analytes on the basis of biospecific recognition. They should provide two coupled functions - the recognition (specific binding) of the target and the transduction of information about the recognition event into a measurable signal. The present review highlights the achievements and prospects in design and operation of molecular biosensors for which the transduction mechanism is based on fluorescence. We focus on the general strategy of fluorescent molecular sensing, construction of sensor elements, based on natural and designed biopolymers (proteins and nucleic acids). Particular attention is given to the coupling of sensing elements with fluorescent reporter dyes and to the methods for producing efficient fluorescence responses.

The problem of self-calibration of fluorescence signal in microscale sensor systems

Most recent developments in fluorescent molecular sensor devices are based on "ON-OFF" switching, which is an operation with a single measurable parameter, commonly the quenching of total intensity. In the meantime, with this approach self-calibration on the molecular level is not achievable. This calibration is strongly needed in all kinds of microscale applications, including microarrays, microfluidic systems and living cell imaging. Different possibilities are discussed for overcoming this difficulty and an "OR-OR" switching concept is suggested that involves the two-channel detection as a promising solution. For achieving the desired efficiency specific conditions are needed: it should be a single reporter dye exhibiting rapid reversible excited-state reaction and providing two-band wavelength ratiometric response.

The future of fluorescence sensor arrays

The rapid progress in sensor and biosensor array technologies needs a general strategy in the design of fluorescence reporters. Such reporters should provide a high density of sensor elements, allow analysis of targets of different affinities, and be internally calibrated, reproducible and have a rapid readout. Several criteria are introduced here for the comparative evaluation of fluorescence-sensing techniques. It is shown that only the two-band wavelength ratiometric sensing with a single reporter dye exhibiting rapid reversible excited-state reaction can satisfy all these criteria and is a prospective candidate for further development.

A triterpene oleanolic acid conjugate with 3-hydroxyflavone derivative as a new membrane probe with two-color ratiometric response

We report on the synthesis by coupling of a triterpenoid oleanolic acid with 4'-diethylamino-3-hydroxyflavone (FE) to produce an environment-sensitive biomembrane probe with two-band ratiometric response in fluorescence emission. The synthesized compound (probe FOT) was tested in a series of model solvents and demonstrated the response to solvent polarity and intermolecular hydrogen bonding very similar to that of parent probe FE. Meantime when incorporated into lipid bilayer membranes, it showed new features differing in response between lipids of different surface charges as well as between glycerophospholipids and sphingomyelin. We observed that in the conditions of coexistence of rafts and non-raft structures the probe is excluded from the rafts.

Molecular insight into the electrostatic membrane surface potential by 14n/31p MAS NMR spectroscopy: nociceptin-lipid association

Exploiting naturally abundant (14)N and (31)P nuclei by high-resolution MAS NMR (magic angle spinning nuclear magnetic resonance) provides a molecular view of the electrostatic potential present at the surface of biological model membranes, the electrostatic charge distribution across the membrane interface, and changes that occur upon peptide association. The spectral resolution in (31)P and (14)N MAS NMR spectra is sufficient to probe directly the negatively charged phosphate and positively charged choline segment of the electrostatic P(-)-O-CH(2)-CH(2)-N(+)(CH(3))(3) headgroup dipole of zwitterionic DMPC (dimyristoylphosphatidylcholine) in mixed-lipid systems. The isotropic shifts report on the size of the potential existing at the phosphate and ammonium group within the lipid headgroup while the chemical shielding anisotropy ((31)P) and anisotropic quadrupolar interaction ((14)N) characterize changes in headgroup orientation in response to surface potential. The (31)P/(14)N isotropic chemical shifts for DMPC show opposing systematic changes in response to changing membrane potential, reflecting the size of the electrostatic potential at opposing ends of the P(-)-N(+) dipole. The orientational response of the DMPC lipid headgroup to electrostatic surface variations is visible in the anisotropic features of (14)N and (31)P NMR spectra. These features are analyzed in terms of a modified "molecular voltmeter" model, with changes in dynamic averaging reflecting the tilt of the C(beta)-N(+)(CH)(3) choline and PO(4)(-) segment. These properties have been exploited to characterize the changes in surface potential upon the binding of nociceptin to negatively charged membranes, a process assumed to proceed its agonistic binding to its opoid G-protein coupled receptor.

Bimodal distribution and fluorescence response of environment-sensitive probes in lipid bilayers

A remarkable heterogeneity is often observed in the spectroscopic properties of environment-sensitive fluorescence probes in phospholipid bilayers. To explain its origin, we provided a detailed investigation of the fluorescence excitation and emission spectra of 4'-dimethylamino-3-hydroxyflavone (probe F) in bilayer vesicles with the variations of fatty acid composition, polar heads, temperature, and cholesterol content. Probe F, due to excited-state intramolecular proton transfer, exhibits two bands in emission that are differently sensitive to intermolecular interactions-thereby allowing us to distinguish universal (dipole-dipole) and specific (H-bonding) interactions within the bilayer. Spectroscopic, quenching, and anisotropy data suggest the presence of two forms of probe F at different locations in the bilayer: an H-bond free form located below sn(1)-carbonyls and an H-bonded form located at the polar membrane interface. We provide a quantitative analysis of the distribution of the probe between these two locations as well as the polarity of these locations, and show that both the distribution and the polarity contribute to the probe response. Moreover, analysis of literature data on other environment-sensitive probes (Prodan, Laurdan, Nile Red, NBD lipids, etc.) in lipid bilayers allows us to suggest that the bimodal distribution in the lipid bilayer is probably a general feature of low-polar molecules with polar groups capable of H-bonding interactions.

Simultaneous probing of hydration and polarity of lipid bilayers with 3-hydroxyflavone fluorescent dyes

The penetration of water into the hydrophobic interior leads to polarity and hydration profiles across lipid membranes which are fundamental in the maintenance of membrane architecture as well as in transport and insertion processes into the membrane. The present paper is an original attempt to evaluate simultaneously polarity and hydration properties of lipid bilayers by a fluorescence approach. We applied two 3-hydroxyflavone probes anchored in lipid bilayers at a relatively precise depth through their attached ammonium groups. They are present in two forms: either in H-bond-free form displaying a two-band emission due to an excited state intramolecular proton transfer reaction (ESIPT), or in H-bonded form displaying a single-band emission with no ESIPT. The individual emission profiles of these forms were obtained by deconvolution of the probes' fluorescence spectra. The polarity of the probe surrounding the bilayer was estimated from the two-band spectra of the H-bond-free form, while the local hydration was estimated from the relative contribution of the two forms. Our results confirm that by increasing the lipid order (phase transition from fluid to gel phase, addition of cholesterol or decrease in the lipid unsaturation), the polarity and to a lesser extent, the hydration of the bilayers decrease simultaneously. In contrast, when fluidity (i.e. lipid order) is kept invariant, increase of temperature and of bilayer curvature leads to a higher bilayer hydration with no effect on the polarity. Furthermore, no correlation was found between dipole potential and the hydration of the bilayers.

Ultrasensitive two-color fluorescence probes for dipole potential in phospholipid membranes

The principle of electrochromic modulation of excited-state intramolecular proton-transfer reaction was applied for the design of fluorescence probes with high two-color sensitivity to dipole potential, Psid, in phospholipid bilayers. We report on the effect of Psid variation on excitation and fluorescence spectra of two new 3-hydroxyflavone probes, which possess opposite orientations of the fluorescent moiety in the lipid bilayer. The dipole potential in the bilayer was modulated by the addition of 6-ketocholestanol or phloretin and by substitution of dimyristoyl phosphatidylcholine lipid with its ether analog 1,2-di-o-tetradecyl-sn-glycero-3-phosphocholine, and its value was estimated by the reference styryl dye 1-(3-sulfonatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine. We demonstrate that after Psid changes, the probe orienting in the bilayer similarly to the reference dye shows similar shifts in the excitation spectra, whereas the probe with the opposite orientation shows the opposite shifts. The new observation is that the response of 3-hydroxyflavone probes to Psid in excitation spectra is accompanied by and quantitatively correlated with dramatic changes of relative intensities of the two well separated emission bands that belong to the initial normal and the product tautomer forms of the excited-state intramolecular proton-transfer reaction. This provides a strong response to Psid by change in emission color.

Spectral and acid-base features of 3,7-dihydroxy-2,8-diphenyl-4H,6H-pyrano[3,2-g]chromene-4,6-dione (diflavonol)--a potential probe for monitoring the properties of liquid phases

Diflavonol is a molecule that can exist in neutral or anionic form and in several tautomeric forms in ground and excited states. Absorption and emission spectroscopy combined with theoretical calculations have shown that only one tautomer of neutral diflavonol exists in the ground state, but two exist in the excited state. In the latter case, one is the tautomer originating from the ground state tautomer, which exists in strongly protic solvents, the other is the phototautomer occurring in weakly protic or aprotic solvents as a result of the intramolecular transfer of one proton. The OH groups present in diflavonol and involved in weak intramolecular hydrogen bonds exhibit a proton-donating ability reflected by the experimental values of acidity constants or theoretical enthalpies and free energies of proton detachment. The electronically excited molecule is a relatively strong acid when it loses one proton. With increasing basicity of the medium, monoanionic and dianionic forms occur which exhibit spectral characteristics and an emission ability different from those of neutral diflavonol. These interesting features of diflavonol open up possibilities for the analytical use of the compound and its application as a spectral probe sensitive to the properties of liquid phases.

Solvent influence on excited-state intramolecular proton transfer in 3-hydroxychromone derivatives studied by cryogenic high-resolution fluorescence spectroscopy

High-resolution Shpol'skii spectra (recorded at 10 K in n-octane) of 3-hydroxychromone (3HC) substituted at the 2-position with a furan (3HC-F), a benzofuran (3HC-BF) or a naphthofuran group (3HC-NF) are presented. Being close analogues of 3-hydroxyflavone (3HF), these compounds can undergo excited-state intramolecular proton transfer (ESIPT). Luminescence can occur from the normal N* state (blue) or from the tautomeric T* state (green). Whether blue or green emission is observed is strongly dependent on hydrogen-bonding interactions with the environment. For all three chromones studied, high-resolution emission spectra in the green region (T*-->T) were obtained in pure n-octane, showing four sites with distinct emission bands and detailed vibrational structures, whereas no blue emission was detected. Contrary to the spectra published for 3HF, the emission lines were very narrow (line-broadening effects beyond detection) which implies that the ESIPT rate constants are >10(12) s(-1), at least 25 times lower than for 3HF. In order to study the effects of hydrogen-bonding solvents, four isomers of octanol (1-, 2-, 3- and 4-octanol) were added, forming 1:1 complexes with the 3HC derivatives. For all the combinations considered both blue and additional green emission was observed and in some cases narrow-banded spectra were obtained, mostly in the green. Only for the 3HC-NF/2-octanol complex, narrow-banded emission was found both in the blue and in the green region. It is demonstrated that these emissions come from different configurations of the complex. Possible structures for the two complex species are proposed, supported by semi-empirical calculations on complex formation enthalpies.

A new ratiometric fluorescence probe as strong sensor of surface charge of lipid vesicles and micelles

We report on the ability of a new fluorescent probe, 4-(2-pyren-1-yl-vinyl) pyridine, 1, to respond to micelles and phospholipid vesicles of different surface charge. The probe gets incorporated into micellar and membranous assemblies and shows a large red-shift in the fluorescence emission maxima especially when the surface charge of the organized media is anionic. The effect on the photo-excitation of the probe is very clear and pronounced as it can be easily visualized. The sample color upon photo-excitation changes from blue to orange/red once the probe experiences negatively charged vesicular or micellar surfaces. These results make the probe molecule useful as a reporter for sensing electrostatic environment in biological membranes and related organized assemblies.