Pyrene Monomer Fluorescence Research Articles

Accurate Determination of the Average Rate Constant of Pyrene Excimer Formation for Pyrene-Labeled Macromolecules from the Analysis of Individual Fluorescence Decays with Sums of Exponentials.

The average rate constant (⟨k⟩) for pyrene excimer formation (PEF) between an excited and a ground-state pyrenyl label covalently attached to a pyrene-labeled macromolecule (PyLM) has been found to be an ideal parameter to probe macromolecular conformations due to its proportionality to the local concentration ([Py]loc) of pyrenyl labels in PyLM. To date, ⟨k⟩ has only been determined with the model-free analysis (MFA) involving the global analysis of the pyrene monomer and excimer fluorescence decays of PyLM. Unfortunately, the MFA is computationally demanding which prevents its widespread use. To circumvent this complication, a methodology is introduced that involves the analysis of individual fluorescence decays with sums of exponentials (SoE), which are commonly used in the analysis packages of commercial time-resolved fluorometers. The individual fluorescence decays of the pyrene monomer acquired with 286 PyLM were analyzed with a SoE to yield ⟨kSoE-M⟩. The strong correlation between ⟨kSoE-M⟩ and ⟨kMF⟩ obtained from the global MFA indicated that ⟨kSoE-M⟩ was a good representation of ⟨k⟩. Furthermore, the AE-/AE+ ratio, equal to the ratio of the sum of the negative pre-exponential factors over the sum of the positive pre-exponential factors, was determined by fitting the individual pyrene excimer fluorescence decays of the 286 PyLM with a SoE. AE-/AE+ was found to take a value between -1.0 and -0.8, indicating that the pyrenyl labels were not aggregated. This result indicated that [Py]loc was well described by ⟨kSoE-M⟩, so that ⟨kSoE-M⟩ could be used to describe the conformation of macromolecules in the same manner as ⟨kMF⟩. Consequently, the methodology based on the analysis of individual fluorescence decays with sums of exponentials to determine ⟨kSoE-M⟩ and AE-/AE+ provides a robust alternative to the use of the MFA for the study of PyLM to many scientists interested in the characterization of macromolecular conformations.

Development of a bis-pyrene phospholipid probe for fluorometric detection of phospholipase A2 inhibition

In this work, we report the design, synthesis, and application of a bis-pyrene phospholipid probe for detection of phospholipase A2 action through changes in pyrene monomer and excimer fluorescence intensities. Continuous fluorometric assays enabled detection of the activities of multiple PLA2 enzymes as well as the decrease in catalysis by PLA2 from honey bee venom caused by the inhibitor p-bromo phenacylbromide. Thin-layer chromatography and mass spectrometry analysis were also used to validate probe hydrolysis by PLA2. Mass spectrometry data also supported cleavage of the probe by phospholipase C and D enzymes, although changes in fluorescence were not observed in these cases. Nevertheless, the bis-pyrene phospholipid probe developed in this work is effective for detection of PLA2 enzyme activity through an assay that enables screening for inhibitor development.

Synthesis and Characterization of a Pyrene-Labeled Gemini Surfactant Sensitive to the Polarity of Its Environment.

The cationic gemini surfactant PyO-3-12 was designed to include two dimethyl ammonium groups, one dodecyl tail, and 1-pyrenemethyl hexyl ether tail into the structure of the surfactant. The pyrenyl label ensured that the fluorescence of pyrene could be employed to probe the behavior of PyO-3-12 at the molecular level. The introduction of the oxygen atom in the β-position to pyrene was found to be critical for restoring the sensitivity of the pyrenyl label to the polarity of its environment. The properties of PyO-3-12 were characterized in water by surface tension and a fluorescence methodology that involved the global model-free analysis (MFA) of the pyrene monomer and excimer fluorescence decays to provide quantitative information about the state (unassociated-vs-aggregated) of PyO-3-12. The MFA was combined with a fluorescence quenching study with 2,6-dinitrotoluene to determine the size of the PyO-3-12 micelles. PyO-3-12 was found to behave like a typical gemini surfactant, exhibiting a critical micelle concentration (CMC) of 0.38 (±0.05) mM and an aggregation number (Nagg) equal to 23 (±2). Besides allowing PyO-3-12 to probe the polarity of its environment, the oxygen atom in the β-position next to pyrene brought some pyrenyl labels closer to the interface between the micellar interior and the aqueous phase, in a process that increased the effective volume of the hydrophobic part of PyO-3-12. This led to an increase in the packing parameter of PyO-3-12 and, consequently, an increase in Nagg compared to the Nagg value of 14 (±0.2) obtained for Py-3-12, a gemini surfactant, whose chemical structure was similar to that of PyO-3-12 but without the oxygen in the β-position to pyrene. The methodology described in this study to prepare and characterize pyrene-labeled surfactants is general and can be applied to study any pyrene-labeled surfactant and its interactions with oppositely charged macromolecules.

A self-assembly/disassembly two-photo ratiometric fluorogenic probe for bacteria imaging

Fluorescence imaging has facilitated fluorescent probes to analyze the subcellular localization and dynamics of biological targets. In this paper, we reported a fluorogenic probe for bacteria imaging. The probe was an imidazolium-derived pyrene compound, which self-assembled to form nano-particles and the pyrene fluorescence was quenched by the aggregation effects. When the self-assembly nano-particles interacted with anionic bacteria surfaces, synergistic effects of electrostatic interaction and hydrophobic force caused competing binding between bacteria surfaces and imidazoliums. This binding resulted in the disassembly of the aggregates to give fluorescence turn-on signal. Meanwhile, the probe bound bacteria surfaces and displayed both pyrene-excimer and pyrene-monomer fluorescence, which gave ratiometric signal. Then, fluorescent labeling by the probe enabled the two-photo ratiometric imaging of bacteria.

Membrane Binding and Oligomerization of the Lipopeptide A54145 Studied by Pyrene Fluorescence

A54145 is a lipopeptide antibiotic related to daptomycin that permeabilizes bacterial cell membranes. Its action requires both calcium and phosphatidylglycerol in the target membrane, and it is accompanied by the formation of membrane-associated oligomers. We here probed the interaction of A54145 with model membranes composed of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol, using the steady-state and time-resolved fluorescence of a pyrene-labeled derivative (Py-A54145). In solution, the labeled peptide was found to exist as a monomer. Its membrane interaction occurred in two stages that could be clearly distinguished by varying the calcium concentration. In the first stage, which was observed between 0.15 and 1 mM calcium, Py-A54145 bound to the membrane, as indicated by a strong increase in pyrene monomer emission. At the same calcium concentration, excimer emission increased also, suggesting that Py-A54145 had oligomerized. A global analysis of the time-resolved pyrene monomer and excimer fluorescence confirmed that Py-A54145 forms oligomers quantitatively and concomitantly with membrane binding. When calcium was raised beyond 1 mM, a distinct second transition was observed that may correspond to a doubling of the number of oligomer subunits. The collective findings confirm and extend our understanding of the action mode of A54145 and daptomycin.

Fundamental Study of Electrospun Pyrene-Polyethersulfone Nanofibers Using Mixed Solvents for Sensitive and Selective Explosives Detection in Aqueous Solution.

Fluorescent pyrene-polyethersulfone (Py-PES) nanofibers were prepared through electrospinning technique using mixed solvents. The effects of mixed solvent ratio and polymer/fluorophore concentrations on electrospun nanofiber's morphology and its sensing performance were systematically investigated and optimized. The Py-PES nanofibers prepared under optimized conditions were further applied for highly sensitive detection of explosives, such as picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) in aqueous phase with limits of detection (S/N = 3) of 23, 160, 400, and 980 nM, respectively. The Stern-Volmer (S-V) plot for Py excimer fluorescence quenching by PA shows two linear regions at low (0-1 μM) and high concentration range (>1 μM) with a quenching constant of 1.263 × 10(6) M(-1) and 5.08 × 10(4) M(-1), respectively. On the contrary, S-V plots for Py excimer fluorescence quenching by TNT, DNT, and RDX display an overall linearity in the entire tested concentration range. The fluorescence quenching by PA can be attributed to the fact that both photoinduced electron transfer and energy transfer are involved in the quenching process. In addition, pyrene monomer fluorescence is also quenched and exhibits different trends for different explosives. Fluorescence lifetime studies have revealed a dominant static quenching mechanism of the current fluorescent sensors for explosives in aqueous solution. Selectivity study demonstrates that common interferents have an insignificant effect on the emission intensity of the fluorescent nanofibers in aqueous phase, while reusability study indicates that the fluorescent nanofibers can be regenerated. Spiked real river water sample was also tested, and negligible matrix effect on explosives detection was observed. This research provides new insights into the development of fluorescent explosive sensor with high performance.

Micelle-assisted signaling of peracetic acid by the oxidation of pyreneboronic acid via monomer–excimer switching

A simple fluorescent probe for the industrial oxidant peracetic acid (PAA) was investigated. PAA-assisted oxidative conversion of pyrene-1-boronic acid into 1-hydroxypyrene was used as the signaling tool. Pyreneboronic acid was found to display selective signaling behavior, being more responsive to PAA than to other commonly used practical oxidants such as H2O2 and HOCl. The changes in pyrene monomer fluorescence to excimer were used in the quantitative analysis of PAA. When using the surfactant hexadecyltrimethylammonium bromide as a micellar additive, the signaling of PAA was markedly enhanced. Selective fluorescence signaling of PAA by pyrene-1-boronic acid with a detection limit of 1.5×10−6M in aqueous environment was successfully achieved.

Direct observation of bis(dicarbollyl)nickel conformers in solution by fluorescence spectroscopy: an approach to redox-controlled metallacarborane molecular motors.

As a continuation of work on metallacarborane-based molecular motors, the structures of substituted bis(dicarbollyl)nickel complexes in Ni(III) and Ni(IV) oxidation states were investigated in solution by fluorescence spectroscopy. Symmetrically positioned cage-linked pyrene molecules served as fluorescent probes to enable the observation of mixed meso-trans/dl-gauche (pyrene monomer fluorescence) and dl-cis/dl-gauche (intramolecular pyrene excimer fluorescence with residual monomer fluorescence) cage conformations of the nickelacarboranes in the Ni(III) and Ni(IV) oxidation states, respectively. The absence of energetically disfavored conformers in solution--dl-cis in the case of nickel(III) complexes and meso-trans in the case of nickel(IV)--was demonstrated based on spectroscopic data and conformer energy calculations in solution. The conformational persistence observed in solution indicates that bis(dicarbollyl)nickel complexes may provide attractive templates for building electrically driven and/or photodriven molecular motors.

Effect of Side-Chain Length on the Polymer Chain Dynamics of Poly(alkyl methacrylate)s in Solution

Eight series of poly(alkyl methacrylate)s bearing different side chains and one series of poly(methyl acrylate) were randomly labeled with pyrene, and their ability to form pyrene excimer was characterized quantitatively by steady-state and time-resolved fluorescence to demonstrate that such measurements provide a quantitative measure of polymer chain dynamics (PCD) in solution. Each series of pyrene-labeled polymer showed increased excimer formation with increasing pyrene content, but the increase was more pronounced for the polymers known to be more flexible based on their reported glass transition temperature (Tg). In the case of the poly(alkyl methacrylate)s with a linear side chain, a shorter side chain resulted in increased excimer formation. Replacing a linear side chain with a more rigid one containing the same number of carbon atoms resulted in decreased mobility of the polymer. Fluorescence Blob Model (FBM) analysis of the fluorescence decays provided a more accurate representation of those pyrenes that formed excimer by diffusion and thus reflected PCD more precisely. Global FBM analysis of the pyrene monomer and excimer fluorescence decays yielded the blob size Nblob and the product kblob × Nblob which reflects PCD. For each series, both Nblob and kblob × Nblob remained constant with pyrene content. Their average value ⟨Nblob⟩ and ⟨kblob × Nblob⟩ decreased substantially with increasing side-chain length, addition of the α-methyl substituent to poly(methyl acrylate) to yield poly(methyl methacrylate), or increased rigidity of the side chain, demonstrating that an increase in bulkiness or stiffening of the side- or main chain is associated with a pronounced decrease in chain mobility. These experiments are the first to demonstrate that pyrene excimer formation can be used to characterize quantitatively PCD in solution in the same manner that Tg is being used to characterize PCD in the bulk.

Fluorescence enhancement effect in pyrene and perylene doped nanoporous polystyrene films: Mechanistic and morphology

AbstractLuminescent porous polystyrene (PS) films containing pyrene and various amounts of perylene were prepared in humid atmosphere by solution casting technique for use as a plastic scintillator. The fluorescence spectra of doped PS films monitored at pyrene excitation show quenching of monomer fluorescence of pyrene with simultaneous sensitization of perylene monomer fluorescence appearing towards red. The observed enhancement in fluorescence of perylene was explained by considering excitation energy transfer (EET) from pyrene to perylene molecules accommodated in the pores generated during film formation. The fluorescence quenching data do not fit into the Stern–Volmer relation, but the sensitized fluorescence is fitted by an exponential growth equation, y = 201 860 433.29 ex/0.64366 + 0.49262, with correlation coefficient R2 = 0.997. The results on absorption and excitation spectroscopy indicated that the pyrene and perylene filled in pores of the PS films exist as isolated molecules. The porosity of the films was examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM), showing randomly oriented holes, and their mean size and size distribution were found to be depending upon the dopant concentration.Pyrene and perylene doped porous PS films prepared by solution casting method in humid atmosphere showed highly intense blue perylene fluorescence by fluorescence resonance energy transfer (FRET) at pyrene excitation. The SEM and AFM images revealed that nanosized pores are perylene concentration dependent, both pyrene and perylene held in the cavity of the pores in close proximal distance less than 10 nm is required to cause efficient energy transfer between them.

Light-responsive microstructures capable of pyrene monomer fluorescence switching

We synthesized an amphiphilic fluorescent compound (PyAzEG) having a pyrene fluorophore directly attached to twisted ortho-alkylated azobenzene (i) to suppress pyrene excimer formation and (ii) to facilitate pyrene monomer fluorescence switching. PyAzEG molecules in a dilute solution underwent azobenzene-based trans ↔ cis photoisomerization under UV and visible light irradiation. When such amphiphilic molecules dispersed in an aqueous solution, they could assemble into spheres through a delicate balance between π–π stacking among aromatic rings and hydrophilic interactions among tetra(ethylene glycol) chains. For a PyAzEG suspension containing micrometer-sized spheres, typical pyrene monomer fluorescence was observed in the range of 370–460 nm, whereas excimer fluorescence did not emerge from the fluorescence spectrum. Monomer fluorescence switching as well as structural transformation from spheres to less packed vesicle-like structures were achieved by changing the wavelength of light. Furthermore, PyAzEG spheres exhibited enhanced fluorescence relative to the monomeric solution, which is probably due to moderate intermolecular interactions among chromophores in a head-to-tail fashion.

Structure-Selective Anisotropy Assay for Amyloid Beta Oligomers

Amyloid β (Abeta) peptides in their oligomeric form have been proposed as major toxic species in Alzheimer's disease (AD). There are a limited number of anti-Abeta antibodies specific to oligomeric forms of Abeta compared to the monomeric form, and accurate measurement of oligomeric forms in biological samples, cerebrospinal fluid (CSF), or brain extracts remains challenging. We introduce an oligomer-specific (in preference to monomers or fibrils) fluorescence assay based on a conformationally sensitive bis-pyrene-labeled peptide that contains amino acid residues 16-35 of the human amyloid beta protein (pronucleon peptide, PP). This peptide exhibits a shift in fluorescence emission from pyrene excimer to pyrene monomer emission resulting from a conformational change. Specific binding of PP to oligomeric forms of Abeta can be monitored in solution by a change in fluorescence spectrum as well as a change in pyrene monomer fluorescence anisotropy (or polarization). The mechanism of binding and its relation to anisotropy and fluorescence lifetime changes are discussed. The development of a simple, rapid, anisotropy assay for measurement of Abeta oligomers is important for further study of the oligomers' role in AD, and specific detection of oligomers in biological samples, such as cerebrospinal fluid.

Solvent-dependent conformational and fluorescence change of an N-phenylbenzohydroxamic acid derivative bearing two pyrene moieties

The conformation of N-phenylbenzohydroxamic acid has been reported to change depending on the solvent properties. Here, we newly synthesized N-phenylbenzohydroxamic acid derivative, which contains two pyrene moieties separated from the phenyl rings by ethylene linkers. It showed solvent-dependent conformational change, and its fluorescence was also solvent-dependent, that is, only monomer fluorescence of pyrene was observed in DMSO or DMF, whereas the excimer fluorescence was observed in CH2Cl2 or CHCl3. Thus, the structural characteristics could be converted to fluorescence change as output, which would be a good candidate as a fluorescent sensor.

Molar Absorbance Coefficient of Pyrene Aggregates in Water Generated by a Poly(ethylene oxide) Capped at a Single End with Pyrene

The molar absorbance coefficient of ground-state pyrene aggregates (ε(E0)(λ)) in water generated by a short poly(ethylene oxide) chain labeled at a single end with a 1-pyrenemethoxide unit, namely, the polymeric construct Py₁-PEO(2.5K), was determined by a combination of UV-vis absorption and time-resolved fluorescence experiments. Since direct excitation of ground-state pyrene aggregates results in instantaneous excimer emission, whereas excimer formation by diffusive encounters between an excited- and a ground-state pyrene is delayed, acquisition of a pyrene monomer and excimer fluorescence decays with 1 nm increments of the excitation wavelength can probe the subtle effect that the excitation wavelength has on the extent of instantaneous excimer formation. These differences were taken advantage of to determine the fraction f(agg)(f)(λ) of the total absorbance of the Py₁-PEO(2.5K) solution that is due to ground-state pyrene aggregates. This was achieved by fitting the pyrene monomer and excimer fluorescence decays globally with the model free analysis. For each wavelength, an f(agg)(f)(λ) value was obtained which was then related to ε(E0)(λ) and f(agg), the molar fraction of aggregated pyrenes that is wavelength independent. These experiments were conducted for three Py₁-PEO(2.5K) concentrations yielding three similar ε(E0)(λ) spectra as expected since ε(E0)(λ) is an intrinsic property of the pyrene aggregates that should not depend on pyrene concentration. The ε(E0)(λ) spectra of the pyrene aggregates of Py₁-PEO(2.5K) in water were substantially broader and red-shifted compared to that of the pyrene monomer. Its measure provides a simple means to calculate the molar fraction of aggregated pyrene units in a Py₁-PEO(2.5K) aqueous solution from the absorption of the solution. This procedure should become widely applicable to determine f(agg) of water-soluble polymers which have been hydrophobically modified with a pyrene derivative and used as models for associative polymers.

Characterization of the Behavior of a Pyrene Substituted Gemini Surfactant in Water by Fluorescence

Time-resolved fluorescence was applied to characterize the behavior in solution of a gemini surfactant substituted with pyrene (Py-3-12). Upon association in water, excimer formation by Py-3-12 can be probed by acquiring pyrene monomer and excimer fluorescence decays which can be fitted globally according to the model free (MF) analysis to yield quantitative information about the internal dynamics of the Py-3-12 surfactant micelles as well as a complete description of the distribution of the different pyrene species in solution either incorporated inside the micelles or free in solution. A proof of procedure for the MF analysis was established by noting that the concentrations of free surfactant in solution, [Py-3-12](free), was found to equal the critical micelle concentration (CMC) for surfactant concentrations larger than the CMC. (I(E)/I(M))(SPC), the ratio of pyrene monomer to excimer fluorescence intensities, was calculated from parameters retrieved from the MF analysis of the fluorescence decays and was found to be independent of sample geometry. This work demonstrates how time-resolved fluorescence can be used to study the properties of pyrene-labeled macromolecules under conditions where large absorptions and inner filter effects usually distort the steady-state fluorescence signals. It was found that the pyrene excimer is formed mostly by diffusion within the Py-3-12 micelles, which suggests that the pyrene microenvironment is fluid, an important feature for future studies on the interactions of Py-3-12 with DNA.

How switching the substituent of a pyrene derivative from a methyl to a butyl affects the fluorescence response of polystyrene randomly labeled with pyrene

A series of polystyrenes randomly labeled with 1-pyrenebutanol were prepared by copolymerizing styrene and 1-pyrenebutylacrylate yielding the CoBuE–PS series. Solutions of CoBuE–PS were prepared in nine organic solvents having viscosities ranging from 0.36 to 5.5 mPa·s and the fluorescence spectra and pyrene monomer and excimer fluorescence decays were acquired. Analysis of the fluorescence spectra yielded the IE/IM ratio, whereas analysis of the fluorescence decays with the fluorescence blob model (FBM) yielded the parameters N blobo , <kblob × Nblob> , and k blobo . These parameters were compared to those obtained with two other series of pyrene-labeled polystyrenes, which had been studied earlier, namely CoA–PS and CoE–PS where pyrene was attached to the polymer backbone via a methylamide and benzyl methylether linker, respectively. Although the parameters IE/IM, N blobo , <kblob × Nblob>, and k blobo took different values according to the specific nature of the linker connecting pyrene to the polystyrene backbone, they exhibited trends that were quite similar for all the pyrene-labeled polystyrene constructs. The excellent agreement between the parameters retrieved for the three different types of pyrene-labeled polystyrenes suggests that the FBM accounts satisfyingly for differences in the nature of the label used, while still retrieving information pertinent to the polymer of interest.

Fluorescent oligonucleotide probe based on G-quadruplex scaffold for signal-on ultrasensitive protein assay

This work reported the G-quadruplex structure of pyrene-labeled G-rich DNA probe and its application in the immunoglobulin E (IgE) detection, providing plausibly an insight into the biological function of human telomere. Based on the intermolecular G-quadruplex, a terminal-single-pyrene-labeled oligonucleotide signaling probe was developed and a novel protein assay strategy was proposed via combining specific DNA cleavage by S1 nuclease with target-recognizing aptamer. This assay platform not only circumvented the optimization of specific sites for reporter attachment and the pyrene monomer fluorescence quenching by flanking nucleotide bases but also presented a signal-on mechanism. Thus, ultrasensitive homogeneous detection of IgE was successfully conducted. A linear dynamic range of 4.72 × 10 −12 to 7.56 × 10 −9 m, a regression coefficient of 0.9941 and a detection limit of 9.45 × 10 −14 m were given. Additionally, a preliminary concept of the single-pyrene-labeled excimer fluorescence probes associated with G-quadruplex for screening biological markers was described. Importantly, the unexpected structural property of G-quadruplex discovered seems to provide valuable information to allow understanding of the structure and function of human telomere and exploring of useful structure-based anticancer drug.

Solvent dependence of pyrene monomer and excimer fluorescence quenching by heavy atoms

Two independent methods have been used for the determination of pyrene monomer (M*) and excimer (D*) quenching rate parameters in eight common solvents ranging in viscosities between 0.3 and 5.9 cP. Heavy atoms (CCl4) were used as quenchers. The discrepancies between the values of the quenching rate parameters reported in the two methods have been explained as due to formation of complexes between the pyrene molecule and CCl4 molecules as a result of using a more intense source, which might lead to larger values of the quenching rate parameters. Assuming that the quenching processes are diffusion-controlled ones, the probabilities of the effective collisions between M* or D* and CCl4 (PP or PP, respectively) were then calculated. These values were found to depend on the nature of the solvents used and in particular on their viscosities (η). At low values of η, the average values of PP and PP are approximately the same for each solvent used. For intermediate values of η, PP becomes much larger than PP. Finally for relatively large values of η, PP becomes much smaller than PP.

Exciplex Fluorescence as a Diagnostic Probe of Structure in Coordination Polymers of Zn2+ and 4,4‘-Bipyridine Containing Intercalated Pyrene and Enclathrated Aromatic Solvent Guests

Coordination polymers based on Zn(II)-4,4'-bipyridine (Zn-bipy) frameworks containing pyrene intercalated between adjacent layers and aromatic solvent molecules enclathrated within the framework cavities have been prepared and characterized for the first time. These compounds are highly fluorescent, and show broad, featureless emission spectra significantly red-shifted relative to pyrene monomer fluorescence; this has been assigned to pyrene-bipy exciplex emission. Single-crystal X-ray structural analysis shows that the presence of the aromatic solvent molecule within the cavities has a profound effect on the architecture of these frameworks: in the case of benzene, toluene, p-xylene, and chlorobenzene, the Zn-bipy framework consists of 1-D ladders, whereas in the case of o-dichlorobenzene (the largest solvent guest), the framework was based on a 2-D square grid. This difference in stoichiometry and architecture was also reflected in significant differences in the fluorescence of these coordination polymers, with three of the four compounds with 1-D ladder geometries having similar fluorescence maxima (ca. 520 nm) and lifetimes (ca. 70 ns), whereas the compound with square grid topology had a significantly blue-shifted maximum (ca. 460 nm) and shorter lifetime (ca. 42 ns). It is proposed that exciplexes form upon excitation of ground-state complexes, involving face-to-face bipy/pyrene complexes (pi-pi stacking interactions) in the case of the 1-D ladder structures, but edge-to-face bipy/pyrene and pyrene/o-dichlorobenzene complexes (C-H...pi interactions) in the case of the 2-D square grid structure.

Pyrene Excimer Fluorescence as a Probe for Parallel G-Quadruplex Formation

The formation and properties of G-quadruplex structures from short single-strand oligonucleotide conjugates possessing two to four guanines and a 5'-terminal pyrenebutanol are reported. The 4-G conjugate forms a stable G-quadruplex under low or high potassium ion concentrations, whereas the 3-G conjugate forms a stable G-quadruplex only in the presence of high potassium. The 2-G conjugate fails to form a stable G-quadruplex even at low temperature and high potassium concentration. Both pyrene monomer and excimer fluorescence are observed for the G-quadruplex structures, whereas only monomer fluorescence is observed for the single-strand conjugates. Thus, pyrene excimer fluorescence can be used as a probe for the formation of G-quadruplex structures. The excimer/monomer intensity ratios for the G-quadruplex structures are dependent upon both the temperature and potassium or lithium salt concentration. The salt effect is attributed to a change in the structure of the hydrophobic pyrene chromophores, which are assembled on the 5'-face of the G-quadruplex as a consequence of electrostriction.