Investigating Tryptophan Quenching of Fluorescein Fluorescence under Protolytic Equilibrium

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Magnetic-Field Dependence of LC-Photo-CIDNP in the Presence of Target Molecules Carrying a Quasi-Isolated Spin Pair

NMR spectroscopy is well known for its superb resolution, especially at high applied magnetic field. However, the sensitivity of this technique is very low. Liquid-state low-concentration photo-chemically-induced dynamic nuclear polarization (LC-photo-CIDNP) is a promising emerging methodology capable of enhancing NMR sensitivity in solution. LC-photo-CIDNP works well on solvent-exposed Trp and Tyr residues, either in isolation or within proteins. This study explores the magnetic-field dependence of the LC-photo-CIDNP experienced by two tryptophan isotopologs in solution upon in situ LED-mediated optical irradiation. Out of the two uniformly 13C,15N-labeled Trp (Trp-U-13C,15N) and Trp-α-13C-β,β,2,4,5,6,7-d7 species employed here, only the latter bears a quasi-isolated 1Hα-13Cα spin pair. Computer simulations of the predicted polarization due to geminate recombination of both species display a roughly bell-shaped field dependence. However, while Trp-U-13C,15N is predicted to show a maximum at ca. 500 MHz (11.7 T) and a fairly weak field dependence, Trp-α-13C-β,β,2,4,5,6,7-d7 is expected to display a much sharper field dependence accompanied by a dramatic polarization increase at lower field (ca. 200 MHz, 4.7 T). Experimental LC-photo-CIDNP studies on both Trp isotopologs at 1μM concentration, performed at selected fields, are consistent with the theoretical predictions. In summary, this study highlights the prominent field-dependence of LC-photo-CIDNP enhancements (ε ) experienced by Trp isotopologs bearing a quasi-isolated spin pair.

Evaluating the interaction of human serum albumin (HSA) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes in different aqueous environments using anisotropy resolved multi-dimensional emission spectroscopy (ARMES)

Studying the interaction between plasma proteins and liposomes is critical, particularly for their use as drug delivery systems. Here, the efficacy of anisotropy resolved multidimensional emission spectroscopy (ARMES) for investigating the interaction of human serum albumin (HSA) with liposomes was explored and compared to conventional spectroscopic techniques. Dynamic Light Scattering (DLS) and absorbance spectroscopy (with Multivariate Curve Resolution (MCR) modeling) indicated that the highest degree of liposome rupturing, and aggregation occurred in water, with less in ammonium bicarbonate buffer (ABC) and phosphate buffered saline (PBS). Fluorescence emission spectra of HSA-liposome mixtures revealed significant hypsochromic shifts for water and ABC, but much less in PBS, where the data suggests a non-penetrating protein layer was formed. Average fluorescence lifetimes decreased upon liposome interaction in water (6.2→5.2 ns) and ABC buffer (6.3→5.6 ns) but increased slightly for PBS (5.6→5.8 ns). ARMES using polarized Total Synchronous Fluorescence Scan measurements with parallel factor (PARAFAC) analysis resolved intrinsic HSA fluorescence into two components for interactions in water and ABC buffer, but only one component for PBS. These components, in water and ABC buffer, corresponded to two different HSA populations, one blue-shifted and penetrating the liposomes (λ_ex/em = ~ 280/320 nm) and a second, similar to free HSA in solution (λ_ex/em = ~ 282/356 nm). PARAFAC scores for water and ABC buffer suggested that a large proportion of HSA interacted in an end on configuration. ARMES provides a new way for investigating protein-liposome interactions that exploits the full intrinsic emission space of the protein and thus avoids the use of extrinsic labels. The use of multivariate data analysis provided a comprehensive and structured framework to extract a variety of useful information (resolving different fluorescent species, quantifying their signal contribution, and extracting light scatter signals) all of which can be used to discriminate between interaction mechanisms.

Monitoring protein conformational changes using fluorescent nanoantennas

Understanding the relationship between protein structural dynamics and function is crucial for both basic research and biotechnology. However, methods for studying the fast dynamics of structural changes are limited. Here, we introduce fluorescent nanoantennas as a spectroscopic technique to sense and report protein conformational changes through noncovalent dye-protein interactions. Using experiments and molecular simulations, we detect and characterize five distinct conformational states of intestinal alkaline phosphatase, including the transient enzyme-substrate complex. We also explored the universality of the nanoantenna strategy with another model protein, Protein G and its interaction with antibodies, and demonstrated a rapid screening strategy to identify efficient nanoantennas. These versatile nanoantennas can be used with diverse dyes to monitor small and large conformational changes, suggesting that they could be used to characterize diverse protein movements or in high-throughput screening applications.

The impact of calcium phosphate on FITC-BSA loading of sonochemically prepared PLGA nanoparticles for inner ear drug delivery elucidated by two different fluorimetric quantification methods

Although therapeutically active proteins are highly efficacious, their content in protective nanoparticles is often too low to elicit adequate plasma levels. A strategy to increase protein loading is the in-situ generation of calcium phosphate as a protein adsorbent. To verify this approach, a highly sensitive and reliable fluorimetric method for quantification of incorporated fluorescein-labelled bovine serum albumin (FITC-BSA) as a model protein drug was developed. Dequenching the fluorescein label by pronase E, which digests the protein backbone, and dissolving the nanoparticle matrix in acetonitrile enabled FITC-BSA quantification in the nanogram per milliliter range. This test was confirmed by a second assay involving alkaline hydrolysis of FITC-BSA and the matrix. Nanoparticles prepared with calcium phosphate contained 40 µg FITC-BSA/mg and nanoparticles without calcium phosphate only 15 µg FITC-BSA/mg, representing a 2.7-fold increase in model protein loading. In this work the nanoparticle preparation procedure was optimized in terms of size for administration in the inner ear, but the range of applications is not limited.

Antibody mix-and-read assays based on fluorescence intensity probes

Antibodies and Fc fusion proteins are a rapidly growing class of pharmaceuticals. Cell culture and purification process development and operation require frequent measurement of product concentrations, commonly by complex enzyme-linked immunosorbent assay and high-performance liquid chromatography methods. Here we report a fast (<30 s), and simple antibody Fc assay based on mix-and-read reporting by fluorescence emission. A soluble fluorescein-labeled Fc-affinity reporter produced by standard peptide synthesis is mixed with an Fc-containing sample to produce an immediate shift in both fluorescence polarization and intensity, compatible with on- and at-line measurements and microbioreactor monitoring. We observed significant shifts in fluorescence intensity in Chinese hamster ovary cell culture fluid spiked with IgG and detected an adalimumab biosimilar down to 100 ng/mL (10^-4 g/L), despite the interferents in the complex sample matrix. Neither the fluorescence polarization nor the fluorescence intensity assay is significantly affected by the addition of clarified lysate of 2 million CHO-k1 cells/mL, suggesting applicability even to cultures of low viability. Biochemical and molecular docking approaches suggest that the fluorescence intensity enhancement is caused by changes in the fluorophore's local microenvironment upon binding to IgG Fc, especially by interactions with Fc His433.Abbreviations: CCF: Cell Culture Fluid; CHO: Chinese Hamster Ovary cells; ELISA: Enzyme Linked Immunosorbent Assay; Fc: Fragment Crystallizable of antibody; HPLC: High-Performance Liquid Chromatography; HPβCD: hydroxypropyl-β-cyclodextrin; IgG: ImmunoglobulinG; mAb: Monoclonal Antibody; PBS: Phosphate-Buffered Saline; PDB: Protein Data Bank; SpA: Staphylococcal protein A; SpG: Staphylococcal protein G.

Multiplexed Graphene Quantum Dots with Excitation-Wavelength-Independent Photoluminescence, as Two-Photon Probes, and in Ultraviolet-Near Infrared Bioimaging

In this study, sorted nitrogen-doped graphene quantum dots were prepared and subsequently conjugated with polymers. The synthesized materials exhibited excitation-wavelength-independent photoluminescence emissions ranging from ultraviolet to near-infrared and were 0.9-8.4 nm in size. The materials also exhibited high-photoluminescence quantum yields and excellent two-photon properties. Therefore, in two-photon bioimaging the materials with different emission spectra can be effective two-photon contrast agents. Specific antibodies were used to label organelles in cancer cells and identify nuclear antigens, thereby enabling the simultaneous detection of four targets in cells at a single two-photon excitation wavelength. The sorted nitrogen-doped graphene quantum dot materials were determined to be considerably more advantageous than organic dyes in identifying multiplexed targets, and they can be effective probes in cellular imaging.

Synthesis of new fluorescent pyrylium dyes and study of their interaction with N-protected amino acids

Six new fluorescent styrylpyrylium dyes have been synthesized and the collisional quenching taking place upon their interaction with Z-protected amino acids has been studied.

Super Stable Fluorescein Isothiocyanate Isomer I Monolayer for Total Internal Reflection Fluorescence Microscopy

Total internal reflection fluorescence microscopy (TIRFM) is an important method in surface science and for the analysis of surface-bound macromolecules. Here, we developed and explored the use of a novel fluorescein isothiocyanate isomer I (FITC)-adsorbed monolayer for alignment and validation of TIRFM measurements and configurations. Aqueous solutions of FITC exist as several different protolytic forms (dianionic, anionic, neutral, and cationic) with each form having different emission characteristics. However, the emission behavior of FITC adsorbed on hydrophilic, hydrophobic, and unmodified glass surfaces at different pH was unknown. TIRFM imaging and spectroscopy were used to study FITC and FITC-labeled bovine serum albumin (BSA-FITC) monolayers generated on three different glass surfaces. Monolayer emission intensity, spectra, and the photobleaching profiles were all dependent on pH and the surface properties of the glass. Very strangely, however, at pH 5.0 on hydrophobic surfaces, the FITC monolayers produced were both bright and apparently unbleachable over ∼20 min of imaging (60 s total exposure). During monolayer formation at pH 5.0, we saw clear evidence for concentration-based quenching, indicating high surface coverage. When the monolayer had been rinsed with buffer to remove unbound FITC, we observed an increase in emission intensity during illumination indicative of some form of photoactivated species being present. Eventually, the fluorescence emission stabilized and remained constant for extended periods of time with no evidence of photobleaching. We hypothesize that during the adsorption process (a hydrophobic-hydrophobic interaction) there was conversion to the fluorescent quinoid form of FITC. In contrast, at pH 7.4 and 9.6 on hydrophobic surfaces, FITC monolayers had well-defined, fast photobleaching kinetics (decay to ∼50% intensity in 5-10 s). The equivalent BSA-FITC monolayers were slightly brighter, with similar photobleaching kinetics. While the precise mechanism for this unusual behavior is still unknown, all these low-cost monolayers were easily prepared, were reproducible, and can serve as convenient test samples for TIRFM alignment, calibration, and validation prior to undertaking measurements with more sensitive biogenic or biological specimens.

Fluorescence Quenching Investigation of Methyl Red Adsorption on Aluminum-Based Metal-Organic Frameworks

The adsorption of methyl red (MR) isomers (ortho, meta, and para) on metal-organic frameworks (MOFs) was investigated by using a fluorescence quenching technique. All three MR isomers were found to quench the fluorescence of MOFs effectively. Nonlinear fluorescence quenching trends were observed in Stern-Volmer plots. A modified nonlinear Stern-Volmer equation with the concepts of multiple adsorption sites, adsorption strength, and quencher accessibility was successfully adopted to fit the fluorescence quenching data. The fitted parameters were correlated with the structural properties of MRs and MOFs. The order of quenching efficiency was found to be m-MR > p-MR > o-MR for all MOFs. This indicates that MR molecules not only adsorb via carboxylate-metal bonding but also adsorb through π-π interactions between the aromatic rings of MR and linker molecules in MOFs. The position of the carboxylate group in MRs and the structure of the linkers in MOFs are the key factors affecting the fluorescence quenching efficiency.

Effect of heavy atoms on photochemically induced dynamic nuclear polarization in liquids

Given its short hyperpolarization time (∼10^-6 s) and mostly non-perturbative nature, photo-chemically induced dynamic nuclear polarization (photo-CIDNP) is a powerful tool for sensitivity enhancement in nuclear magnetic resonance. In this study, we explore the extent of ¹H-detected ¹³C nuclear hyperpolarization that can be gained via photo-CIDNP in the presence of small-molecule additives containing a heavy atom. The underlying rationale for this methodology is the well-known external-heavy-atom (EHA) effect, which leads to significant enhancements in the intersystem-crossing rate of selected photosensitizer dyes from photoexcited singlet to triplet. We exploited the EHA effect upon addition of moderate amounts of halogen-atom-containing cosolutes. The resulting increase in the transient triplet-state population of the photo-CIDNP sensitizer fluorescein resulted in a significant increase in the nuclear hyperpolarization achievable via photo-CIDNP in liquids. We also explored the internal-heavy-atom (IHA) effect, which is mediated by halogen atoms covalently incorporated into the photosensitizer dye. Widely different outcomes were achieved in the case of EHA and IHA, with EHA being largely preferable in terms of net hyperpolarization.

Efficient two-photon luminescence for cellular imaging using biocompatible nitrogen-doped graphene quantum dots conjugated with polymers

Nitrogen-doped graphene quantum dot (N-GQD) nanomaterials conjugated with polyethylenimine (PEI)-polystyrene sulfonate (PSS)-anti-epidermal growth factor receptor (Ab_EGFR) antibody (N-GQD-PEI-PSS-Ab_EGFR) demonstrated impressive two-photon properties and stability, signifying that they can serve as an effective two-photon contrast agent in two-photon bioimaging. Furthermore, they provided high intensity, brightness, and signal-to-noise ratios at an ultra-low two-photon excitation (TPE) power level in an observation extending to a deep, three-dimensional depth.

Interaction between triethanolamine and singlet or triplet excited state of xanthene dyes in aqueous solution

Triethanolamine (TEOA) has been often used as a hole-scavenger in dye-sensitized semiconductor photocatalytic systems. However, the femtosecond time-resolved kinetics of the interaction between a sensitized dye and TEOA has not been reported in literatures. Herein, we selected four commonly used xanthene dyes, such as fluorescein, dibromofluorescein, eosin Y, and erythrosine B, and studied their ultrafast fluorescence quenching dynamics in the presence of TEOA in aqueous solution, respectively, by using both femtosecond transient absorption and time-resolved fluorescence measurements. We obtained the electron transfer rate from TEOA to each photoexcited xanthene dye in 2.0 M TEOA solution. We also obtained the intersystem crossing rate of each xanthene dye in aqueous solution with fluorescence quantum yield and lifetime measurements. Finally we found that TEOA mainly interacts with the singlet excited-state of fluorescein, dibromofluorescein, and eosin Y, and that TEOA can interact with both the singlet and triplet excited-states of erythrosine B in high concentration of TEOA aqueous solution.

The interaction of amino acids with macrocyclic pH probes of pseudopeptidic nature

The fluorescence quenching, by a series of amino acids, of pseudopeptidic compounds acting as probes for cellular acidity has been investigated.

Fluorescein: A Photo-CIDNP Sensitizer Enabling Hypersensitive NMR Data Collection in Liquids at Low Micromolar Concentration

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is a powerful approach for sensitivity enhancement in NMR spectroscopy. In liquids, intermolecular photo-CIDNP depends on the transient bimolecular reaction between photoexcited dye and sample of interest. Hence the extent of polarization is sample-concentration dependent. This study introduces fluorescein (FL) as a photo-CIDNP dye whose performance is exquisitely tailored to data collection at extremely low sample concentrations. The photo-CIDNP resonance intensities of tryptophan in the presence of either FL or FMN (i.e., the routinely employed flavin mononucleotide photosensitizer) in the liquid state show that FL yields superior sensitivity and enables rapid data collection down to an unprecedented 1 μM concentration. This result was achieved on a conventional spectrometer operating at 14.1 T and equipped with a room-temperature probe (i.e., noncryogenic). Kinetic simulations show that the excellent behavior of FL arises from its long excited-state triplet lifetime and superior photostability relative to conventional photo-CIDNP sensitizers.

Novel antioxidant capacity assay for lipophilic compounds using electron paramagnetic resonance spectroscopy

A novel antioxidant capacity assay for lipophilic compounds was developed using electron paramagnetic resonance (EPR) spectroscopy. The assay is based on antioxidant’s scavenging ability against the tert-butoxyl radical generated photolytically from di-tert-butyl peroxide in ethyl acetate, and named the tert-butoxyl-based antioxidant capacity (BAC) assay. The radical was trapped by spin trap, 5,5-dimethyl-1-pyrroline-N-oxide, and EPR signal intensity of the spin adduct was used as a quantitative marker of radical levels. Signal intensity decreased in a dose-dependent manner in the presence of an antioxidant that competitively reacts with the radical, which was utilized to evaluate BAC values.

The BAC method enabled the accurate estimation of antioxidant capacity for lipophilic materials that may counteract lipid peroxidation in biological membranes. The BAC values for quercetin and caffeic acid are 0.639 ± 0.020 and 0.118 ± 0.012 trolox equivalents, respectively, which are much smaller than values obtained by other aqueous methods such as H-ORAC and ORAC-EPR. Thus, antioxidants present in a non-aqueous environment should be evaluated using a non-aqueous system. In combination with in situ ascorbate reduction, the BAC method was capable of accurately determining the antioxidant capacity of water-insoluble materials that may be reduced in living cells.

Bright or dark immune complexes of anti-TAMRA antibodies for adapted fluorescence-based bioanalysis

Fluorescence labels, for example fluorescein or rhodamin derivatives, are widely used in bioanalysis applications including lateral-flow assays, PCR, and fluorescence microscopy. Depending on the layout of the particular application, fluorescence quenching or enhancement may be desired as the detection principle. Especially for multiplexed applications or high-brightness requirements, a tunable fluorescence probe can be beneficial. The alterations in the photophysics of rhodamine derivatives upon binding to two different anti-TAMRA antibodies were investigated by absorption and fluorescence-spectroscopy techniques, especially determining the fluorescence decay time and steady-state and time-resolved fluorescence anisotropy. Two monoclonal anti-TAMRA antibodies were generated by the hybridoma technique. Although surface-plasmon-resonance measurements clearly proved the high affinity of both antibodies towards 5-TAMRA, the observed effects on the fluorescence of rhodamine derivatives were very different. Depending on the anti-TAMRA antibody either a strong fluorescence quenching (G71-DC7) or a distinct fluorescence enhancement (G71-BE11) upon formation of the immune complex was observed. Additional rhodamine derivatives were used to gain further information on the binding interaction. The data reveal that such haptens as 5-TAMRA could generate different paratopes with equal binding affinities but different binding interactions, which provide the opportunity to adapt bioanalysis methods including immunoassays for optimized detection principles for the same hapten depending on the specific requirements.

Tailoring of optical properties of fluorescein using green synthesized gold nanoparticles

The anisotropy in plasmonic field makes star shaped particles as an optical ruler that can probe larger distances as compared to spherical gold nanoparticles, for which dipoles are parallel to the surface act as more efficient quencher for fluorescein dye.

Thioamide quenching of fluorescent probes through photoinduced electron transfer: mechanistic studies and applications

Previously we have shown that thioamides can be incorporated into proteins as minimally perturbing fluorescence-quenching probes to study protein dynamics, folding, and aggregation. Here, we show that the spontaneity of photoinduced electron transfer between a thioamide and an excited fluorophore is governed by the redox potentials of each moiety according to a Rehm-Weller-type model. We have used this model to predict thioamide quenching of various common fluorophores, and we rigorously tested more than a dozen examples. In each case, we found excellent agreement between our theoretical predictions and experimental observations. In this way, we have been able to expand the scope of fluorophores quenched by thioamides to include dyes suitable for microscopy and single-molecule studies, including fluorescein, Alexa Fluor 488, BODIPY FL, and rhodamine 6G. We describe the photochemistry of these systems and explore applications that demonstrate the utility of thioamide quenching of fluorescein to studying protein folding and proteolysis.

Spectroscopic studies on the interaction of fluorescein and safranine T in PC liposomes

In this study, the fluorescence quenching of fluorescein by safranine T in liposome media had been investigated systematically by fluorescence spectroscopy, UV-vis absorption spectroscopy and fluorescence decay lifetime measurements. The spectroscopic data were analyzed using a Stern-Volmer equation to determine the quenching process. The experimental results showed that the intrinsic fluorescence of fluorescein was strongly quenched by safranine T, and that the quenching mechanism was considered as static quenching by forming a ground-complex. The Stern-Volmer quenching constant Ksv, and the bimolecular quenching constant Kq were estimated. The distances between the donor (fluorescein) and the acceptor (safranine T) were calculated according to the Förster non-radiation energy transfer theory. In addition, the partition coefficient of the safranine T (Kp) in the L-egg lecithin phosphatidylcholine liposomes was also calculated by utilizing the fluorescence quenching.

Peptide-based Targeting of Fluorescent Zinc Sensors to the Plasma Membrane of Live Cells

Combining fluorescent zinc sensors with the facile syntheses and biological targeting capabilities of peptides, we created green- and blue-emitting probes that, (i) are readily prepared on the solid-phase, (ii) retain the photophysical and zinc-binding properties of the parent sensor, and (iii) can be directed to the extracellular side of plasma membranes in live cells for detection of mobile zinc.

Characteristics of the spin-trapping reaction of a free radical derived from AAPH: further development of the ORAC-ESR assay

The characteristics of the spin-trapping reaction in the oxygen radical absorbance capacity (ORAC)-electron spin resonance (ESR) assay were examined, focusing on the kind of spin traps. 2,2-Azobis(2-amidinopropane) dihydrochloride (AAPH) was used as a free radical initiator. The spin adducts of the AAPH-derived free radical were assigned as those of the alkoxyl radical, RO· (R=H(2)N(HN)C-C(CH(3))(2)). Among the spin traps tested, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5,5-dimethyl-4-phenyl-1-pyrroline N-oxide (4PDMPO), 5-(2,2-dimethyl-1,3-propoxycyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO), and 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) were applicable to the ORAC-ESR assay. Optimal formation of spin-trapped radical adduct was observed with 1 mM AAPH, 10 mM spin trap, and 5 s UV irradiation. The calibration curve (the Stern-Volmer's plot) for each spin trap showed good linearity, and their slopes, k (SB)/k (ST), were estimated to be 87.7±2.3, 267±15, 228±9, and 213±16 for DMPO, 4PDMPO, CYPMPO, and DEPMPO, respectively. Though the k (SB)/k (ST) values for selected biosubstances varied with various spin traps, their ratios to Trolox (the relative ORAC values) were almost the same for all spin traps tested. The ORAC-ESR assay also had a very good reproducibility. The ORAC-ESR assay was conducted under stoichiometric experimental conditions. The present results demonstrate the superiority of the ORAC-ESR assay.

A fluorescein-containing, small-molecule, water-soluble receptor for cytosine free bases

In this study, we synthesized small-molecule, water-soluble, fluorescein-containing ureido compounds 6 and 8 as target receptors for cytosine free bases and then investigated the binding of cytosine free bases with the receptors using (15)N NMR spectroscopy and partially labeled cytosine-2,4-(13)C-1,3,4-(15)N-cytosine. Binding with the receptor 6a (the disodium form of 6) caused the chemical shift of the nitrogen atom of the amino group of cytosine to move downfield; binding of the receptor 8a (the disodium form of 8), which is possessing no corresponding aryl nitrogen atom, had no effect on this signal. Fluorescence spectroscopy revealed that binding of cytosine and its derivatives led to quenching of the fluorescence of receptor 6a; in contrast, the quenching of receptor 8a was only slightly affected by cytosine. Because the fluorescence of 6a was not quenched by either deoxycytidine or uracil, it appears that this receptor is a specific for cytosine among the DNA bases. We used the fluorescence of 6a to measure the apparent binding constants for various cytosine derivatives, including the anticancer prodrug 5-fluorocytosine. Receptor 6a is the first small-molecule, water-soluble fluorescent receptor for the specific binding of cytosine free bases in aqueous solution.

"Turn-on" fluorescent polymeric microparticle sensors for the determination of ammonia and amines in the vapor state

Cross-linked acrylic ester microparticles (EG50OH) with absorbed fluorescent probe molecules, such as fluorescein and acridine orange were successfully fabricated and employed as "turn-on" fluorescent sensors for the detection and determination of ammonia and organic amine vapor concentrations. Using EG50OH microparticles that had fluorescein (fluorescein fluorescent microparticle, FL-FMP sensor) incorporated as the fluorescent probe molecule (with lambda(ex) = 450 nm; lambda(em) = 528 nm), the detection limit achieved for ammonia vapor was 0.73 ppm, the response being linearly dependent on concentration over the range of 1.0-250 ppm gaseous ammonia. The FL-FMP responded to organic amines with the relative signal response following the order: triethylamine > tert-butylamine > diethylamine > n-propylamine > ammonia. A limit of detection of 0.048 ppm triethylamine vapor was achieved using this FL-FMP sensor. The sensor response is based on the acid-base properties of the fluorescent probe molecules. The fluorescent probes immobilized in/on the EG50OH are in a microenvironment such that they are in their neutral or protonated states and only exhibit weak fluorescence. Upon exposure to ammonia or amine vapor, the fluorescent species are deprotonated and exhibit much greater fluorescence emission ("turned-on" due to exposure to these basic analytes). The ease of fabrication and aforementioned properties of these fluorescent microparticle sensors are such that they should be amenable for use in a variety of situations requiring the detection or monitoring of ammonia and amines in the vapor state.