Sharmistha Dutta Choudhury

Cooperative Metal Ion Binding to a Cucurbit[7]uril−Thioflavin T Complex: Demonstration of a Stimulus-Responsive Fluorescent Supramolecular Capsule

We report an intriguing noncovalent interaction of thioflavin T (ThT), a fibril diagnostic dye, with the versatile macrocyclic host molecule cucurbit[7]uril (CB7) in the presence of metal cations. ThT forms both 1:1 (CB7.ThT) and 2:1 [(CB7)(2).ThT] complexes with CB7 host, leading to specific structural arrangements. Addition of competitive guests like metal cations to the 1:1 stoichiometric complex displays expected competitive binding interactions with CB7, leading to decreased fluorescence intensity from ThT. However, addition of metal ions to the 2:1 complex leads to unusual enhancement in the fluorescence emission ( approximately 270-fold in the presence of Ca(2+) and approximately 160-fold in the presence of Na(+)). These contrasting observations on the fluorescence enhancement with change in the stoichiometric equilibrium have been investigated explicitly for a feasible binding model. Detailed photophysical characterization with supporting data from NMR and anisotropy measurements has led to the revelation of a novel stimulus-responsive cooperative metal ion binding to the stoichiometrically selected (CB7)(2).ThT complex, demonstrating a highly fluorescent supramolecular nanocapsule. The first example of a noncovalently packed fluorescent complex became feasible due to the structural arrangement of the host-guest complex in the 2:1 stoichiometry with two CB7 portals providing strong negative charge density for the metal ions to group and seal the complex, thus protecting the incorporated dye. To further strengthen the usefulness of the supramolecular capsule established here, rupture of the capsular complex has been demonstrated with a strong competitive guest, 1-amantadine hydrochloride, which helped in disrupting the capsule to release the dye. It is proposed here that by judicious design of the chromophore (guest) structure, such capsular assemblies can be explored for the binding and release of drug molecules, for fluorescence on-off systems, and as building blocks for molecular architectures displaying unique properties.

Noncovalent interaction of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin with Cucurbit[7]uril: a supramolecular architecture.

Noncovalent interaction of two water-soluble synthetic macromolecules, Cucurbit[7]uril (CB7) and 5,10,15,20-tetrakis(4- N-methylpyridyl)porphyrin (TMPyP), has been studied from the viewpoint of organizing them through supramolecular interactions and thereby modulating their functional activities. Steady-state and time-resolved fluorescence measurements along with NMR results establish that CB7 crowns the N-methylpyridyl group of the TMPyP in a 1:4 stoichiometry. The overall binding constant was evaluated to be approximately 4.5 x 10 (19) M (-4). The high binding affinity, promoting a stable and extendable molecular assembly in aqueous solution, could open new frontiers in the design and synthesis of higher-order supramolecular structures with photofunctional moieties and project their utility in therapeutic applications.

Contrasting guest binding interaction of cucurbit[7-8]urils with neutral red dye: controlled exchange of multiple guests

Interactions among macrocyclic hosts and dyes/drugs have been explored extensively for their direct usage in controlled uptake and release of large number of potential drug molecules. In this paper we report the non-covalent interaction of cucurbit[8]uril macrocycle (CB8) with a biologically important dye, neutral red, by absorption and fluorescence spectroscopy. A comparative analysis with the complexation behaviour of the dye with CB7, the lower homologue of CB8, indicates contrasting guest binding behaviour with significant changes in the photophysical characteristics of the dye. While CB7 interaction leads to a 1 ratio 1 stoichiometry resulting in approximately 6 fold enhancement in the fluorescence emission of the dye, CB8 displays signatures for a 1 ratio 2 host-guest stoichiometry with drastic reduction in the fluorescence emission. Apart from the evaluation of approximately 2 unit shift in the protolytic equilibrium on complexation (pK(a) shift), the measurements with tryptophan established a selective guest exchange to favour a co-localized dimer inside the CB8 cavity. In a protein medium (BSA), the 1 ratio 2 complex was converted to a 1 ratio 1 ratio 1 CB8-NRH(+)-BSA complex. The finding that NRH(+) can be transferred from CB8 to BSA, even though the binding constant for NRH(+)-CB8 is much higher than NRH(+)-BSA, is projected for a controlled slow release of NRH(+) towards BSA. Since the release and activity of drugs can be controlled by regulating the protolytic equilibrium, the macromolecular encapsulation and release of NRH(+) demonstrated here provide information relevant to host-guest based drug delivery systems and its applications.

Photophysical studies on the noncovalent interaction of thioflavin T with cucurbit[n]uril macrocycles.

Noncovalent interaction of Thioflavin T (ThT) with versatile macrocyclic host molecules, namely, cucurbit[7]uril (CB7) and cucurbit[5]uril (CB5), has been investigated in aqueous solutions by photophysical methods. Steady-state and time-resolved fluorescence studies illustrate significant enhancements/modifications in the ThT fluorescence yield, lifetime, and spectral features on interaction with the CBs and are assigned due to the formation of 1:1 and 2:1 complexes between the CBs and the ThT. The high binding constant values for the 1:1 complex (K(1) approximately 10(5) M(-1)) indicate the strong ion-dipole interaction between the host and guest molecules, whereas the 2:1 complex formation is mainly driven by weaker forces like hydrophobic interaction as evident from the lower binding constants (K(2) approximately 10(3) M(-1)). From the characteristic differences in the photophysical properties of the CB7-ThT and CB5-ThT complexes, it has been adjudged that ThT forms an inclusion complex with CB7 whereas with CB5, the interaction is through an exclusion complex formation. These contentions have been further verified by the rotational relaxation dynamics, NMR, and quantum chemical calculations on CB-ThT systems. The present results have also been compared with those reported for the dye in the presence of cyclodextrin hosts.

Control of the supramolecular excimer formation of thioflavin T within a cucurbit[8]uril host: a fluorescence on/off mechanism.

On or off? A new excimer band at lambda = 570 nm was visualized during the noncovalent host-guest interaction between thioflavin T (ThT) and cucurbit[8]uril (CB8). Controlled dissociation of this assembly in the presence of Ca(2+) was demonstrated as an on/off fluorescence switch (see picture).

Recognition-Mediated Light-Up of Thiazole Orange with Cucurbit[8]uril: Exchange and Release by Chemical Stimuli

This article reports a convenient supramolecular strategy to construct fluorescent photoswitchable molecular assemblies between a macrocyclic host, cucurbit[8]uril (CB8), and a fluorogenic dye, thiazole orange (TO). The interaction mechanism and the stable stoichiometric host-guest arrangements have been claimed on the basis of the optical absorption, steady-state and time-resolved fluorescence lifetime and anisotropy measurements, and also the geometry optimization studies. The CB8 recognized TO in its 2:2 stoichiometry exhibited spectacular fluorescence enhancement of the order of 1700 fold, which is the largest directly determined value so far reported for a dye in an organic macrocyclic system. This prospective 2CB8:2TO assembly responded to selected chemical stimuli such as metal ions, adamantylamine, and tryptophan, providing different dissociation mechanisms and demonstrating a controlled exchange and release action desired with such noncovalently linked assemblies. Positively, considering the aqueous solubility and biocompatibility of the host-guest constituents, this methodology can evolve into a general approach to deliver and operate intracellularly functional molecular components under chemical/thermal/optical trigger control, especially for therapeutic applications.

Modulation of Excited-State Proton-Transfer Reactions of 7-Hydroxy-4-methylcoumarin in Ionic and Nonionic Reverse Micelles

The prototropic behavior of the dye, 7-hydroxy-4-methylcoumarin (7H4MC), has been studied in cationic benzyldimethylhexadecylammonium chloride (BDHC) and nonionic poly(oxyethylene)(tetramethylbutyl)phenyl ether (TritonX-100, TX-100) reverse micelles using ground-state absorption and steady-state and time-resolved fluorescence measurements. The results have been compared with the previous results in the anionic sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Although the probe dye, 7H4MC, is indicated to reside in the interfacial region in all of the reverse micelles studied, significant differences have been observed in the evolution of the different prototropic species. In BDHC reverse micelles, the anionic form is favored over the tautomeric form at the higher w0 values, which is contrary to the observation in AOT reverse micelles where both of these forms are almost equally produced. The higher propensity for the formation of the anionic form in BDHC reverse micelles has been explained on the basis of the additional electrostatic stabilization of the anionic species in the cationic BDHC reverse micelles compared to that in the anionic AOT reverse micelles. On the other hand, in TX-100 reverse micelles, the anionic form is not very evident, but interestingly, the tautomer form begins to appear beyond w0=2. The appearance of the tautomeric species apparently coincides with the formation of the water pool in the TX-100 reverse micelles. However, due to the more restricted nature of the water molecules within this reverse micelle (mostly dispersed around the oxyethylene chains), deprotonation of the 7H4MC dye and the consequential stabilization of the anionic form are considerably reduced. The results clearly reveal that the aqueous environment in the vicinity of the probe is quite different for the reverse micelles considered, and these differences largely modulate the prototropic processes of the excited dye.

Surfactant-induced aggregation patterns of thiazole orange: a photophysical study.

The aggregation behavior of the DNA marker dye thiazole orange (TO), has been investigated in two types of surfactant assemblies, namely, premicelles/micelles of sodium dodecyl sulfate (SDS) and pre reverse micelles/reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT). In the case of an SDS/water system, absorption spectral changes of TO signify the formation of H-aggregates and H-dimers of the dye at premicellar concentrations, which subsequently convert to the monomeric form beyond the critical micellar concentration (cmc). Interestingly, the observed changes in the absorption and emission characteristics due to the surfactant-induced formation of H-aggregates/dimers of TO are found to be useful to estimate the surfactant concentration parameters for premicellar aggregation of SDS. In the case of an AOT/n-heptane system, similarly, H-aggregates/dimers are observed at low AOT concentrations, below the cmc. However, in this case, the H-dimers persist even beyond the cmc. This is attributed to the strong tendency of TO for self-aggregation and its favorable electrostatic interactions with the AOT head groups. With increasing water content in the AOT reverse micelles, the hydration of the dye leads to the conversion of H-dimers to the monomeric form. The steady-state fluorescence results are nicely corroborated with those from time-resolved fluorescence studies and demonstrate the interesting behavior of the surfactant-induced aggregation of TO dye.

Directing fluorescence with plasmonic and photonic structures.

Fluorescence technology pervades all areas of chemical and biological sciences. In recent years, it is being realized that traditional fluorescence can be enriched in many ways by harnessing the power of plasmonic or photonic structures that have remarkable abilities to mold the flow of optical energy. Conventional fluorescence is omnidirectional in nature, which makes it difficult to capture the entire emission. Suitably designed emission directivity can improve collection efficiency and is desirable for many fluorescence-based applications like sensing, imaging, single molecule spectroscopy, and optical communication. By incorporating fluorophores in plasmonic or photonic substrates, it is possible to tailor the optical environment surrounding the fluorophores and to modify the spatial distribution of emission. This promising approach works on the principle of near-field interaction of fluorescence with spectrally overlapping optical modes present in the substrates. In this Account, we present our studies on directional emission with different kinds of planar metallic, dielectric, and hybrid structures. In metal-dielectric substrates, the coupling of fluorescence with surface plasmons leads to directional surface-plasmon-coupled emission with characteristic dispersion and polarization properties. In one-dimensional photonic crystals (1DPC), fluorophores can interact with Bloch surface waves, giving rise to sharply directional Bloch surface wave-coupled emission. The interaction of fluorescence with Fabry-Pérot-like modes in metal-dielectric-metal substrates and with Tamm states in plasmonic-photonic hybrid substrates provides beaming emission normal to the substrate surface. These interesting features are explained in the context of reflectivity dispersion diagrams, which provide a complete picture of the mode profiles and the corresponding coupled emission patterns. Other than planar substrates, specially fabricated plasmonic nanoantennas also have tremendous potential in controlling and steering fluorescence beams. Some representative studies by other research groups with various nanoantenna structures are described. While there are complexities to near-field interactions of fluorescence with plasmonic and photonic structures, there are also many exciting possibilities. The routing of each emission wavelength along a specific direction with a given angular width and polarization will allow spatial and spectral multiplexing. Directional emission close to surface normal will be particularly useful for microscopy and array-based studies. Application-specific angular emission patterns can be obtained by varying the design parameters of the plasmonic/photonic substrates in a flexible manner. We anticipate that the ability to control the flow of emitted light in the nanoscale will lead to the development of a new generation of fluorescence-based assays, instrumentation, portable diagnostics, and emissive devices.

Photoinduced bimolecular electron transfer kinetics in small unilamellar vesicles

Photoinduced electron transfer (ET) from N,N-dimethylaniline to some coumarin derivatives has been studied in small unilamellar vesicles (SUVs) of the phospholipid, DL-alpha-dimyristoyl-phosphatidylcholine, using steady-state and time-resolved fluorescence quenching, both below and above the phase transition temperature of the vesicles. The primary interest was to examine whether Marcus inversion [H. Sumi and R. A. Marcus, J. Chem. Phys. 84, 4894 (1986)] could be observed for the present ET systems in these organized assemblies. The influence of the topology of SUVs on the photophysical properties of the reactants and consequently on their ET kinetics has also been investigated. Absorption and fluorescence spectral data of the coumarins in SUVs and the variation of their fluorescence decays with temperature indicate that the dyes are localized in the bilayer of the SUVs. Time-resolved area normalized emission spectra analysis, however, reveals that the dyes are distributed in two different microenvironments in the SUVs, which we attribute to the two leaflets of the bilayer, one toward bulk water and the other toward the inner water pool. The microenvironments in the two leaflets are, however, not indicated to be that significantly different. Time-resolved anisotropy decays were biexponential for all the dyes in SUVs, and this has been interpreted in terms of the compound motion model according to which the dye molecules can experience a fast wobbling-in-cone type of motion as well as a slow overall rotating motion of the cone containing the molecule. The expected bimolecular diffusion-controlled rates in SUVs, as estimated by comparing the microviscosities in SUVs (determined from rotational correlation times) and that in acetonitrile solution, are much slower than the observed fluorescence quenching rates, suggesting that reactant diffusion (translational) does not play any role in the quenching kinetics in the present systems. Accordingly, clear inversions are observed in the correlation of the fluorescence quenching rate constants k(q) with the free energy change, DeltaG(0) of the reactions. However, the coumarin dyes, C152 and C481 (cf. Scheme 1), show unusually high k(q) values and high activation barriers, which is not expected from Marcus ET theory. This unusual behavior is explained on the basis of participation of the twisted intramolecular charge transfer states of these two dyes in the ET kinetics.