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Designing extrinsic fluorescence sensors for amyloid fibrils is a very active and important area of research. Recently, an ultrafast molecule rotor dye, Auramine O (AuO), has been projected as a fluorescent amyloid marker. It has been claimed that AuO scores better than the most extensively utilized gold-standard amyloid probe, Thioflavin-T (ThT). This advantage arises from the fact that AuO, in addition to its usual emission band (∼500 nm), also displays a large red-shifted emission band (∼560 nm), exclusively in the presence of human insulin fibril medium and not in the native protein or buffer media. On the contrary, for ThT, the emission maximum (∼490 nm) largely remains unchanged while going from protein to fibril. This otherwise unknown large red-shifted emission band of AuO, observed in the presence of human insulin fibrils, was tentatively attributed to a species formed upon fast proton dissociation from excited AuO. It was proposed that because of the long excited-state lifetime (∼1.8 ns) of AuO upon association with human insulin fibrils, this fast proton dissociation from excited AuO could be observed, which is otherwise not observed in buffer or native protein media, owing to its very short excited-state lifetime (∼1 ps). Herein, we show that despite the long excited-state lifetime of AuO in other fibrillar media (human serum albumin and lysozyme), the new red-shifted emission band at 560 nm is not observed, thus possibly suggesting a different origin of the red-shifted emission band of AuO in human insulin fibril medium. We convincingly show that this red-shifted band of AuO (∼560 nm) could be observed under conditions that promote dye aggregation, such as a premicellar concentration of surfactants and polyelectrolytes. These AuO aggregates display strong emission wavelength dependence of transient decay traces, similar to that for AuO in human insulin fibril medium. Detailed time-resolved emission spectral (TRES) measurements suggest that the AuO/premicellar surfactant and AuO/human insulin fibril system share similar features, such as a dynamic red-shift in TRES and an isoemissive point in the time-resolved area-normalized emission spectra, suggesting that the characteristic red-shifted emission band of AuO in human insulin fibril medium may arise from AuO aggregates.

Triblock copolymer and surfactant based supramolecular assemblies have been used for the controlled sequestration of the DNA intercalator. The triblock copolymer micelles do not affect the molecules that are intercalated in the DNA. However, on addition of charged surfactant to the triblock copolymer micellar solution, sequestration of the intercalated molecules from DNA to the polymer-surfactant supramolecular assemblies takes place. Such sequestration of the intercalated molecules in the polymer-surfactant supramolecular assemblies has been explained on the basis of the charged surface formed in the polymer micelles due to the addition of surfactants. Sequestration of the intercalated molecules from the DNA to the polymer-surfactant supramolecular assemblies has been monitored through the ground state absorption, steady state, and time-resolved emission measurements. It is shown that the extent of sequestration of the intercalated molecules can be finely tuned by tuning the concentration of the surfactant in the triblock copolymer solution. Quantitative sequestration of the intercalated molecules by the supramolecular assemblies has been achieved. Such controlled sequestration of the DNA intercalated molecules by polymer-surfactant supramolecular assemblies can be used to study the binding of drug with DNA and may be useful in applications like detoxification in the case of drug overdose.

Controlled and targeted release of an active small molecule at the site of demand is very crucial in pharmaceutical applications. In the present article, we have reported a very simple yet unique chemical system which can be used for the controlled and quantitative transfer of a small molecule from a nanocarrier to natural DNA using an external stimulus. Due to the high sensitivity of emission intensity toward its microenvironments, an ultrafast molecular rotor has been used as a spectroscopic probe. SDS micelle has been used as a nanocarrier and the cyclodextrin molecules are used as an external stimulus. The molecular recognition property of the stimulus toward the hydrophobic chain of the surfactant molecules has been utilized for controlled transfer of the small molecule from the nanocarrier to DNA. Through detailed steady state and time-resolved spectroscopic studies, it has been demonstrated that quantitative transfer of the small molecules from nanocarrier to the natural DNA molecules could be achieved. The present chemical system might be very promising in the field of controlled and targeted drug delivery.

The degenerate transition corresponding to asymmetric stretches of the D(3h) tricyanomethanide anion, C(CN)(3)(-), in aqueous solution was investigated by linear FTIR spectroscopy, femtosecond pump–probe spectroscopy, and 2D IR spectroscopy. Time resolved vibrational spectroscopy shows that water induces vibrational energy transfer between the degenerate asymmetric stretch modes of tricyanomethanide. The frequency–frequency correlation function and the vibrational energy transfer show two significantly different ultrafast time scales. The system is modeled with molecular dynamics simulations and ab initio calculations. A new model for theoretically describing the vibrational dynamics of a degenerate transition is presented. Microscopic models, where water interacts axially and radially with the ion, are suggested for the transition dipole reorientation mechanism.

Ultrafast photoinduced intermolecular electron transfer (ET) dynamics involving 7-aminocoumarin derivatives as electron donor and pyridinium moiety of surfactant molecules in cetylpyridinium chloride (CPC) micelle as electron acceptor has been investigated to understand the role of separation and orientation of reactants on micellar ET reactions. Unlike in noninteracting micelles (like Triton-X-100, sodium dodecyl sulfate, dodecyltrimethylammonium bromide, etc.), where surfactant-separated donor-acceptor pairs are understood to give the ultrafast ET component with the shortest time constant in the range of approximately 4 ps, in CPC micelles with pyridinium moiety as the intrinsic acceptor the ultrafast ET component is found to be in the subpicosecond time scale (of around 240 fs). This time scale is very similar to the values reported in the cases of ultrafast ET reactions involving coumarin dyes in electron-donating solvents. The ultrafast ET times in CPC micelles are significantly faster than the diffusive solvation dynamics in the micellar media. Correlation of the observed ET rates in the present cases with the free-energy changes of the reactions shows the inverse-bell-shaped correlation, predicted by Marcus ET theory. Interestingly, the onset of the Marcus inversion appears at a relatively lower exergonicity, which is attributed to the nonequilibrium solvent configuration during the ultrafast ET reaction, as envisaged from two-dimensional ET (2DET) model. Along with the ultrafast ET component, there are also slower ET components in these systems, which are attributed to those close-contact donor-acceptor populations in the micelles that have relatively weaker electronic coupling due to improper orientation of the interacting donor-acceptor pairs. The present results suggest that, along with the shifting of Marcus inversion at lower exergonicity, the ET rates can also be maximized in a micellar media by using surfactant molecule as an intrinsic reactant.

The femtosecond fluorescence upconversion technique is used to study the effect of viscosity on the excited state relaxation dynamics of an amyloid fibril sensor, thioflavin-T, in different solvent media. The excited state decay in all of the solvents is seen to be dependent on the emission wavelength. From the constructed time-resolved emission spectra, it is seen that the present system shows dynamic Stokes' shift as well as an appreciable increase in the spectral width with time. These temporal spectral characteristics of time-resolved emission spectra have been assigned to the formation of a new emissive species from the locally excited state of the thioflavin-T molecule. The formation of the new emissive state from the locally excited state is also supported by the fact that an iso-emissive point appears in the time-resolved area normalized emission spectra. From the detailed study on the excited state dynamics of thioflavin-T as a function of solvent viscosity, it is concluded that the new emissive state is formed due to the twisting around the central C-C single bond in the excited state of thioflavin-T. The formation rate of the twisted emissive state from the locally excited state is found to be nicely correlated with the viscosity of the medium.

The fundamental process of bond twisting that is responsible for the fluorescence sensing activity of the most extensively used amyloid fibril sensor, Thioflavin T, has been revealed using ultrafast time-resolved fluorescence spectroscopy. From the wavelength-dependent fluorescence decay kinetics and the subsequently constructed time-resolved emission spectra (TRES), the dynamic Stokes shift and the change in the spectral width were observed. These results are rationalized on the basis of the proposition that, following photoexcitation, Thioflavin T undergoes ultrafast bond twisting to form a twisted intramolecular charge-transfer state that is weakly emissive in nature. Formation of the twisted state from the local excited state was found to occur in the subpicosecond time domain (time constant approximately = 570 fs). Quantum chemical calculations support the proposition of the bond twisting process in the photoexcited Thioflavin T and suggest that the twisting around the central C-C single bond, rather than the C-N single bond, of the Thioflavin T molecule is mainly responsible for the observed ultrafast dynamics in the excited state. Detailed time-resolved fluorescence studies of Thioflavin T incorporated in amyloid fibril show substantial retardation in the bond twisting dynamics, suggesting the involvement of this process in the sensor activity of the dye.

Photophysical properties and ultrafast excited state torsional dynamics of the protein binding dye, Thioflavin-T, have been investigated in a nanoconfined water pool of reverse micelles using both steady-state fluorescence and the femtosecond fluorescence upconversion technique. It is seen that due to the confined environment in the reverse micelle, the fluorescence yield of Thioflavin-T is dramatically enhanced approximately 250-fold as compared to that in bulk water. The fluorescence yield decreases nonlinearly with the increase in the water pool size of the reverse micelles. Fluorescence lifetime of Thioflavin-T is also seen to decrease sharply with an increase in the water pool size. The results have been rationalized on the basis of the effect of confinement on the ultrafast torsional motion in the Thioflavin-T. The rate constants for the torsional motion in Thioflavin-T molecule in confined water pool have been estimated.

Effect of cosurfactant concentration on the location of a dissolved solute in a block copolymer-surfactant supramolecular system has been investigated using time-resolved fluorescence anisotropy and dynamic Stokes' shift measurements. Pluronic F88 and cosurfactant CTAC have been used to form a supramolecular assembly. The anion of coumarin 343 dye has been used as the solute/probe. It is seen that as the CTAC concentration is increased in the F88-CTAC supramolecular assembly, the microviscosity around the probe gradually increases. The result suggests that the probe undergoes a gradual migration from micellar surface to the interior of the micelle as the concentration of the CTAC is increased. This is also supported by the dynamic Stokes' shift results. It is seen that as the CTAC concentration is increased in the system, the observed Stokes' shift gradually increases due to the movement of the probe away from the bulk water. By comparing the present results with those reported in another pluronic-surfactant system, namely, P123-CTAC, it is indicated that the extent of modulation in the position of the probe in such supramolecular systems is largely determined by the composition of the pluronic, especially on the length of its hydrophilic ethyleneoxide block.

Effects of constituent block size of triblock copolymers on the nature of the water molecules in the corona region of their micelles have been investigated using time-resolved fluorescence measurements. The physical nature of the water molecules in the micellar corona region of the block copolymer, Pluronic F88 ([ethylene oxide (EO)]103-[propylene oxide (PO)]39-EO103), has been studied using a solubilized coumarin dye. Solvent reorientation time and rotational correlation time have been measured and compared with another block copolymer, Pluronic P123 (EO20-PO70-EO20), which has a different composition of the constituent PO and EO blocks. It is noted that due to the presence of larger number of EO blocks in F88 as compared with P123, the corona region of the former micelle is more hydrated than that of the latter. The solvent reorientation time and rotational correlation time are found to be relatively shorter for F88 as compared with P123. This indicates that the water molecules in the corona of the F88 micelle are more labile than those of P123, which is also supported from the estimated number of water molecules associated with each EO unit, measured from the size of each type of micelle and its aggregation number. To understand the effect of block size on the chemical reactions in these microheterogeneous media, electron transfer reactions have been carried out between different coumarin acceptors and N, N-dimethylaniline donor. The electron transfer results obtained in F88 micelles have been compared with those obtained in P123, and the results are rationalized on the basis of the relative hydration of the two triblock copolymer micelles.

The vibrational population relaxation and hydration shell dynamics of the symmetric tricyanomethanide (TCM) anion are investigated in a sodium bis(2-ethylhexyl)sulfosuccinate reverse micelle as a function of the water pool radius. Two-dimensional infrared (IR) spectroscopy in combination with linear absorption and ultrafast IR pump–probe spectroscopy is utilized in this study. Spectroscopic measurements show that the anion has two bands in the 2160–2175 cm–1 region, each with its own spectroscopic signatures. Analysis of the vibrational dynamics shows that the two vibrational bands are consistent with the anion located either at the interface or in the water pool. The sensitivity of the TCM anion to the environment allows us to unequivocally monitor the vibrational and hydration dynamics of the anion in those two different environments. A TCM anion located at the interface does not show any significant variation of the vibrational dynamics with the water pool size. On the contrary, the TCM anion inside th...