Symmetric and asymmetric squarylium dyes as noncovalent protein labels: a study by fluorimetry and capillary electrophoresis

Squaraine Dyes Derived from Indolenine and Benzo[e]indole as Potential Fluorescent Probes for HSA Detection and Antifungal Agents

Four squaraine dyes derived from 2,3,3-trimethylindolenine and 1,1,2-trimethyl-1H-benzo[e]indole with different combinations of barbituric groups attach to the central ring, having ester groups and alkyl chains in the nitrogen atoms of heterocyclic rings were synthesized. These dyes were fully characterized and their photophysical behavior was studied in ethanol and phosphate-buffered saline solution. Absorption and emission bands between 631 and 712 nm were detected, with the formation of aggregates in aqueous media, which is typical of this class of dyes. Tests carried out with 1,3-diphenylisobenzofuran allowed us to verify the ability of the dyes to produce singlet oxygen. The interaction of synthesized dyes with human serum albumin (HSA) was also evaluated, being demonstrated a linear correlation between fluorescence intensity and protein concentration. The antifungal potential of the dyes against the yeast Saccharomyces cerevisiae was evaluated using a broth microdilution assay. In order to test the photosensitizing capacity of the synthesized dyes, tests were carried out in the dark and with irradiation, using a custom-built light-emitting diode that emits close to the absorption wavelength of the studied dyes. The results showed that the interaction of dyes with HSA and the antifungal activity depends on the different structural modifications of the dyes.

Fluorescence sensing technology for the rapid detection of haze-forming proteins in white wine

The methods currently available for determining haze proteins in wine are time-consuming, expensive, and often not sufficiently accurate. The latter may lead to bentonite over-fining of a wine, which might strip wine phenolics and aroma compounds, or wine under-fining, which increases the risk of protein instability. In this work, an efficient and rapid fluorescence-based technology to detect haze-forming proteins in white wines was developed. A fluorescent compound was synthesised to selectively bind haze-forming proteins. Studies involving HPLC demonstrated a linear dependence over a range of relevant haze protein concentrations and a low detection limit of 2 mg/L. Forty-eight control and bentonite fined wines were analysed to validate the analytical performance of the fluorescent dye in the detection of haze-forming proteins. The method can be deployed rapidly, without sample preparation, presenting an opportunity to use in routine testing and overcome limitations of the "heat test" currently used in the wine industry.

Squaraine dyes as serum albumins probes: Synthesis, photophysical experiments and molecular docking studies

Three barbiturate squaraine dyes derived from indolenine or benzothiazole, with different barbituric acid derivatives were prepared, characterized and photophysically evaluated by standard spectroscopic methods. As expectable for squaraines, these dyes showed narrow and intense absorption and emission bands in the Vis/NIR region. The interaction of synthesized dyes with bovine and human serum albumins (BSA and HSA) was also evaluated in phosphate buffer (PB). The results revealed that upon the addition of BSA or HSA the complex dye-protein emit more fluorescence, and the emission intensity is directly proportional to the concentration of protein used (0-3.5 µM). The titration tests allowed to calculate the binding constants, in an order of magnitude of 10⁴-10⁶ M, as well as the limits of detection and quantification in the nanomolar tens range. All dyes showed a good response to the interaction with both proteins, but the most pronounced envisioning their use as protein labeling was observed for the squaraine dye derived from the indolenine with a 1,3-dimethylbarbituric acid moiety. The molecular docking studies revealed the existence of a binding between the compounds and four sites on the HSA molecule, where one of these four locations is a new binding site with which this series of dye interacts.

Fluorescent, colourimetric, and ratiometric probes based on diverse fluorophore motifs for mercuric(II) ion (Hg2+) sensing: highlights from 2011 to 2019

Though it has not been shown to deliver any biological importance, mercuric(II) ion (Hg²⁺) is a deleterious cation which poses grievous effects to the human body and/or the ecosystem, hence, the need for its sensitive and selective monitoring in both environmental and biological systems. Over the years, there has been a great deal of work in the use of fluorescent, colourimetric, and/or ratiometric probes for Hg²⁺ recognition. Essentially, the purpose of this review article is to give an overview of the advances made in the constructions of such probes based on the works reported in the period from 2011 to 2019. Discussion in this review work has been tailored to the kinds of fluorophore scaffolds used for the constructions of the probes reported. Selected examples of probes under each fluorophore subcategory were discussed with mentions of the typically determined parameters in an analytical sensing operation, including modulation in fluorescence intensity, optimal pH, detection limit, and association constant. The environmental and biological application ends of the probes were also touched where necessary. Important generalisations and conclusions were given at the end of the review. This review article highlights 196 references.

Research Progress of Near-Infrared Fluorescence Immunoassay

Near-infrared fluorescence probes (NIFPs) have been widely used in immunoassay, bio-imaging and medical diagnosis. We review the basic principles of near-infrared fluorescence and near-infrared detection technology, and summarize structures, properties and characteristics of NIFPs (i.e., cyanines, xanthenes fluorescent dyes, phthalocyanines, porphyrin derivates, single-walled carbon nanotubes (SWCNTs), quantum dots and rare earth compounds). We next analyze applications of NIFPs in immunoassays, and prospect the application potential of lateral flow assay (LFA) in rapid detection of pathogens. At present, our team intends to establish a new platform that has highly sensitive NIFPs combined with portable and simple immunochromatographic test strips (ICTSs) for rapid detection of food-borne viruses. This will provide technical support for rapid detection on the port.

Aggregation behavior of novel heptamethine cyanine dyes upon their binding to native and fibrillar lysozyme

Novel cyanine dyes can be used for amyloid fibril detection.

Properties, theoretical study and crystal structure of 3-benzothiazole-9-ethyl carbazole

The title compound of 3-benzothiazole-9-ethyl carbazole was synthesized by the reaction of 3-aldehyde-9-ethyl carbazole and 2-aminothiophenol. The compound was characterized by (1) H nuclear magnetic resonance (NMR) and mass spectrometry (MS). Its crystal structure was obtained and determined by single crystal X-ray diffraction. The results showed that the crystal belongs to the orthorhombic crystal system and the cell parameters of space group P2(1)2(1)2(1) were a = 5.6626 (12) Å, b = 12.606 (3) Å, c = 22.639 (5) Å, α = 90°, β = 90°, γ = 90°, V = 1616.0 (6) Å(3) , Z = 4, Dc = 1.350 mg/m(3) . The UV-vis and fluorescence spectra were also studied preliminarily. The fluorescence spectra of the title compound with bovine serum albumin (BSA) showed that BSA could be marked with the compound and the stability constant between them was 0.82 × 10(7)  M(-1) . Meanwhile, the crystal and molecule were theoretically surveyed by density functional tight-binding (DFTB). The results showed that there was an orbital overlap for lowest unoccupied molecular orbital (LUMO) between the neighbouring molecules for the crystal, which is different from the molecule structure. It was also showed that the crystal structure is a non-conductor. Copyright © 2016 John Wiley & Sons, Ltd.

In-gel staining of proteins in native poly acryl amide gel electrophoresis using tetrakis(4-sulfonato phenyl)porphyrin

Protein identification in polyacrylamide gel electrophoresis (PAGE) requires post-electrophoretic steps like fixing, staining and destaining of the gel, which are time-consuming and cumbersome. We have developed a method for direct visualization of protein bands in PAGE using tetrakis(4-sulfonato phenyl)porphyrin (TPPS) as a dye without the need for any post electrophoretic steps, where separation and recovery of enzymes become much easier for further analysis. Activity staining was done to prove that the biochemical activity of the enzymes was preserved after electrophoresis.

Metallo Supramolecular Assemblies of Bis-squaraines by Allosteric Ca2+ Ion Binding

Alkyl chain tethered bis-squaraines bind to Ca2+ ions through the participation of the negatively charged oxygen of the central cyclobutene moiety to form folded H-type aggregates. The initially formed Ca2+ complex is preorganized to facilitate cooperative allosteric binding of Ca2+, resulting in the formation of extended supramolecular arrays. The electronic absorption, IR, and ESI-MS studies support the formation of metallo supramolecular architectures of the folded H-type dimers of the bis-squaraines.

LIF detection of peptides and proteins in CE

CE- and microchip-based separations coupled with LIF are powerful tools for the separation, detection and determination of biomolecules. CE with certain configurations has the potential to detect a small number of molecules or even a single molecule, thanks to the high spatial coherence of the laser source which permits the excitation of very small sample volumes with high efficiency. This review article discusses the use of LIF detection for the analysis of peptides and proteins in CE. The most common laser sources, basic instrumentation, derivatization modes and set-ups are briefly presented and special attention is paid to the different fluorogenic agents used for pre-, on- and postcapillary derivatization of the functional groups of these compounds. A table summarizing major applications of these derivatization reactions to the analysis of peptides and proteins in CE-LIF and a bibliography with 184 references are provided which covers papers published to the end of 2005.

Investigating noncovalent squarylium dye–protein interactions by capillary electrophoresis–frontal analysis

Noncovalent interactions between fluorescent probe molecules and protein analyte molecules, which typically occur with great speed and minimal sample handling, form the basis of many high sensitivity analytical techniques. Understanding the nature of these interactions and the composition of the resulting complexes represents an important area of study that can be facilitated by capillary electrophoresis (CE). Specifically, we will present how frontal analysis (FA) and Hummel-Dreyer (HD) methods can be implemented with CE to determine association constants and stoichiometries of noncovalent complexes of the red luminescent squarylium dye Red-1c with bovine serum albumin (BSA) and beta-lactoglobulin A. By adjusting solution conditions, such as pH or ionic strength, it is possible to selectively modify the binding process. As such, conditions for optimal selectivity for labeling reactions can be established by capillary electrophoresis-frontal analysis (CE-FA) investigations.

Synthesis of water-soluble, ring-substituted squaraine dyes and their evaluation as fluorescent probes and labels

A series of ring-substituted squaraines absorbing and emitting in the red and NIR spectral region was synthesized and their spectral and photophysical properties (quantum yields, fluorescence lifetimes) and photostabilities were measured and compared to Cy5, a commonly used fluorescent label. The absorption maxima in aqueous media were found to be between 628 and 667 nm and the emission maxima are between 642 and 685 nm. Squaraine dyes exhibit high extinction coefficients (163,000-265,000 M(-1) cm(-1)) and lower quantum yields (2-7%) in aqueous buffer but high quantum yields (up to 45%) and long fluorescence lifetimes (up to 3.3 ns) in presence of BSA. Dicyanomethylene- and thio-substituted squaraines exhibit an additional absorption around 400 nm with extinction coefficients between 21,500 and 44,500 M(-1) cm(-1). These dyes are excitable not only with red but also with blue diode lasers or light emitting diodes. Due to the favourable spectral and photophysical properties these dyes can be used as fluorescent probes and labels for intensity- and fluorescence lifetime-based biomedical applications.

Two-Site Ionic Labeling with Pyranine: Implications for Structural Dynamics Studies of Polymers and Polypeptides by Time-Resolved Fluorescence Anisotropy

Time-resolved fluorescence anisotropy (TRFA) is widely used to study dynamic motions of biomolecules in a variety of environments. However, depolarization due to rapid side chain motions often complicates the interpretation of anisotropy decay data and interferes with the accurate observation of segmental motions. Here, we demonstrate a new method for two-point ionic labeling of polymers and biomolecules that have appropriately spaced amino groups using the fluorescent probe 8-hydroxyl-1,3,6-trisulfonated pyrene (pyranine). TRFA analysis shows that such labeling provides a more rigid attachment of the fluorophore to the macromolecule than the covalent or single-point ionic labeling of amino groups, leading to time-resolved anisotropy decays that better reflect the backbone motion of the labeled polymer segment. Optimal coupling of pyranine to biomolecule dynamics is shown to be obtained for appropriately spaced Arg groups, and in such cases the ionic binding is stable up to 150 mM ionic strength. TRFA was used to monitor the behavior of pyranine-labeled poly(allylamine) (PAM) and poly-d-lysine (PL) in sodium silicate derived sol-gel materials and revealed significant restriction of backbone motion upon entrapment for both polymers, an observation that was not readily apparent in a previous study with entrapped fluorescein-labeled PAM and PL. The implications of these findings for fluorescence studies of polymer and biomolecule dynamics are discussed.

Protein labeling with red squarylium dyes for analysis by capillary electrophoresis with laser-induced fluorescence detection

Two new red luminescent asymmetric squarylium dyes (designated "Red-1c and Red-3") have been shown to exhibit absorbance shifts to longer wavelengths upon the addition of protein, along with a concomitant increase in fluorescence emission. Specifically, the absorbance maxima for Red-1c and Red-3 dyes are 607 and 622 nm, respectively, in the absence of HSA, and 642 and 640 nm in the presence of HSA, making the excitation of their protein complexes feasible with inexpensive and robust diode lasers. Fluorescence emission maxima, in the presence of HSA, are 656 and 644 nm for Red-1c and Red-3, respectively. Because of the inherently low fluorescence of the dyes in their free state, Red-1c and Red-3 were used as on-column labels (that is, with the dye incorporated into the separation buffer), thus eliminating the need for sample derivatization prior to injection and separation. A comparison of precolumn and on-column labeling of proteins with these squarylium dyes revealed higher efficiencies and greater sensitivities for on-column labeling, which, when conducted with a basic, high-salt content buffer, permitted baseline resolution of a mixture of five model proteins. LOD for model proteins, such as transferrin, alpha-lactalbumin, BSA, and beta-lactoglobulin A and B, labeled with these dyes and analyzed by CE with LIF detection (CE-LIF) were found to be dependent upon dye concentration and solution pH, and are as low as 5 nM for BSA. Satisfactory linear relationships between peak height (or peak area) and protein concentration were obtained by CE-LIF for this on-column labeling method with Red-3 and Red-1c.

Capillary electrophoresis of proteins 2003-2005

This review article with 304 references describes recent developments in CE of proteins, and covers the two years since the previous review (Hutterer, K., Dolník, V., Electrophoresis 2003, 24, 3998-4012) through Spring 2005. It covers topics related to CE of proteins, including modeling of the electrophoretic migration of proteins, sample pretreatment, wall coatings, improving separation, various forms of detection, special electrophoretic techniques such as affinity CE, CIEF, and applications of CE to the analysis of proteins in real-world samples including human body fluids, food and agricultural samples, protein pharmaceuticals, and recombinant protein preparations.

Fluorimetric studies and noncovalent labeling of protein with the near-infrared dye HITCI for analysis by CE-LIF

1,1',3,3,3',3'-Hexamethylindotricarbocyanine iodide (HITCI) is a commercially available, positively charged, indocarbocyanine dye used typically as a laser dye in the near infrared (NIR). The absorbance and fluorescence properties of HITCI in a variety of solvent systems were determined. Results indicate that the fluorescence of HITCI is not significantly affected by the pH. Titration of HITCI with human serum albumin (HSA) and trypsinogen was carried out to investigate the interactions between this dye and proteins. These studies revealed that the absorbance and fluorescence properties of the dye change upon binding to protein in a wide range of solution pH's. The potential use of HITCI as a noncovalent protein labeling probe, therefore, was explored. Determination and separation of HITCI and HITCI-protein complexes was performed by capillary electrophoresis with diode-laser induced fluorescence detection (CE-LIF). Both pre-column and on-column noncovalent labeling methods are demonstrated.

Detection of myelination using a novel histological probe

Current methods for myelin staining in tissue sections include both histological and immunohistochemical techniques. Fluorescence immunohistochemistry, which uses antibodies against myelin components such as myelin basic protein, is often used because of the convenience for multiple labeling. To facilitate studies on myelin, this paper describes a quick and easy method for direct myelin staining in rodent and human tissues using novel near-infrared myelin (NIM) dyes that are comparable to other well-characterized histochemical reagents. The near-infrared fluorescence spectra of these probes allow fluorescent staining of tissue sections in multiple channels using visible light fluorophores commonly used in immunocytochemistry. These dyes have been used successfully to detect normal myelin structure and myelin loss in a mouse model of demyelination disease.