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A straightforward two-step synthesis protocol affords a series of chiral amide-based bis-pyridine substituted polyether macrocycles. One ligand is particularly able to complex Terbium(III) ion spontaneously. Upon complexation, interesting chiroptical properties are observed both in absorbance (ECD) and in fluorescence (CPL). In ligand-centered electronic circular dichroism, a sign inversion coupled with a signal enhancement is measured; while an easily detectable metal-centered circularly polarized luminescence with a glum of 0.05 is obtained for the main 5D4 → 7F5 terbium transition. The coordination mode and structure of the complex was studied using different analysis methods (NMR analysis, spectrophotometric titration and solid-state elucidation).

HydroFlippers are introduced as the first fluorescent membrane tension probes that report simultaneously on membrane compression and hydration. The probe design is centered around a sensing cycle that couples the mechanical planarization of twisted push-pull fluorophores with the dynamic covalent hydration of their exocyclic acceptor. In FLIM images of living cells, tension-induced deplanarization is reported as a decrease in fluorescence lifetime of the dehydrated mechanophore. Membrane hydration is reported as the ratio of the photon counts associated to the hydrated and dehydrated mechanophores in reconvoluted lifetime frequency histograms. Trends for tension-induced decompression and hydration of cellular membranes of interest (MOIs) covering plasma membrane, lysosomes, mitochondria, ER, and Golgi are found not to be the same. Tension-induced changes in mechanical compression are rather independent of the nature of the MOI, while the responsiveness to changes in hydration are highly dependent on the intrinsic order of the MOI. These results confirm the mechanical planarization of push-pull probes in the ground state as most robust mechanism to routinely image membrane tension in living cells, while the availability of simultaneous information on membrane hydration will open new perspectives in mechanobiology.

Fluorescent probes are known for their ability to sense changes in their direct environment. We introduce here the idea that common red-emitting fluorophores recommended for biological labeling and typically used for simple visualization of biomolecules can also act as reporters of the water content in their first solvent sphere by a simple measurement of their fluorescence lifetime. Using fluorescence spectroscopy, we investigated the excited-state dynamics of seven commercially available fluorophores emitting between 650 and 800 nm that are efficiently quenched by H2O. The amount of H2O in their direct surrounding was modulated in homogeneous H2O-D2O mixtures or, in heterogeneous systems, by confining them into reverse micelles, by encapsulating them into host-guest complexes with cyclodextrins, or by attaching them to peptides and proteins. We found that their fluorescence properties can be rationalized in terms of the amount of H2O in their direct surroundings, which provides a general mechanism for protein-induced fluorescence enhancements of red-emitting dyes and opens perspectives for directly counting water molecules in key biological environments or in polymers.

Since the non-linear optical (NLO) response of matter to incident excitation light does not require long-lived intermediate excited states working as relays, the conversion of low-energy photons into high energy light beams using second-harmonic generation (second-order NLO process) or two-photon absorption (third-order NLO process) can be implemented either in low-phonon macroscopic solids or in molecules containing high-energy vibrations. However, harnessing the very weak non-linear absorption coefficients requires (very) intense excitation sources, typically lasers, for getting reasonable emitted intensities. In contrast, the piling of successive near-infrared photons to get visible emission using linear optics, i.e. upconversion, is much more efficient, but it depends on the existence of intermediate excited states possessing long residence lifetimes. Therefore, upconversion usually occurs in low-phonon ionic solids or nanoparticles doped with pertinent activators. The recent recognition that trivalent erbium coordination complexes possessing high-frequency oscillators may act as dual visible/near-infrared activators, which implies the existence of at least one long-lived intermediate excited state in these complexes, paved the way for the implementation of the first upconversion processes within isolated molecules. Beyond a justification for using trivalent lanthanides, and especially erbium, for the manipulation of the energy of photons in molecules using linear optics, this tutorial review summarizes the current level of developments in the field of molecular-based upconversion and discusses some forthcoming challenges.

The piling up of low-energy photons to produce light beams of higher energies while exploiting the nonlinear optical response of matter was conceived theoretically around 1930 and demonstrated 30 years later with the help of the first coherent ruby lasers. The vanishingly small efficacy of the associated light- upconversion process was rapidly overcome by the implementation of powerful successive absorptions of two photons using linear optics in materials that possess real intermediate excited states working as relays. In these systems, the key point requires a favorable competition between the rate constant of the excited-state absorption (ESA) and the relaxation rate of the intermediate excited state, the lifetime of which should be thus maximized. Chemists and physicists therefore selected long-lived intermediate excited states found (i) in trivalent lanthanide cations doped into ionic solids or into nanoparticles (2S+1LJ spectroscopic levels) or (ii) in polyaromatic molecules (triplet states) as the logical activators for designing light upconverters using linear optics. Their global efficiency has been stepwise optimized during the past five decades by using indirect intermolecular sensitization mechanisms (energy transfer upconversion = ETU) combined with large absorption cross sections. The induction of light-upconversion operating in a single discrete entity at the molecular level is limited to metal-based units and remained a challenge for a long time because coordination complexes possess high-frequency oscillators incompatible with the existence of (i) scales of accessible excited relays with long lifetimes and (ii) final high-energy emissive levels with noticeable intrinsic quantum yields. In contrast to intermolecular energy transfer processes operating in metal-based doped solids, which require statistical models, the combination of sensitizers and activators within the same molecule limits energy transfers to easily tunable intramolecular processes with first-order kinetic rate constants. Their successful programming in a trinuclear CrErCr complex in 2011 led to the first detectable near-infrared to green light upconversion induced in a molecular unit under reasonable excitation intensity. The subsequent progress in the modeling and understanding of the key factors controlling metal-based light upconversion operating in molecular complexes led to a burst of various designs exploiting different mechanisms, excited-state absorption (ESA), energy transfer upconversion (ETU), cooperative luminescence (CL), and cooperative upconversion (CU), which are discussed in this Account.

Single-center linear excited state absorption (ESA) can be implemented in isolated mononuclear erbium(iii) coordination complexes, thus fixing the zero-level of quantum yields for lanthanide-based molecular light upconversion., Single-center light upconversion corresponds to the piling up of low-energy photons via successive linear absorptions: a phenomenon commonly observed in lanthanide-doped low-phonon ionic solids or nanoparticles. Its ultimate miniaturization in molecular complexes opens challenging perspectives in terms of improved reproducibility, chemical control and optical programming. However, high-energy vibrations inherent in coordination complexes severely limit the efficiency of successive excited-state absorptions (ESAs) responsible for the gain in photon energy. By carefully wrapping three polyaromatic ligand strands around trivalent erbium, we managed to induce low-power room temperature near-infrared (λexc = 801 nm or 966 nm) to visible green (λem = 522 nm and 545 nm) light upconversion within mononuclear coordination complexes [Er(Lk)3]3+ operating either in the solid state or in non-deuterated solution. The calculated upconversion quantum yields set the zero-level of an elemental erbium-centered molecular ESA mechanism, a value which favorably compares with cooperative upconversion (CU) previously implemented in sophisticated multisite Yb2Tb supramolecular assemblies. The various dependences of the upconverted emission on the incident excitation power imply different mechanisms, which can be tuned by molecular design.

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., Overtones and combinations of O–H vibrations in the solvent efficiently quench red-emitting fluorophores by resonant energy transfer.

International audience; The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4–L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triple-stranded helical [LnM(Lk)3]5+ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)3]5+ (strictly high-spin), [LnFe(L5)3]5+ (spin-crossover (SCO) around room temperature), and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

We report the design, synthesis, and evaluation of fluorescent flipper probes for single-molecule super-resolution imaging of membrane tension in living cells. Reversible switching from bright-state ketones to dark-state hydrates, hemiacetals, and hemithioacetals is demonstrated for twisted and planarized mechanophores in solution and membranes. Broadband femtosecond fluorescence up-conversion spectroscopy evinces ultrafast chalcogen-bonding cascade switching in the excited state in solution. According to fluorescence lifetime imaging microscopy, the new flippers image membrane tension in live cells with record red shifts and photostability. Single-molecule localization microscopy with the new tension probes resolves membranes well below the diffraction limit.

This article describes four fluorescent membrane tension probes that have been designed, synthesized, evaluated, commercialized and applied to current biology challenges in the context of the NCCR Chemical Biology. Their names are Flipper-TR®, ER Flipper-TR®, Lyso Flipper-TR®, and Mito Flipper-TR®. They are available from Spirochrome.

On June 14, 2019, nearly 50 photochemists from all over Switzerland and beyond gathered together at the Haute Ecole d'Ingénierie et d'Architecture in Fribourg (HEIA-FR) for the annual SCS Photochemistry Section meeting to discuss their latest findings in the field. The organizing committee consisting of the board of the SCS Photochemistry Section put together a program consisting of 3 invited talks, 9 oral communications and a poster session with 24 posters to revive this event which, they hope, will take place annually. In addition, the general assembly of the Section was held at the premise during the day.

On June 14, 2019, nearly 50 photochemists from all over Switzerland and beyond gathered together at the Haute Ecole d'Ingénierie et d'Architecture in Fribourg (HEIA-FR) for the annual SCS Photochemistry Section meeting to discuss their latest findings in the field. The organizing committee consisting of the board of the SCS Photochemistry Section put together a program consisting of 3 invited talks, 9 oral communications and a poster session with 24 posters to revive this event which, they hope, will take place annually. In addition, the general assembly of the Section was held at the premise during the day.

The chemokine receptor CCR5 is a drug target to prevent transmission of HIV/AIDS. We studied four analogs of the native chemokine RANTES (CCL5) that have anti-HIV potencies of around 25 pM, which is more than four orders-of-magnitude higher than that of RANTES itself. It has been hypothesized that the ultra-high potency of the analogs is due to their ability to bind populations of receptors not accessible to native chemokines. To test this hypothesis, we developed a homogeneous dual-color fluorescence cross-correlation spectroscopy (FCCS) assay for saturation and competition binding experiments. The FCCS assay has the advantage that it does not rely on competition with radioactively labeled native chemokines used in conventional assays. We prepared site-specifically labeled fluorescent analogs using native chemical ligation of synthetic peptides, followed by bioorthogonal fluorescent labeling. We engineered a mammalian cell expression construct to provide fluorescently labeled CCR5, which was purified using a tandem immunoaffinity and size-exclusion chromatography approach to obtain monomeric fluorescent CCR5 in detergent solution. We found subnanomolar binding affinities for the two analogs 5P12-RANTES and 5P14-RANTES, and about twenty-fold reduced affinities for PSC-RANTES and 6P4-RANTES. Using homologous and heterologous competition experiments with unlabeled chemokine analogs, we conclude that the analogs all bind at the same binding site; whereas, the native chemokines (RANTES and MIP1α) fail to displace bound fluorescent analogs even at tens of micromolar concentrations. Our results can be rationalized with de novo structural models of the N-terminal tails of the synthetic chemokines that adopt a different binding mode as compared to the parent compound.

The chemokine receptor CCR5 is a drug target to prevent transmission of HIV/AIDS. We studied four analogs of the native chemokine regulated, on activation, normal T-cell-expressed, and secreted (RANTES) (CCL5) that have anti-HIV potencies of around 25 pM, which is more than four orders of magnitude higher than that of RANTES itself. It has been hypothesized that the ultrahigh potency of the analogs is due to their ability to bind populations of receptors not accessible to native chemokines. To test this hypothesis, we developed a homogeneous dual-color fluorescence cross-correlation spectroscopy assay for saturation- and competition-binding experiments. The fluorescence cross-correlation spectroscopy assay has the advantage that it does not rely on competition with radioactively labeled native chemokines used in conventional assays. We prepared site-specifically labeled fluorescent analogs using native chemical ligation of synthetic peptides, followed by bioorthogonal fluorescent labeling. We engineered a mammalian cell expression construct to provide fluorescently labeled CCR5, which was purified using a tandem immunoaffinity and size-exclusion chromatography approach to obtain monomeric fluorescent CCR5 in detergent solution. We found subnanomolar binding affinities for the two analogs 5P12-RANTES and 5P14-RANTES and about 20-fold reduced affinities for PSC-RANTES and 6P4-RANTES. Using homologous and heterologous competition experiments with unlabeled chemokine analogs, we conclude that the analogs all bind at the same binding site, whereas the native chemokines (RANTES and MIP-1α) fail to displace bound fluorescent analogs even at tens of micromolar concentrations. Our results can be rationalized with de novo structural models of the N-terminal tails of the synthetic chemokines that adopt a different binding mode as compared to the parent compound.

Chemokines and some chemical analogs of chemokines prevent cellular HIV-1 entry when bound to the HIV-1 coreceptors C-C chemokine receptor 5 (CCR5) or C-X-C chemokine receptor 4 (CXCR4), which are G protein–coupled receptors (GPCRs). The ideal HIV-1 entry blocker targeting the coreceptors would display ligand bias and avoid activating G protein–mediated pathways that lead to inflammation. We compared CCR5-dependent activation of second messenger pathways in a single cell line. We studied two endogenous chemokines [RANTES (also known as CCL5) and MIP-1α (also known as CCL3)] and four chemokine analogs of RANTES (5P12-, 5P14-, 6P4-, and PSC-RANTES). We found that CCR5 signaled through both G i/o and G q/11 . IP 1 accumulation and Ca 2+ flux arose from G q/11 activation, rather than from Gβγ subunit release after G i/o activation as had been previously proposed. The 6P4- and PSC-RANTES analogs were superagonists for G q/11 activation, whereas the 5P12- and 5P14-RANTES analogs displayed a signaling bias for G i/o . These results demonstrate that RANTES analogs elicit G protein subtype–specific signaling bias and can cause CCR5 to couple preferentially to G q/11 rather than to G i/o signaling pathways. We propose that G protein subtype–specific signaling bias may be a general feature of GPCRs that can couple to more than one G protein family.

International audience; Intrigued by the unexpected room-temperature dual visible/near-infrared (NIR) luminescence observed for fast-relaxing erbium complexes embedded in triple-stranded helicates, in this contribution, we explore a series of six tridentate N-donor receptors L4-L9 with variable aromaticities and alkyl substituents to extricate the stereoelectronic features responsible for such scarce optical signatures. Detailed solid-state (X-ray diffraction, differential scanning calorimetry, optical spectroscopy) and solution (speciations and thermodynamic stabilities, spectrophotometry, NMR and optical spectroscopy) studies of mononuclear unsaturated [Er(Lk) ] and saturated triple-helical [Er(Lk) ] model complexes reveal that the stereoelectronic changes induced by the organic ligands affect inter- and intramolecular interactions to such an extent that 1) melting temperatures in solids, 2) the affinity for trivalent erbium in solution, and 3) optical properties in luminescent complexes can be rationally varied and controlled. With this toolkit in hand, mononuclear erbium complexes with low stabilities displaying only NIR emission can be transformed into molecular-based dual Er-centered visible/NIR emitters operating at room temperature in both solid and solution states.

International audience

To date, the piling up of successive photons of low energies (near infrared; NIR) using a single lanthanide center and linear optics to ultimately produce upconverted visible emission was restricted to low-phonon solid materials and nanoparticles. Now we show that the tight helical wrapping of three terdentate N-donor ligands around a single nine-coordinate trivalent erbium cation provides favorable conditions for a mononuclear molecular complex to exhibit unprecedented related upconverted emission. Low power NIR laser excitations into the metal-centered transitions Er(4 I11/2 ←4 I15/2 ) at 801 nm or Er(4 I13/2 ←4 I15/2 ) at 966 nm result in upconverted blue-green emissions, where two or three photons respectively are successively absorbed by a molecular lanthanide complex possessing high-energy vibrations.

Wnt proteins are secreted, hydrophobic, lipidated proteins found in all animals that play essential roles in development and disease. Lipid modification is thought to facilitate the interaction of the protein with its receptor, Frizzled, but may also regulate the transport of Wnt protein and its localization at the cell membrane. Here, by employing single-molecule fluorescence techniques, we show that Wnt proteins associate with and diffuse on the plasma membranes of living cells in the absence of any receptor binding. We find that labeled Wnt3A transiently and dynamically associates with the membranes of Drosophila Schneider 2 cells, diffuses with Brownian kinetics on flattened membranes and on cellular protrusions, and does not transfer between cells in close contact. In S2 receptor-plus (S2R+) cells, which express Frizzled receptors, membrane diffusion rate is reduced and membrane residency time is increased. These results provide direct evidence of Wnt3A interaction with living cell membranes, and represent, to our knowledge, a new system for investigating the dynamics of Wnt transport.

Historically, many of the classical organic fluorescent dyes were developed as laser dyes and characterized and optimized in organic solvents. Since then, fluorescence has, however, found a vast range of applications in the life sciences in which the fluorophores are usually surrounded by water and not by organic solvents. The omnipresence of water in biomolecular fluorescence spectroscopy and imaging leads to some unwanted but nonetheless unavoidable consequences on the photophysical properties of the dyes, which may impact the quality and complicate quantitative interpretation of the experiments. This paper discusses and illustrates with examples two such water-induced phenomena, namely chromophore aggregation in water and fluorescence quenching by water, as well as some ways to overcome them.

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.

The interaction of a series of chiral cationic [4]helicene derivatives, which differ by their substituents, with double-stranded DNA has been investigated by using a combination of spectroscopic techniques, including time-resolved fluorescence, fluorescence anisotropy, and linear dichroism. Addition of DNA to helicene solutions results to a hypochromic shift of the visible absorption bands, an increase of fluorescence quantum yield and lifetime, a slowing down of fluorescence anisotropy decay, and a linear dichroism in flow-oriented DNA, which unambiguously points to the binding of these dyes to DNA. Both helicene monomers and dimeric aggregates, which form at higher concentration, bind to DNA, the former most probably upon intercalation and the latter upon groove binding. The binding constant depends substantially on the dye substituents and is, in all cases, larger with the M than the P enantiomer, by factors ranging from 1.2 to 2.3, depending on the dye.

Clustering of arrestins upon G protein-coupled receptor stimulation is a phenomenon that is well-known but difficult to describe quantitatively due to the size of the clusters close to the diffraction limit of visible light. We introduce a general method to quantitatively investigate the clustering of arrestin following stimulation of the C-C chemokine receptor 5 (CCR5) using single-molecule super-resolution imaging and coordinate and image-based cluster analysis. We investigated the effect of potent anti-HIV ligands of CCR5 with different pharmacological profiles on arrestin2 cluster formation and found that only the ligands capable of inducing CCR5 internalization induced arrestin2 recruitment and clustering. We further demonstrate that the fraction of arrestin2 molecules found in clusters larger than 100nm correlates with the magnitude of ligand-induced CCR5 internalization, but not with G protein activation, indicating that recruitment of arrestin2 to CCR5 is independent of G protein activation. Pre-treatment of the cells with the drug cytochalasin D, which blocks actin polymerization, led to the formation of larger clusters, whereas the inhibitor of microtubule polymerization nocodazole had little effect on arrestin2 recruitment, suggesting an active role of actin in the organization and dynamics of these aggregates.

The superfamily of G protein-coupled receptors (GPCRs) mediates a wide range of physiological responses and serves as an important category of drug targets. Earlier biochemical and biophysical studies have shown that GPCRs exist temporally in an ensemble of interchanging conformations. Single-molecule techniques are ideally suited to understand the dynamic signaling and conformational complexity of G protein-coupled receptors (GPCRs). Here, we review the progress in single-molecule studies on GPCRs. We introduce the fundamental technical aspects of single-molecule fluorescence. We also survey the methodologies for labeling GPCRs with biophysical probes, particularly fluorescent dyes, and highlight the relevant chemical biology innovations that can be instrumental for studying GPCRs. Finally, we illustrate how the optical techniques and the labeling schemes have been combined to investigate GPCR signaling and dynamics at the single-molecule level.

G protein-coupled receptors (GPCRs) are desensitized and internalized following activation. They are then subjected to post-endocytic sorting (degradation, slow recycling or fast recycling). The majority of research on post-endocytic sorting has focused on the role of sequence-encoded address structures on receptors. This study focuses on trafficking of CCR5, a GPCR chemokine receptor and the principal entry coreceptor for HIV. Using Chinese Hamster Ovary cells stably expressing CCR5 we show that two different anti-HIV chemokine analogs, PSC-RANTES and 5P14-RANTES, direct receptor trafficking into two distinct subcellular compartments: the trans-Golgi network and the endosome recycling compartment, respectively. Our results indicate that a likely mechanism for ligand-directed sorting of CCR5 involves capacity of the chemokine analogs to elicit the formation of durable complexes of CCR5 and arrestin2 (beta-arrestin-1), with PSC-RANTES eliciting durable association in contrast to 5P14-RANTES, which elicits only transient association.

The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L-3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraaza-cyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2](2+) (Chart 1). By contrast, Stern-Volmer analysis of intermolecular quenching of [ZnL2](2+) by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted.

The emission from two photoactive 14-membered macrocyclic ligands, 6-((naphthalen-1-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L1) and 6-((anthracen-9-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L2) is strongly quenched by a photoinduced electron transfer (PET) mechanism involving amine lone pairs as electron donors. Time-correlated single photon counting (TCSPC), multiplex transient grating (TG), and fluorescence upconversion (FU) measurements were performed to characterize this quenching mechanism. Upon complexation with the redox inactive metal ion, Zn(II), the emission of the ligands is dramatically altered, with a significant increase in the fluorescence quantum yields due to coordination-induced deactivation of the macrocyclic amine lone pair electron donors. For [ZnL2]2+, the substituted exocyclic amine nitrogen, which is not coordinated to the metal ion, does not quench the fluorescence due to an inductive effect of the proximal divalent metal ion that raises the ionization potential. However, for [ZnL1]2+, the naphthalene chromophore is a sufficiently strong excited-state oxidant for PET quenching to occur.

The photophysics of the dye Lucifer Yellow ethylenediamine (LYen) has been investigated in various polar solvents. The main deactivation pathways of its first singlet excited state are the fluorescence and the intersystem crossing. In water, non-radiative decay by intermolecular proton transfer becomes a significant deactivation channel. The early fluorescence dynamics, which was investigated in liquids and in reverse micelles, was found to depend substantially on the environment. An important static quenching of LYen by tryptophan and indole occurring in the subpicosecond timescale was observed. The use of the fluorescence dynamics of LYen as a local probe is illustrated by preliminary results obtained with a biotinylated Lucifer Yellow derivative complexed with avidin.

G protein-coupled receptors (GPCRs) are the largest superfamily of membrane receptors in the human genome and they are targets for a quarter of all prescription drugs. Activation of a GPCR by an agonist ligand results in G protein-mediated downstream signaling, followed by kinase action and arrestin-mediated desensitization, internalization/sequestration, and recycling. Selective manipulation of these individual steps of the GPCR activation cycle is often desired when creating drugs targeting a given receptor. We are interested in the C-C chemokine receptor CCR5 that is the major HIV coreceptor used in person-to-person transmission. Globally, the HIV/AIDS pandemic has caused nearly 30 million deaths and a similar number of people are currently infected. Certain analogues of the chemokine RANTES/CCL5 are highly potent entry inhibitors against R5-tropic HIV-1 strains, in vitro and in vivo. Three such analogues, 5P12-, 5P14-, and 6P4-RANTES, are particularly interesting because while they differ only slightly in structure they show strikingly different pharmacological profiles (G protein-linked signaling activity, stimulation of receptor internalization). We have recently developed a general, simple, and robust method for stoichiometric, site-specific fluorescence labeling of expressed GPCRs. The method is based on bioorthogonal conjugation of a fluorescent reporter group to a genetically encoded azido group introduced into expressed GPCRs using amber codon suppression.[1] We have adopted a similar strategy for fluorescent labeling of chemokines with azido groups introduced by chemical synthesis. Here we present our progress towards automated, multi-color, single-molecule fluorescence studies of the compositional and conformational dynamics of GPCR signaling complexes ("signalosomes") using fluorescently labeled chemokines and receptors in biochemically defined systems.[1] H Tian, TP Sakmar, & T Huber (2013) Site-specific labeling of genetically encoded azido groups for multi-color, single-molecule fluorescence imaging of GPCRs. Methods in Cell Biology, 117, in press.

Rigid p -octiphenyl rods were used to create helical tetrameric π-stacks of blue, red-fluorescent naphthalene diimides that can span lipid bilayer membranes. In lipid vesicles containing quinone as electron acceptors and surrounded by ethylenediaminetetraacetic acid as hole acceptors, transmembrane proton gradients arose through quinone reduction upon excitation with visible light. Quantitative ultrafast and relatively long-lived charge separation was confirmed as the origin of photosynthetic activity by femtosecond fluorescence and transient absorption spectroscopy. Supramolecular self-organization was essential in that photoactivity was lost upon rod shortening (from p -octiphenyl to biphenyl) and chromophore expansion (from naphthalene diimide to perylene diimide). Ligand intercalation transformed the photoactive scaffolds into ion channels.

Advanced fluorescence microscopy techniques includingsingle-molecule and super-resolution imaging require brightand photostable fluorophores that can be selectively attachedto biomolecules. There is therefore an ongoing interest in thedevelopment of improved chromophores for biology, espe-cially ones that absorb and emit in the near-infrared owing tothe reduced cellular autofluorescence and phototoxicity atthese wavelengths.

G protein-coupled receptors (GPCRs) comprise a large family of seven-helix transmembrane proteins which regulate cellular signaling by sensing light, ligands, and binding proteins. The GPCR activation process, however, is not a simple on-off switch; current models suggest a complex conformational landscape in which the active, signaling state includes multiple conformations with similar downstream activity. The present study probes the conformational dynamics of single β(2)-adrenergic receptors (β(2)ARs) in the solution phase by Anti-Brownian ELectrokinetic (ABEL) trapping. The ABEL trap uses fast electrokinetic feedback in a microfluidic configuration to allow direct observation of a single fluorescently labeled β(2)AR for hundreds of milliseconds to seconds. By choosing a reporter dye and labeling site sensitive to ligand binding, we observe a diversity of discrete fluorescence intensity and lifetime levels in single β(2)ARs, indicating a varying radiative lifetime and a range of discrete conformational states with dwell times of hundreds of milliseconds. We find that the binding of agonist increases the dwell times of these states, and furthermore, we observe millisecond fluctuations within states. The intensity autocorrelations of these faster fluctuations are well-described by stretched exponential functions with a stretching exponent β ~ 0.5, suggesting protein dynamics over a range of time scales.

The Anti-Brownian ELectrokinetic (ABEL) trap uses voltage feedback to electrokinetically cancel the Brownian motion of single particles in solution in microfluidic geometries. This allows trapping of single high density lipoprotein (HDL) particles for extended observation.

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.

The objective with synthetic multifunctional nanoarchitecture is to create large suprastructures with interesting functions. For this purpose, lipid bilayer membranes or conducting surfaces have been used as platforms and rigid-rod molecules as shape-persistent scaffolds. Examples for functions obtained by this approach include pores that can act as multicomponent sensors in complex matrices or rigid-rod π-stack architecture for artificial photosynthesis and photovoltaics.

We discuss and illustrate by several examples how the ultrafast excited-state dynamics of a chromophore can be altered when changing its environment from a homogenous solution to a biological molecule such as proteins or nucleic acids.

The fluorescence enhancement mechanisms of a series of DNA stains of the oxazole yellow (YO) family have been investigated in detail using steady-state and ultrafast time-resolved fluorescence spectroscopy. The strong increase in the fluorescence quantum yield of these dyes upon DNA binding is shown to originate from the inhibition of two distinct processes: 1) isomerisation through large-amplitude motion that non-radiatively deactivates the excited state within a few picoseconds and 2) formation of weakly emitting H-dimers. As the H-dimers are not totally non-fluorescent, their formation is less efficient than isomerisation as a fluorescent contrast mechanism. The propensity of the dyes to form H-dimers and thus to reduce their fluorescence contrast upon DNA binding is shown to depend on several of their structural parameters, such as their monomeric (YO) or homodimeric (YOYO) nature, their substitution and their electric charge. Moreover, these parameters also have a substantial influence on the affinity of the dyes for DNA and on the ensuing sensitivity for DNA detection. The results give new insight into the development and optimisation of fluorescent DNA probes with the highest contrast.

Design, synthesis and evaluation of advanced rigid-rod pi-stack photosystems with asymmetric scaffolds are reported. The influence of push-pull rods on self-organization, photoinduced charge separation and photosynthetic activity is investigated and turns out to be surprisingly small overall.

The excited-state dynamics of the DNA intercalator YO-PRO-1 and of three derivatives has been investigated in water and in DNA using ultrafast fluorescence spectroscopy. In the free form, the singly charged dyes exist both as monomers and as H-dimers, while the doubly charged dyes exist predominantly as monomers. Both forms are very weakly fluorescent: the monomers because of ultrafast nonradiative deactivation, with a time constant on the order of 3-4 ps, associated with large amplitude motion around the methine bridge, and the H-dimers because of excitonic interaction. Upon intercalation into DNA, large amplitude motion is inhibited, H-dimers are disrupted, and the molecules become highly fluorescent. The early fluorescence dynamics of these dyes in DNA exhibits substantial differences compared with that measured with their homodimeric YOYO analogues, which are ascribed to dissimilarities in their local environment. Finally, the decay of the fluorescence polarization anisotropy reveals ultrafast hopping of the excitation energy between the intercalated dyes. In one case, a marked change of the depolarization dynamics upon increasing the dye concentration is observed and explained in terms of a different binding mode.

The excited-state dynamics of the DNA bisintercalator YOYO-1 and of two derivatives has been investigated using ultrafast fluorescence up-conversion and time-correlated single photon counting. The free dyes in water exist in two forms: nonaggregated dyes and intramolecular H-type aggregates, the latter form being only very weakly fluorescent because of excitonic interaction. The excited-state dynamics of the nonaggregated dyes is dominated by a nonradiative decay with a time constant of the order of 5 ps associated with large amplitude motion around the monomethine bridge of the cyanine chromophores. The strong fluorescence enhancement observed upon binding of the dyes to DNA is due to both the inhibition of this nonradiative deactivation of the nonaggregated dyes and the dissociation of the aggregates and thus to the disruption of the excitonic interaction. However, the interaction between the two chromophoric moieties in DNA is sufficient to enable ultrafast hopping of the excitation energy as revealed by the decay of the fluorescence anisotropy. Finally, these dyes act as solvation probes since a dynamic fluorescence Stokes shift was observed both in bulk water and in DNA. Very similar time scales were found in bulk water and in DNA.

Influence of solute-solvent interactions on the quenching dynamics of perylene derivatives in an electron donating solvent

The electron transfer quenching dynamics of excited perylene and cyanoperylene in various donating solvents has been investigated by using ultrafast fluorescence up-conversion and multiplex transient grating. The strongly nonexponential fluorescence decays have been analyzed by using the orientational model described in the first article of this series ( J. Phys. Chem. A 2003, 107, 5375). It appears that the solvent dependence of the quenching dynamics is strongly connected to the number of surrounding donor molecules enabling ultrafast electron transfer. This number depends mainly on the driving force for electron transfer, on steric interactions, and on the occurrence of dipole-dipole interactions with the acceptor. The quenching product is an exciplex with a strong charge-transfer character. The complicated wavelength dependence of the fluorescence dynamics in the exciplex region, as well as the spectral dynamics observed in the transient grating data, is attributed to dipolar solvation, which leads to an increase of the charge-transfer character of the exciplex. The strong donor dependence of the exciplex lifetime is very similar to that reported earlier for the charge recombination time of geminate ion pairs in acetonitrile, and can be rationalized in terms of different intramolecular reorganization energies and electronic coupling constants.

The electron transfer (ET) quenching dynamics of excited perylene (Pe), cyanoperylene (PeCN), methanolperylene (PeOH), and methylperylene (PeMe) in N,N-dimethylaniline (DMA) has been investigated using ultrafast fluorescence up-conversion. Measurements of the rotational dynamics of PeCN and PeMe in nonpolar and polar inert solvents using optically heterodyned polarization spectroscopy are also presented. The fluorescence decay in DMA is strongly nonexponential and about 10 times faster with PeCN than with the other electron acceptors. The quenching dynamics has been analyzed with a model distinguishing three types of donor molecules surrounding the acceptor: those with optimal orientation for ET and those requiring orientational or translational diffusion prior to ET. According to this model, which can account for the whole fluorescence decay, the faster quenching dynamics of PeCN is not due to a larger ET rate constant, but to a larger number of donor molecules, typically three to four, with an optimal orientation. This is explained by the effect of dipole−dipole interaction between PeCN and the donor molecules, which favors mutual orientations with a large electronic coupling. With the other acceptors, this interaction is either not present or does not lead to ET active geometries. The occurrence of this interaction is substantiated by the rotational dynamics measurements.

Several aspects of ultrafast photochemistry in the condensed phase are discussed and illustrated by three examples from our laboratory.

Fluorescence imaging beyond the diffraction limit has grown into a method of choice to elucidate questions related to biological structure and organisation. Among super-resolution techniques, imaging based on the localization of individual photoswitchable fluorescent probes has become particularly popular due to its relative ease of implementation and the nature of qualitative and quantitative answers it can offer. We review the field of single-molecule localization microscopy (SMLM) by providing an overview of its underlying principles and of different categories of photoswitchable fluorophores. In addition to summarizing target-specific labelling strategies and presenting examples of successful applications of SMLM in fixed and living systems, we show how SMLM data offer unique opportunities for quantitative biomolecular counting and distribution analysis.