Fluorescence Quenching of UVITEX‐OB by Aniline in Alcohols and Alkanes

Fluorescence quenching of UVITEX‐OB [2,5‐thiophenediylbis(5‐tert‐butyl‐1,3‐benzoxazole)] by aniline in different polar and nonpolar solvents was examined at room temperature by steady‐state fluorescence measurements. Positive deviations from the nonlinear Stern–Volmer plots were observed in most of the solvents indicating the extent of quenching to be large. The quencher concentration dependence data were analyzed using ground‐state complex and sphere of action static quenching models in order to interpret the results. The magnitudes of the quenching rate parameters suggest that a sphere of action static quenching model is expected to describe the data most accurately. Also, the results are suggestive of both static and dynamic quenching processes being responsible for the observed positive deviation in the Stern–Volmer plot. Experimental results are described by an equation derived using the finite sink approximation model, which allows the evaluation of diffusion‐limited interaction and the estimation of encounter distance and mutual diffusion coefficient independently.     

Photodegradation Studies by Laser‐Induced Fluorescence of the Reaction Product of 1,2‐Indanedione And Glycine

Photodegradation of the reaction product of 1,2‐indanedione with glycine in methanol at room temperature has been studied using laser-induced fluorescence. Samples were liquid solution and developed fingerprints on papers. Continuous laser excitation of fresh solution of 1,2‐indanedione–glycine dissolved in methanol shows that the emission peak at 564 nm reached a quite stable low level after almost 1 hr of continuous excitation. For latent fingerprints on papers developed with 1,2‐indanedione dissolved in methanol solution, the luminescence peak decreases for the first week and reached a stable level for almost 2 weeks. A long‐term study is needed to reach a conclusion on the stability of the samples (liquid and developed fingerprints) at room temperature conditions.     

Fluorescence Quenching of Eu3+ in Organic Solvents. Quenching Effect of the →C-H and -O-H Groups

The extremely weak fluorescence of the rare-earth ions in aqueous solutions is enhanced considerably by replacing H₂O with D₂O (1,2).     

Fluorescence Quenching of 2,2″-dimethyl-p-terphenyl by Carbon Tetrachloride in Binary Mixtures

The fluorescence quenching of 2,2″-dimethyl-p-terphenyl (DMT) by carbon tetrachloride (CCl₄) was investigated in different solvent mixtures of benzene and acetonitrile at room temperature (300 K). A positive deviation from linearity was observed in the Stern-Volmer plots for all the solvent mixtures. This could be explained satisfactorily by static and dynamic quenching models. The nonlinearities in the S-V plots are interpreted in terms of ground state complex model and the sphere of action static quenching model. The results suggest that positive deviations in the S-V plot are due to the presence of both static and dynamic quenching processes. To explain that bimolecular reactions are diffusion limited, we have used finite sink approximation model. Various rate parameters for the quenching process have been determined by static and dynamic quenching models. The dynamic quenching constant depends on the solvent polarity and indicates that quenching reaction is diffusion limited.     

Mechanisms of Fluorescence Quenching in Donor—Acceptor Labeled Antibody—Antigen Conjugates

The cyanine dyes Cy5 and Cy5.5 are presented as a new long wavelength-excitable donor-acceptor dye pair for homogeneous fluoroimmunoassays. The deactivation pathways responsible for the quenching of the fluorescence of the antibody-bound donor are elucidated. Upon binding of the donor dye to the antibodies at low dye/protein ratios, its fluorescence quantum yield rises to unity. Higher dye/protein ratios lead to progressive aggregation of the dyes, which results in quenching of monomer fluorescence due to resonance energy transfer (RET) from the monomers to the nonfluorescent dimers. The dependence of the quenching efficiency on the labeling ratio is described quantitatively by assuming a Poisson distribution of the dyes over the antibodies. The maximum fluorescence intensity per antibody is obtained at a labeling ratio of 4. Upon formation of the antibody-antigen complex, electron transfer and RET to the antigen-bound acceptor dye occur. Steady-state and time-resolved fluorescence measurements reveal that approximately 50% of the donor quenching is due to RET, while the residual quenching effect is caused by the static quenching process.     

RRS Spectra and Mechanisms for β-Nitro-Tetraphenylporphyrin Fluorescence Quenching

Optics and Spectroscopy - The study of the excited states and photophysical characteristics of β-nitro-tetraphenylporphyrin (TPP–NO2) has been carried out using resonance Raman...     

Solid‐State NMR Studies of Pharmaceutical Systems

Abstract

High‐and low‐resolution solid‐state nuclear magnetic resonance (SSNMR) applications to the study of pharmaceuticals are reviewed. Examples are shown involving the use of mono‐and bidimensional SSNMR techniques based on different nuclear interactions and the measurement of several nuclear parameters, such as chemical shifts, line widths, and relaxation times (T₁, T₂, T_1ρ). The systems investigated include pure active pharmaceutical ingredients (APIs), substances used as drug excipients, and solid dispersions formed by APIs and excipients, up to final drug formulations. The most important aspects treated concern structural, dynamic, and morphological properties, and, in particular, identification, characterization, and quantitation of polymorphs and related forms, conformational and crystalline packing behavior, amorphous phase properties and stability, effects of drug processing, molecular motions, API‐excipient and excipient‐excipient chemical and physical interactions, and phase mixing in heterophasic systems.     

Quenching of the Intrinsic Fluorescence of Human Serum Albumin by Trimethoxyphenylfluorone–Mo(VI) Complex

The interaction mechanism between trimethoxyphenylfluorone (TM‐PF)–molybdenum [Mo(VI)] complex and human serum albumin (HSA) has been investigated using fluorescence method. The binding constants were measured at different temperature. Based on the theory of Förster energy transfer, the binding distance and the energy transfer efficiency between TM‐PF–Mo(VI) complex and HSA were obtained. According to the thermodynamic parameters, the main sorts of binding force can be judged. The results indicate that HSA and TM‐PF–Mo(VI) complex have strong interactions. The mechanism of quenching belongs to static quenching, and the main sorts of binding force are van der Waals force and H‐bonding.     

Steady‐State and Time‐Resolved Emission Studies of 6‐Methoxy Quinoline

Abstract

Steady‐state absorption, fluorescence excitation, and emission spectra of 6‐methoxy quinoline (6‐MQ) were measured at room temperature in cyclohexane, dioxane, ethanol, acetonitrile, water, and water–dioxane solvents. Absorption spectra of cyclohexane, n‐hexane, and isopentane solutions show resolved vibronic structure at room temperature. However, the excitation spectrum of cyclohexane solution is structureless and is found to be emission wavelength dependent, indicating the formation of at least two distinct species in the ground state. Similar behavior was observed in dioxane and water–dioxane solutions. For all other solutions, the fluorescence excitation spectrum of 6‐MQ was found to be the same for different emissions. Emission of 6‐MQ in all solvents consisted of two bands with their maxima around 355 nm (I) and 430 nm (II), the actual positions and the relative intensities being dependent on the solvent used. The bands I and II were respectively attributed to normal and protonated/H‐bonded species of either ¹L_a or ¹L_b states or mixed (¹L_a/¹L_b) state of ππ* character. Fluorescence decay of this dye in all solvents monitored over each emission maximum showed biexponential behavior, and the analysis yielded two different lifetime components for each emission band. The short and long fluorescence decay components were respectively in the range of 0.30–3.00 ns and 18–20 ns. The observed emission characteristics coupled with the nature of the fluorescence polarization spectra and two different decay components for each emission suggest the existence of two different conformers having two different excited electronic states.     

A Negative Deviation from Stern–Volmer Equation in Fluorescence Quenching

A negative deviation from the normal Stern-Volmer equation shown in the fluorescence quenching of doxorubicin by adenosine 5' monophosphate is interpreted in terms of doxorubicin exists in two different conformers in the ground state. An estimate of the Stem-Volmer constant for the excited-state quenching is about 218 M(-1). The fluorescence decay of free doxorubicin is a bi-exponential in polar protic and polar aprotic solvents. In the presence of adenosine 5' monophosphate, doxorubicin shows a tri-exponential decay in water.     

Determination of the Elemental Composition of Chalcogenide Powders by the X‐Ray Fluorescence Method

A procedure of xray spectral fluorescence analysis of the elemental composition of As _x S_100–x chalcogenide powder samples is developed and its metrological characteristics are established. In determining the content of the components in an As₅₀S₅₀ sample, the relative standard deviation was 0.0030 for As and 0.0035 for S. The results of the xray spectral fluorescence analysis are in good agreement with the data of a gravimetric method.     

Fluorescence Studies of Host–Guest Interaction of a Dansyl Amide Labelled Calix[6]arene

The fluorescence behavior of a calix[6]arene with a dansyl group as fluorescence marker (C6-DA) was investigated with respect to the inclusion properties of alkaloids as atropine and cocaine. A strong hypsochromic shift of the fluorescence band and a strong increase in fluorescence intensity is connected with the interaction of atropine to the lower rim of the C6-DA. The fluorescence increase is saturated at an atropine concentration above the 1:1 complex formation. Addition of cocaine to the complexed atropine-C6-DA leads to a decrease of the fluorescence intensity which could be explained by an exchange mechanism of the complexed molecules. The complexation of the atropine (the “belladonna effect”) is interpreted by electrostatic interaction (proton transfer from the carboxylic group to the nitrogen) with important contribution from hydrogen bonding by the guest OH-group.     

Analysis of the Structure of Thin Films of Zn‐Porphyrins by the Kinetics of the Fluorescence Anisotropy

The structure of thin films of Zntetraoctylphenylporphyrin (ZnTOPP) obtained by the spinning method is investigated. The kinetics of the decay of the fluorescence anisotropy of the films is analyzed in the form of a sum of exponents and by simulating the orientation of ZnTOPP complexes on the substrate (quartz) surface with allowance for the processes of the electron excitation energy transfer. The ZnTOPP films have a lamellar structure where individual layers form ordered domains. Within a domain, linear nonintersecting stacks of molecules are formed. In each stack the planes of the molecules are collinear, oriented perpendicularly to the surface of the substrate base, and form an angle of 45° with the directing axis of a stack.     

Numerical Simulations of Thione S2→ S0 Fluorescence Decay: An Aid for Modeling Fast Quenching Kinetics

Numerical simulations designed to illuminate the opportunities for and limitations on recovering the parameters in the Smoluchowski–Collins–Kimball (SCK) model of time-dependent fluorescence quenching from the data of time-correlated single-photon counting experiments have been carried out. Two donors with lifetimes of 880 and 410 ps, corresponding to the unquenched S₂ lifetimes of 2,2,3,3-tetramethylindanethione (TMIT) and 9,9-dimethylanthrathione (DMAT) in inert perfluoroalkane solvents at room temperature, were used for purposes of illustration. The failure of the single-exponential decay model due to the transient effect can be detected at lower quencher concentrations for longer-lived fluorophores. While good values of the diffusion coefficient, effective encounter distance, and scaling parameters can be recovered at sufficiently high quencher concentrations, the simulations show that values of the specific rate constant, , can be expected to vary by up to two orders of magnitude when it is recovered from typical time-correlated single-photon counting data even if the data can be reliably described by the SCK model. The ability to differentiate between SCK kinetics and single-exponential kinetics by using fluorophores of varying unquenched lifetimes has also been explored.     

Fine‐Structure Fluorescence Spectrum of a Dication Form of Porphin in an Inorganic Gel Matrix

It is shown that the molecules of the free base of porphin (H₂P) incorporated into an inorganic tetraethoxysilane (TEOS) gel matrix produce a dication form H₄P²⁺. A finestructure spectrum of the fluorescence of H₄P²⁺ is obtained at 4.2 K on selective photoexcitation. A conclusion on the manifestation of the outofplane vibrations of the porphyrin macrocycle in the lowfrequency region of the vibronic spectrum of porphin dication is drawn and the assignments of the corresponding frequencies to the vibrations of a certain form are suggested.     

Fluorescence Quenching and Time-resolved Fluorescence studies of α-Mannosidase from <Emphasis Type="Italic">Aspergillus fischeri</Emphasis> (NCIM 508)

Apart from the vital role in glycoprotein biosynthesis and degradation, α-mannosidase is currently an important therapeutic target for the development of anticancer agents. Fluorescence quenching and time-resolved fluorescence of α-mannosidase, a multitryptophan protein from Aspergillus fischeri were carried out to investigate the tryptophan environment. The tryptophans were found to be differentially exposed to the solvent and were not fully accessible to the neutral quencher indicating heterogeneity in the environment. Quenching of the fluorescence by acrylamide was collisional. Surface tryptophans were found to have predominantly positively charged amino acids around them and differentially accessible to the ionic quenchers. Denaturation led to more exposure of tryptophans to the solvent and consequently in the significant increase in quenching with all the quenchers. The native enzyme showed two different lifetimes, τ ₁ (1.51 ns) and τ ₂ (5.99 ns). The average lifetime of the native protein (τ) (3.187 ns) was not affected much after denaturation (τ) (3.219 ns), while average lifetime of the quenched protein samples was drastically reduced (1.995 ns for acrylamide and 1.537 ns for iodide). This is an attempt towards the conformational studies of α-mannosidase.     

Displacement Reaction Using Ibuprofen in a Mixture of Bioactive Imidazole Derivative and Bovine Serum Albumin—a Fluorescence Quenching Study

The mutual interaction of imidazole derivative (PIPP) with bovine serum albumin (BSA) was investigated using photoluminescent studies. The fluorescence quenching mechanism of BSA by PIPP was analyzed and the binding constant was calculated. The binding distance between PIPP and BSA was obtained based on the theory of Forester’s non-radiation energy transfer. Displacement experiments were performed by using ibuprofen to identify PIPP binding site in BSA. The effect of some common ions on the binding constant between PIPP and BSA was also examined.     

Enhancement of the Fluorescence Quenching Efficiency of DPPH<Superscript>•</Superscript> on Colloidal Nanocrystalline Quantum Dots in Aqueous Micelles

Luminescent CdS quantum dots capped with thioglycolic acid (CdS-TGA QDs) were demonstrated to serve as a fluorescence probe for a model organic radical, 2,2-diphenyl-1-picrylhydrazyl radical (DPPH^•), employing the quenching of the CdS-TGA QDs emission signal by the radical. Under the optimum conditions, the quenching efficiency of DPPH^• on CdS-TGA QDs was proportional to the concentration of DPPH^•, following Stern-Volmer relationship. Different types of surfactants (cationic, anionic and neutral surfactants) were introduced to CdS-TGA QDs in order to increase the detection sensitivity. The fluorescence intensity of CdS-TGA QDs was greatly enhanced by cationic and neutral surfactants. Moreover, the quenching efficiency of DPPH^• on the QDs in the presence of micelles was remarkably ca. 13 times higher than that in the system without micelles. Effects of pH and concentration of surfactants on the fluorescence quenching of CdS-TGA QDs were investigated. Electron spin resonance (ESR) spectroscopy was also used to monitor the DPPH radical species in CdS-TGA QDs mixtures with and without micelles. Fluorescence quenching mechanisms of CdS-TGA QDs by DPPH^• in the presence and in the absence of CTAB were proposed.     

Fluorescence Spectrum Characteristics of Gold Nanorods

Aqueous solutions containing a high yield of suspended gold nanorods with mean aspect ratio 2.5 have been synthesized via an electrochemical method. The fluorescence emission peaks fixed at 37Onm and 670nm are due to the local field enhancement via coupling to the transverse and longitudinal surface plasmon resonance.The quasi-static calculation results indicate that with the increasing aspect ratio of the nanorods, the longer wavelength emission peak decreases and red shifts, whereas the shorter wavelength emission peak blue shifts slightly.