Fluorescein-based amino acids for solid phase synthesis of fluorogenic protease substrates

Aminofluoresceins Versus Fluorescein: Ascertained New Unusual Features of Tautomerism and Dissociation of Hydroxyxanthene Dyes in Solution

Within the course of this spectroscopic research, we revealed novel features of the protolytic behavior, which extend the knowledge of the chemistry of xanthene dyes and rationalize the utilization of these compounds. In addition to the well-known tautomerism of the molecular form, H₂R, of fluorescein dyes, new aspects of tautomeric transformation of anions are disclosed. First, for the dyes bearing the substituents in the phthalic acid residue, 4'- and 5'-aminofluoresceins and 4'-fluorescein isothiocyanate, the monoanion HR^- exists in non-hydrogen-bond donor solvents not only as a tautomer with the ionized carboxylic and nonionized OH group but also as a "phenolate" ion with a nonionized COOH group. Such state of HR^- ions is typical for dyes bearing halogen atoms or NO₂ groups in the xanthene moiety but was not observed until now in the case of substitution in the phthalic residue. Second, the possibility of the existence of the HR^- species in DMSO in the form of colorless lactone is deduced for the 5'-aminofluorescein using the visible and infrared spectra. This results in a dramatic difference in medium effects. For instance, whereas for fluorescein in DMSO, the inversion of the stepwise ionization constants takes place and the K_a1/K_a2 value equals 0.08, the same ratio for 5'-aminofluorescein is as high as ∼800. In addition, the pK_a values of sulfonefluorescein, erythrosin, methyl ether of fluorescein, and phenol red were obtained to verify the acidity scale in DMSO and to support the detailed scheme of protolytic equilibria of fluorescein dyes.

Enzyme molecules in solitary confinement

Large arrays of homogeneous microwells each defining a femtoliter volume are a versatile platform for monitoring the substrate turnover of many individual enzyme molecules in parallel. The high degree of parallelization enables the analysis of a statistically representative enzyme population. Enclosing individual enzyme molecules in microwells does not require any surface immobilization step and enables the kinetic investigation of enzymes free in solution. This review describes various microwell array formats and explores their applications for the detection and investigation of single enzyme molecules. The development of new fabrication techniques and sensitive detection methods drives the field of single molecule enzymology. Here, we introduce recent progress in single enzyme molecule analysis in microwell arrays and discuss the challenges and opportunities.

Target-to-background enhancement in multispectral endoscopy with background autofluorescence mitigation for quantitative molecular imaging

Fluorescence molecular imaging with exogenous probes improves specificity for the detection of diseased tissues by targeting unambiguous molecular signatures. Additionally, increased diagnostic sensitivity is expected with the application of multiple molecular probes. We developed a real-time multispectral fluorescence-reflectance scanning fiber endoscope (SFE) for wide-field molecular imaging of fluorescent dye-labeled molecular probes at nanomolar detection levels. Concurrent multichannel imaging with the wide-field SFE also allows for real-time mitigation of the background autofluorescence (AF) signal, especially when fluorescein, a U.S. Food and Drug Administration approved dye, is used as the target fluorophore. Quantitative tissue AF was measured for the ex vivo porcine esophagus and murine brain tissues across the visible and nearinfrared spectra. AF signals were then transferred to the unit of targeted fluorophore concentration to evaluate the SFE detection sensitivity for sodium fluorescein and cyanine. Next, we demonstrated a real-time AF mitigation algorithm on a tissue phantom, which featured molecular probe targeted cells of high-grade dysplasia on a substrate containing AF species. The target-to-background ratio was enhanced by more than one order of magnitude when applying the real-time AF mitigation algorithm. Furthermore, a quantitative estimate of the fluorescein photodegradation (photobleaching) rate was evaluated and shown to be insignificant under the illumination conditions of SFE. In summary, the multichannel laser-based flexible SFE has demonstrated the capability to provide sufficient detection sensitivity, image contrast, and quantitative target intensity information for detecting small precancerous lesions in vivo.

The encapsulation of an amphiphile into polystyrene microspheres of narrow size distribution

Encapsulation of compounds into nano- or microsized organic particles of narrow size distribution is of increasing importance in fields of advanced imaging and diagnostic techniques and drug delivery systems. The main technology currently used for encapsulation of molecules within uniform template particles while retaining their size distribution is based on particle swelling methodology, involving penetration of emulsion droplets into the particles. The swelling method, however, is efficient for encapsulation only of hydrophobic compounds within hydrophobic template particles. In order to be encapsulated, the molecules must favor the hydrophobic phase of an organic/aqueous biphasic system, which is not easily achieved for molecules of amphiphilic character.

The following work overcomes this difficulty by presenting a new method for encapsulation of amphiphilic molecules within uniform hydrophobic particles. We use hydrogen bonding of acid and base, combined with a pseudo salting out effect, for the entrapment of the amphiphile in the organic phase of a biphasic system. Following the entrapment in the organic phase, we demonstrated, using fluorescein and (antibiotic) tetracycline as model molecules, that the swelling method usually used only for hydrophobes can be expanded and applied to amphiphilic molecules.

Activatable Optical Probes for the Detection of Enzymes

The early detection of many human diseases is crucial if they are to be treated successfully. Therefore, the development of imaging techniques that can facilitate early detection of disease is of high importance. Changes in the levels of enzyme expression are known to occur in many diseases, making their accurate detection at low concentrations an area of considerable active research. Activatable fluorescent probes show immense promise in this area. If properly designed they should exhibit no signal until they interact with their target enzyme, reducing the level of background fluorescence and potentially endowing them with greater sensitivity. The mechanisms of fluorescence changes in activatable probes vary. This review aims to survey the field of activatable probes, focusing on their mechanisms of action as well as illustrating some of the in vitro and in vivo settings in which they have been employed.

Detection of proteases using an immunochemical method with haptenylated-gelatin as a solid-phase substrate

A simplified method for the measurement of proteases utilising solid-phase substrates incorporating an ELISA end-point detection step is described. Gelatin-hapten conjugates adsorbed onto polystyrene surfaces were found to be efficient substrates for proteases. Digestion of the solid-phase protein-hapten complexes resulted in proportional desorption of the attached conjugates and decrease in the detectable hapten species. Gelatin-cholic acid conjugates, affinity-purified sheep anti-cholic acid antibody-HRP and a chromogenic substrate were incorporated into a convenient and highly sensitive solid-phase immunochemical method. The detectable signal is inversely proportional to enzyme activity. Bacterial proteases (alpha-chymotrypsin Type II, Type IX from Bacillus polymyxa, Type XIV from Streptomyces griseus, Type XXIV from Bacillus licheniformens) were assayed. Dose-response curves for enzyme activities were measured within ranges of 0-550 microunits mL(-1) for chymotrypsin, 0-12 microunits mL(-1) for type IX, 0-35 microunits mL(-1) for type XIV and 0-100 microunits mL(-1) for type XXIV. The detection limits of the proteases studied were 89 microunits mL(-1) for chymotrypsin, 0.26 microunits mL(-1) for type IX, 5.8 microunits mL(-1) for type XIV and 6.5 microunits mL(-1) for type XXIV. It was demonstrated that the two-step immunochemical method combines the simplicity and sensitivity of solid-phase enzyme immunoassays, the broad specificity of gelatin as a protease substrate and the flexibility of the solid-phase format.

Assembly and selective “in synthesis” labeling of quenched fluorogenic protease substrates

Because impaired cellular protease activities are linked to many diseases, such as cancer, inflammation, neurodegeneration, and infection, internally quenched fluorescent peptides have recently been developed as tools for analyzing the specificities of these enzymes. Here we report convenient and cost-effective approaches for the selective "in synthesis" assembly of such substrate peptides for protease assays. Fluorescein and Dabcyl groups were covalently and selectively attached during synthesis to epsilon-amino groups of internal lysines. Functionality was then tested by digestion with leucine aminopeptidase, chymotrypsin, and microsomal vesicles. All peptides proved to be appropriate substrates of the enzymes tested and of the endogenous peptidases in the microsomal vesicles. In summary, we describe an innovative and cheap method to develop completely functional quenched fluorescent peptides that are usable in specific detection of individual proteases, in particular aminopeptidases, in both in vitro and in vivo systems.

Fluorogenic ester substrates to assess proteolytic activity

The synthesis of a new type of fluorogenic ester substrates is described. Prepared from fluorescein in three steps with common commercially available precursors, they all generate bright green fluorescence upon proteolysis. Their particular structure allows the same substrate be used to report enzymatic activity of various proteases from serine and cysteine superfamilies. The substrate cleavage is sensitive to specific protease inhibitors providing a tool for inhibitor screening.

Aminopropargyl derivative of terpyridine-bis(methyl-enamine) tetraacetic acid chelate of europium (Eu (TMT)-AP3): a new reagent for fluorescent labelling of proteins and peptides

The synthesis and photophysical properties of a new terpyridine-based europium(III) chelate (Eu (TMT)-AP3) designed for peptide and protein labelling in aqueous solution phase is described. In order to obtain a stable, easy to handle, versatile and efficient labelling agent, a reactive aminopropargyl arm has been introduced onto the terpyridine moiety. As preliminary biochemical applications the chelate has been 1) efficiently covalently attached onto a representative biomolecule-monoclonal antibody-and 2) converted into iodoacetamido and aldehyde derivatives, and the photoluminescent Eu (TMT)-AP3 was grafted onto cysteine and lysine amino acid residues respectively. These two different solution phase labelling methods yielded original fluorogenic FRET based probes suitable for "in vitro" detection of caspase-3 protease, a key mediator of apoptosis of mammalian cells.