I2/I--mediated fluorescence quenching of an Ag+-doped gold nanocluster-based immunoassay for sensitive detection of Escherichia coli O157:H7 in milk

Escherichia coli O157:H7 is a type of hazardous bacteria in the field of food safety. A sensitive and effective method is urgently needed to detect it, avoiding enormous harm for the human health. In this study, we synthesized stable Ag⁺-doped gold nanoclusters (Ag-AuNC) with a fluorescence intensity 4.8 times stronger than that of AuNC. It was further demonstrated that Ag⁰ existing in the AuNC core and a fraction of Ag⁺ anchored on the AuNC shell eliminated the surface defects and improved the luminescent properties of AuNC. A combination of I₂ and I^- was used to quench fluorescence-enhanced Ag-AuNC, which was first applied in ELISA for detecting E. coli O157:H7 to improve the sensitivity. In the presence of E. coli O157:H7, the biotinylated anti-E. coli O157:H7 mAb and streptavidin-alkaline phosphatase would be immobilized and catalyze l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate to produce ascorbic acid. After addition of KIO₃, I₂/I^- were generated. The I₂ could trigger oxidative etching of Ag-AuNC and I^- could combine with Ag⁺ to decrease the Ag⁺ concentration of Ag-AuNC, which resulted in fluorescence quenching of Ag-AuNC. Under optimal conditions, the linear range of I₂/I^--mediated fluorescence quenching of Ag-AuNC-based immunoassay for detecting E. coli O157:H7 was 3.3 × 10³ to 10⁶ cfu/mL, with a detection limit of 9.2 × 10² cfu/mL, 10.7-fold lower than that of the traditional ELISA. The proposed immunoassay exhibits excellent sensitivity, specificity, recovery, and accuracy, which is useful for quantitative detection of E. coli O157:H7 in food safety.

Interaction between miR4749 and Human Serum Albumin as Revealed by Fluorescence, FRET, Atomic Force Spectroscopy and Computational Modelling

The interaction of Human Serum Albumin (HSA) with the microRNA, miR4749, was investigated by Atomic Force Spectrscopy (AFS), static and time-resolved fluorescence spectroscopy and by computational methods. The formation of a HSA/miR4749 complex with an affinity of about 10⁴ M⁻¹ has been assessed through a Stern–Volmer analysis of steady-state fluorescence quenching of the lone Trp residue (Trp214) emission of HSA. Förster Resonance Energy Transfer (FRET) measurements of fluorescence lifetime of the HSA/miR4749 complex were carried out in the absence and in the presence of an acceptor chromophore linked to miR4749. This allowed us to determine a distance of 4.3 ± 0.5 nm between the lone Trp of HSA and the dye bound to miR4749 5p-end. Such a distance was exploited for a screening of the possible binding sites between HSA and miR4749, as predicted by computational docking. Such an approach, further refined by binding free energy calculations, led us to the identification of a consistent model for the structure of the HSA/miR4749 complex in which a positively charged HSA pocket accommodates the negatively charged miRNA molecule. These results designate native HSA as a suitable miRNA carrier under physiological conditions for delivering to appropriate targets.

Recent Efforts and Milestones for Simulating Nucleic Acid FRET Experiments through Computational Methods

Computational methods can greatly aid nucleic acid fluorescence experiments by either offering fully detailed atomic insights into the conformations and interactions present in the studied system or by providing accurate simulations of the fundamental parameters. Fluorescence-based optical biosensors show great potential for clinical diagnosis of life-altering diseases with a very high specificity. Many of the designs for such rely on the concept of Förster resonance energy transfer (FRET). Currently, the methods used experimentally make use of theoretical assumptions which fundamentally affect the results. Having a detailed atomistic overview or significant simulated parameters could improve the understanding of the calculations and provide much more accurate outcomes. However, there are many challenges that need to be addressed before standardized computational protocols can be employed. This review is meant to highlight the progress made for computational methods used to simulate FRET experiments for nucleic acid probes. Recent advances have been made in computational tools, such as force field parametrizations and improved protocols. Complementary simulations to experimental data are also comprised in the this review.

On the Effect of pH, Temperature, and Surfactant Structure on Bovine Serum Albumin-Cationic/Anionic/Nonionic Surfactants Interactions in Cacodylate Buffer-Fluorescence Quenching Studies Supported by UV Spectrophotometry and CD Spectroscopy

Due to the fact that surfactant molecules are known to alter the structure (and consequently the function) of a protein, protein–surfactant interactions are very important in the biological, pharmaceutical, and cosmetic industries. Although there are numerous studies on the interactions of albumins with surfactants, the investigations are often performed at fixed environmental conditions and limited to separate surface-active agents and consequently do not present an appropriate comparison between their different types and structures. In the present paper, the interactions between selected cationic, anionic, and nonionic surfactants, namely hexadecylpyridinium chloride (CPC), hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polyethylene glycol sorbitan monolaurate, monopalmitate, and monooleate (TWEEN 20, TWEEN 40, and TWEEN 80, respectively) with bovine serum albumin (BSA) were studied qualitatively and quantitatively in an aqueous solution (10 mM cacodylate buffer; pH 5.0 and 7.0) by steady-state fluorescence spectroscopy supported by UV spectrophotometry and CD spectroscopy. Since in the case of all studied systems, the fluorescence intensity of BSA decreased regularly and significantly under the action of the surfactants added, the fluorescence quenching mechanism was analyzed thoroughly with the use of the Stern–Volmer equation (and its modification) and attributed to the formation of BSA–surfactant complexes. The binding efficiency and mode of interactions were evaluated among others by the determination, comparison, and discussion of the values of binding (association) constants of the newly formed complexes and the corresponding thermodynamic parameters (ΔG, ΔH, ΔS). Furthermore, the influence of the structure of the chosen surfactants (charge of hydrophilic head and length of hydrophobic chain) as well as different environmental conditions (pH, temperature) on the binding mode and the strength of the interaction has been investigated and elucidated.

Binding Mechanism between Acetylcholinesterase and Drugs Pazopanib and Lapatinib: Biochemical and Biophysical Studies

Tyrosine kinase inhibitors (TKIs) are antitumor compounds that prevent the phosphorylation of proteins in a biological environment. However, the multitarget performance of TKIs promotes them as possible candidates for drug repositioning. In this work, interaction and inhibition studies through spectroscopic and computational techniques to evaluate the binding effectiveness of lapatinib and pazopanib TKIs to acetylcholinesterase (AChE) are reported. The results indicated potent inhibition at the μM level. The types of inhibition were identified, with pazopanib acting through non-competitive inhibition and lapatinib through acompetitive inhibition. The fluorescence suppression studies indicate a static mechanism for lapatinib-AChE and pazopanib-AChE systems, with a binding constant in the order of 10⁵ M^-1. The obtained thermodynamic parameters reveal interactions driven by van der Waals forces and hydrogen bonds in the lapatinib-AChE system (ΔH° and ΔS° < 0). In contrast, the pazopanib-AChE system shows positive ΔH° and ΔS°, characteristic of hydrophobic interactions. The Foster resonance energy transfer study supports the fluorescence studies performed. The 3D fluorescence studies suggest changes in the microenvironment of the tryptophan and tyrosine residues of the protein in contact with lapatinib and pazopanib. The results suggest effective inhibition and moderate interaction of the drugs with AChE, making them interesting for conducting more in-depth repositioning studies as AChE inhibitors.

Multi-Marker Immunofluorescent Staining and PD-L1 Detection on Circulating Tumour Cells from Ovarian Cancer Patients

Simple Summary

Circulating tumour cells (CTCs) have the potential to serve as a rich source of information for cancer diagnostic and therapeutic decisions. To fully exploit this minimally invasive diagnostic resource requires techniques that aid in enriching heterogenous populations of CTCs and markers to efficiently characterise these cells as tumour derived. In the present study we eva-luated the microfluidic enrichment of CTCs and a multi-marker staining methodology for the identification of heterogeneous CTCs in ovarian cancer (OC) patients and evaluation of PD-L1 expression. We showed, for the first time, the existence of hybrid CTCs with an epithelial/mesenchymal phenotype and their association with PD-L1 in OC. Incorporation of this method in future clinical trials can help predict immunotherapy responsiveness in OC patients.

Abstract

Detection of ovarian cancer (OC) circulating tumour cells (CTCs) is primarily based on targeting epithelial markers, thus failing to detect mesenchymal tumour cells. More importantly, the immune checkpoint inhibitor marker PD-L1 has not been demonstrated on CTCs from OC patients. An antibody staining protocol was developed and tested using SKOV-3 and OVCA432 OC cell lines. We targeted epithelial (cytokeratin (CK) and EpCAM), mesenchymal (vimentin), and OC-specific (PAX8) markers for detection of CTCs, and CD45/16 and CD31 were used for the exclusion of white blood and vascular endothelial cells, respectively. PD-L1 was used for CTC characterisation. CTCs were enriched using the Parsortix™ system from 16 OC patients. Results revealed the presence of CTCs in 10 (63%) cases. CTCs were heterogeneous, with 113/157 (72%) cells positive for CK/EpCAM (epithelial marker), 58/157 (37%) positive for vimentin (mesenchymal marker), and 17/157 (11%) for both (hybrid). PAX8 was only found in 11/157 (7%) CTCs. In addition, 62/157 (39%) CTCs were positive for PD-L1. Positivity for PD-L1 was significantly associated with the hybrid phenotype when compared with the epithelial (p = 0.007) and mesenchymal (p = 0.0009) expressing CTCs. Characterisation of CTC phenotypes in relation to clinical outcomes is needed to provide insight into the role that epithelial to mesenchymal plasticity plays in OC and its relationship with PD-L1.

Structural Characterization of Ectodomain G Protein of Respiratory Syncytial Virus and Its Interaction with Heparan Sulfate: Multi-Spectroscopic and In Silico Studies Elucidating Host-Pathogen Interactions

The global burden of disease caused by a respiratory syncytial virus (RSV) is becoming more widely recognized in young children and adults. Heparan sulfate helps in attaching the virion through G protein with the host cell membrane. In this study, we examined the structural changes of ectodomain G protein (edG) in a wide pH range. The absorbance results revealed that protein maintains its tertiary structure at physiological and highly acidic and alkaline pH. However, visible aggregation of protein was observed in mild acidic pH. The intrinsic fluorescence study shows no significant change in the λ_max except at pH 12.0. The ANS fluorescence of edG at pH 2.0 and 3.0 forms an acid-induced molten globule-like state. The denaturation transition curve monitored by fluorescence spectroscopy revealed that urea and GdmCl induced denaturation native (N) ↔ denatured (D) state follows a two-state process. The fluorescence quenching, molecular docking, and 50 ns simulation measurements suggested that heparan sulfate showed excellent binding affinity to edG. Our binding study provides a preliminary insight into the interaction of edG to the host cell membrane via heparan sulfate. This binding can be inhibited using experimental approaches at the molecular level leading to the prevention of effective host–pathogen interaction.

Evaluation of the Correlation between Porphyrin Accumulation in Cancer Cells and Functional Porphyrin Positions of the Phenyl Group

Porphyrin selectively shows tumour accumulation and has attracted attention as a carrier molecule for drug delivery systems (DDS). Porphyrin has two functional sites termed the meso- and β-positions. In previous work, meso-porphyrin derivatives with an alkyl group were found to exhibit greater accumulation in human breast cancer cells (MCF-7). To identify the correlation between porphyrin accumulation and functional porphyrin positions of other functional groups, the accumulation of porphyrin derivatives with a phenyl group was investigated. The β-porphyrin derivative with a phenyl group showed higher accumulation in MCF-7 cells and greater affinity for albumin than the meso-porphyrin derivative. The results of density functional theory (DFT) calculations suggest that the β-porphyrin derivative with a phenyl group had higher planarity across the total structure than the meso-porphyrin derivative. It was concluded that the greater planarity of the β-porphyrin derivative with a phenyl group might lead to superior MCF-7 cell accumulation.

Transmembrane topology and oligomeric nature of an astrocytic membrane protein, MLC1

MLC1 is a membrane protein mainly expressed in astrocytes, and genetic mutations lead to the development of a leukodystrophy, megalencephalic leukoencephalopathy with subcortical cysts disease. Currently, the biochemical properties of the MLC1 protein are largely unknown. In this study, we aimed to characterize the transmembrane (TM) topology and oligomeric nature of the MLC1 protein. Systematic immunofluorescence staining data revealed that the MLC1 protein has eight TM domains and that both the N- and C-terminus face the cytoplasm. We found that MLC1 can be purified as an oligomer and could form a trimeric complex in both detergent micelles and reconstituted proteoliposomes. Additionally, a single-molecule photobleaching experiment showed that MLC1 protein complexes could consist of three MLC1 monomers in the reconstituted proteoliposomes. These results can provide a basis for both the high-resolution structural determination and functional characterization of the MLC1 protein.

Tracking the movement of discrete gating charges in a voltage-gated potassium channel

Positively charged amino acids respond to membrane potential changes to drive voltage sensor movement in voltage-gated ion channels, but determining the displacements of voltage sensor gating charges has proven difficult. We optically tracked the movement of the two most extracellular charged residues (R1 and R2) in the Shaker potassium channel voltage sensor using a fluorescent positively charged bimane derivative (qBBr) that is strongly quenched by tryptophan. By individually mutating residues to tryptophan within the putative pathway of gating charges, we observed that the charge motion during activation is a rotation and a tilted translation that differs between R1 and R2. Tryptophan-induced quenching of qBBr also indicates that a crucial residue of the hydrophobic plug is linked to the Cole-Moore shift through its interaction with R1. Finally, we show that this approach extends to additional voltage-sensing membrane proteins using the Ciona intestinalis voltage-sensitive phosphatase (CiVSP).

Rapid Estimation of Membrane Protein Orientation in Liposomes

The topological organization of proteins embedded in biological membranes is crucial for the tight interplay between these enzymes and their accessibility to substrates in order to fulfil enzymatic activity. The orientation of a membrane protein reconstituted in artificial membranes depends on many parameters and is hardly predictable. Here, we present a convenient approach to assess this important property independent of the enzymatic activity of the reconstituted protein. Based on cysteine‐specific chemical modification of a target membrane protein with a cyanine fluorophore and a corresponding membrane‐impermeable fluorescence quencher, the novel strategy allows rapid evaluation of the distribution of the two orientations after reconstitution. The assay has been tested for the respiratory complexes bo₃ oxidase and ATP synthase of Escherichia coli and the results agree well with other orientation determination approaches. Given the simple procedure, the proposed method is a powerful tool for optimization of reconstitution conditions or quantitative orientation information prior to functional measurements.

Crystal Structure of VpsR Revealed Novel Dimeric Architecture and c-di-GMP Binding Site: Mechanistic Implications in Oligomerization, ATPase Activity and DNA Binding

VpsR, the master regulator of biofilm formation in Vibrio cholerae, is an atypical NtrC1 type bEBP lacking residues essential for σ⁵⁴-RNAP binding and REC domain phosphorylation. Moreover, transcription from P_vpsL, a promoter of biofilm biosynthesis, has been documented in presence of σ⁷⁰-RNAP/VpsR/c-di-GMP complex. It was proposed that c-di-GMP and VpsR together form an active transcription complex with σ⁷⁰-RNAP. However, the impact of c-di-GMP imparted on VpsR that leads to transcription activation with σ⁷⁰-RNAP remained elusive, largely due to the lack of the structure of VpsR and knowledge about c-di-GMP:VpsR interactions. In this direction we have solved the crystal structure of VpsR^RA, containing REC and AAA⁺ domains, in apo, AMPPNP/GMPPNP and c-di-GMP bound states. Structures of VpsR^RA unveiled distinctive REC domain orientation that leads to a novel dimeric association and noncanonical ATP/GTP binding. Moreover, we have demonstrated that at physiological pH VpsR remains as monomer having no ATPase activity but c-di-GMP imparted cooperativity to convert it to dimer with potent activity. Crystal structure of c-di-GMP:VpsR^RA complex reveals that c-di-GMP binds near the C-terminal end of AAA⁺ domain. Trp quenching studies on VpsR^R, VpsR^A, VpsR^RA, VpsR^AD with c-di-GMP additionally demonstrated that c-di-GMP could potentially bind VpsR^D. We propose that c-di-GMP mediated tethering of VpsR^D with VpsR^A could likely favor generating the specific protein-DNA architecture for transcription activation.

Live-cell visualization of cytochrome c: a tool to explore apoptosis

Apoptosis dysfunction is associated with several malignancies, including cancer and autoimmune diseases. Apoptosis restoration could be an attractive therapeutic approach to those diseases. Mitochondrial outer membrane permeabilization is regarded as the point of no return in the 'classical' apoptosis triggering pathway. Cytoplasmic release of cytochrome c (cyt c), a mitochondrial electron transporter, is a prominent indicator of such critical step. Therefore, visualizing cyt c efflux in living cells is a convenient approach to address apoptosis triggering and monitor performance of apoptosis restoration strategies. Recent years have been prolific in the development of biosensors to visualize cyt c mitochondrial efflux in living cells, by fluorescence microscopy. These biosensors specifically detect endogenous, untagged cyt c, while showing efficient cellular uptake and reduced cell toxicity. A common aspect is their fluorescence quenching in the absence or presence of bound cyt c, resulting in two main biosensor types: 'turn ON' and 'turn OFF'. In some of these systems, fluorescence intensity of fluorophore-bound aptamers is enhanced upon cyt c binding. In others, cyt c binding to quantum dots quenches their fluorescence. In the present minireview, I describe these biosensors and briefly introduce some hypotheses that could be addressed using these novel tools.

Interaction Study between ESIPT Fluorescent Lipophile-Based Benzazoles and BSA

In this study, the interactions of ESIPT fluorescent lipophile-based benzazoles with bovine serum albumin (BSA) were studied and their binding affinity was evaluated. In phosphate-buffered saline (PBS) solution these compounds produce absorption maxima in the UV region and a main fluorescence emission with a large Stokes shift in the blue–green regions due to a proton transfer process in the excited state. The interactions of the benzazoles with BSA were studied using UV-Vis absorption and steady-state fluorescence spectroscopy. The observed spectral quenching of BSA indicates that these compounds could bind to BSA through a strong binding affinity afforded by a static quenching mechanism (K_q~10¹² L·mol⁻¹·s⁻¹). The docking simulations indicate that compounds 13 and 16 bind closely to Trp134 in domain I, adopting similar binding poses and interactions. On the other hand, compounds 12, 14, 15, and 17 were bound between domains I and III and did not directly interact with Trp134.

Effect of ascorbic acid on tyrosinase and its anti-browning activity in fresh-cut Fuji apple

Tyrosinase (polyphenol oxidase) is the key enzyme of enzymatic browning in fruits and vegetables. In this research, the impact of ascorbic acid on tyrosinase and its anti-browning effect on fresh-cut Fuji apple were investigated. Ascorbic acid had a dual effect on tyrosinase with a half inhibitory concentration (IC₅₀ ) of 13.40 ± 0.05 µM. Fluorescence assay demonstrated that ascorbic acid interacted with tyrosinase in a dynamic contaction caused by Förster's resonance energy transfer (FRET) and induced a conformational change of the enzyme. Thermodynamic analysis, copper interaction, and molecular docking further confirmed that ascorbic acid could chelate the copper ions located in active center and interact with amino acid residues of tyrosinase via hydrophobic interaction. In addition, ascorbic acid prevented the browning of fresh-cut apples by increasing APX activity and inhibiting PPO and POD activities which reduce the oxidation of total phenolics and flavonoids. PRACTICAL APPLICATIONS: The present study demonstrated that ascorbic acid had a strong inhibitory activity against tyrosinase (IC₅₀ = 13.40 ± 0.05 µM) and anti-browning activity against fresh-cut Fuji apple. It could delay the browning degree of apple juice, increase APX activity, inhibit PPO and POD activities, and reduce the oxidation of total phenolics and flavonoids. These findings provided a basis for the feasible application of ascorbic acid on the preservation of fruits.

Chromium (VI) detection by microbial carbon dots: Microwave synthesis and mechanistic study

Functionalized carbon dots (CDs) derived from Citrobacter freundii bacterial cells were used for selective detection of Cr(VI). A microwave-heating-based green synthesis approach is adopted to produce functionalized CDs from C. freundii bacterial cells (CF-CDs). The reaction was carried out in a 500 W microwave digester at 200°C for 20 min. The supernatant was filtered with a 0.2 µm filter and highly monodisperse CDs were obtained. Inherent functionalization of CF-CDs with nitrogen and oxygen-containing functional groups made them extremely selective toward Cr(VI) with a lower limit of detection of ~1.7 ppm. More importantly, CF-CDs could distinguish between Cr(VI) and Cr(III), which is highly desirable for practical applications. The fabricated sensor had a dual linear response range between 0 and 50 µM and 50-250 µM. The synthesized CDs were inherently functionalized which made them highly selective for Cr(VI) detection. CF-CDs also possess high stability over long storage period. This study reports the facile synthesis and characterization of a highly selective sensor for Cr(VI). As opposed to similar CDs sensors reported for Cr(VI), based on inner-filter effect, CF-CDs exhibit a strong, specific interaction toward Cr(VI), indicated by the large binding constant.

Ultrasensitive detection of UO2 2+ based on dopamine-functionalized MoOx quantum dots

Uranium is an important nuclear fuel and the risk of human exposure to uranium increases as increasing amounts of uranium-containing waste enter the environment due to the rapid growth of nuclear power. Therefore, rapid, sensitive, and portable uranium detection is a promising approach to effectively control and monitor uranium contamination. To achieve this goal, abundant oxygen- and nitrogen-containing groups were introduced to molybdenum oxide quantum dot (MoO_x QDs) surfaces with dopamine (DA) modification. Due to the excellent coordination ability of oxygen- and nitrogen-containing groups with uranium, the obtained DA-functionalized MoO_x QDs (DA-MoOx QDs) showed a strong binding affinity for uranium and sensitivity was increased nearly 1000-fold compared with MoO_x QDs alone. The limit of detection was 3.85 nM, which is higher than most of the reported nanomaterials. Moreover, the DA-MoOx QD-based method showed high selectivity and uranium could be clearly detected under masking with ethylenediaminetetraacetic acid even when the concentration of other metal ions was 100-fold higher than that of uranium, showing a very promising method for uranium contamination control and monitoring.

Resolving Cross-Sensitivity Effect in Fluorescence Quenching for Simultaneously Sensing Oxygen and Ammonia Concentrations by an Optical Dual Gas Sensor

Simultaneous sensing of multiple gases by a single fluorescent-based gas sensor is of utmost importance for practical applications. Such sensing is strongly hindered by cross-sensitivity effects. In this study, we propose a novel analysis method to ameliorate such hindrance. The trial sensor used here was fabricated by coating platinum(II) meso-tetrakis(pentafluorophenyl)porphyrin (PtTFPP) and eosin-Y dye molecules on both sides of a filter paper for sensing O₂ and NH₃ gases simultaneously. The fluorescent peak intensities of the dyes can be quenched by the analytes and this phenomenon is used to identify the gas concentrations. Ideally, each dye is only sensitive to one gas species. However, the fluorescent peak related to O₂ sensing is also quenched by NH₃ and vice versa. Such cross-sensitivity strongly hinders gas concentration detection. Therefore, we have studied this cross-sensitivity effect systematically and thus proposed a new analysis method for accurate estimation of gas concentration. Comparing with a traditional method (neglecting cross-sensitivity), this analysis improves O₂-detection error from −11.4% ± 34.3% to 2.0% ± 10.2% in a mixed background of NH₃ and N₂.

An AIE-Active Ultrathin Polymeric Self-Assembled Monolayer Sensor for Trace Volatile Explosive Detection

This work has prepared polymeric self-assembled monolayer (SAM) sensors for the detection of trace volatile nitroaromatic compound (NAC) explosives by fluorescence quenching. A typical aggregation-induced emission (AIE) luminogen 1,1,2,2-tetraphenylethene (TPE) polymerizes into PTPE to increase the fluorescence intensity in the SAMs, and the phosphoric acid acts as the anchor group to form stable covalent bonds with the Al₂ O₃ substrate. This design takes advantage of the high sensitivity and good stability of SAMs, and high fluorescence intensity, and "wire effect" of the conjugated polymers. The polymeric SAM sensors are prepared on the Al₂ O₃ silicon wafer and testing paper. Both of them show good response speed, reversibility, selectivity, and sensitivity. The detection limits down to 0.07, 0.35, and 4.11 ppm for TNT, DNB, and NB, respectively, are achieved on the inorganic testing paper. Furthermore, due to the higher fluorescence intensity by interlacing and overlapping of fibers, the detection of the paper can be distinguished by naked eyes even with a low-power handheld UV lamp, which provides an experimental basis for the development of cheap and easy trace NAC explosive sensors.

Tetraphenylethene Functionalized Polyhedral Oligomeric Silsesquioxane Fluorescent Probe for Rapid and Selective Trifluralin Sensing in Vegetables and Fruits

A novel fluorescent probe was designed and synthesized from tetraphenylethene (TPE) and polyhedral oligomeric silsesquioxanes (POSS) via Heck-palladium catalyzed cross-coupling reaction. The as-synthesized TPE functionalized probe performed good solvent stability and selectively preconcentration capability towards target analyte due to its stable structure and the adsorption property. The morphology as well as the physical and chemical properties of the POSS@TPE were carefully characterized. The POSS@TPE was employed to develop an effective fluorescent probe for trifluralin, with a response range of 0.1-80 mg/kg and a detection limit of 0.102 mg/kg. The mixed mechanisms of inner-filter effect (IFE) and photoinduced electron transfer (PET) explain the selectivity of POSS@TPE. Rapid detection for trifluralin in tomato and celery has been achieved with recoveries between 99.4-120.7% (RSD≤3.4%), and the results were verified compared with GC-MS method.

Molecularly Imprinted Silica-Coated CdTe Quantum Dots for Fluorometric Determination of Trace Chloramphenicol

A dual recognition system with a fluorescence quenching of quantum dots (QDs) and specific recognition of molecularly imprinted polymer (MIP) for the detection of chloramphenicol (CAP) was constructed. MIP@SiO₂@QDs was prepared by reverse microemulsion method with 3-aminopropyltriethoxysilane (APTS), tetraethyl orthosilicate (TEOS) and QDs being used as the functional monomer, cross-linker and signal sources, respectively. MIP can specifically recognize CAP, and the fluorescence of QDs can be quenched by CAP due to the photo-induced electron transfer reaction between CAP and QDs. Thus, a method for the trace detection of CAP based on MIP@SiO₂@QDs fluorescence quenching was established. The fluorescence quenching efficiency of MIP@SiO₂@QDs displayed a desirable linear response to the concentration of CAP in the range of 1.00~4.00 × 10² μmol × L⁻¹, and the limit of detection was 0.35 μmol × L⁻¹ (3σ, n = 9). Importantly, MIP@SiO₂@QDs presented good detection selectivity owing to specific recognition for CAP, and was successfully applied to quantify CAP in lake water with the recovery ranging 102.0~104.0%, suggesting this method has the promising potential for the on-site detection of CAP in environmental waters.

Evaluation of Bioactive Metabolites and Antioxidant-Rich Extracts of Amaranths with Possible Role in Pancreatic Lipase Interaction: In Silico and In Vitro Studies

Fat/carbohydrate-rich diet consumption or elevated secretion of pancreatic lipase (PL) in pancreatic injury results in increased fat digestion and storage. Several metabolites in plant-based diets can help achieve the requirements of nutrition and fitness together. Presently, nutritional metabolites from Amaranthus tricolor, A. viridis, and Achyranthes aspera were assessed and predicted for daily intake. The volatile-metabolite profiling of their extracts using GC-MS revealed various antioxidant and bioactive components. The implication of these specialized components and antioxidant-rich extracts (EC₅₀ free radical scavenging: 34.1 ± 1.5 to 166.3 ± 14.2 µg/mL; FRAP values: 12.1 ± 1.0 to 34.0 ± 2.0 µg Trolox Equivalent/mg) in lipolysis regulation by means of interaction with PL was checked by in silico docking (Betahistine and vitamins: ΔGbind -2.3 to -4.4 kcal/mol) and in vitro fluorescence quenching. Out of the various compounds and extracts tested, Betahistine, ATRA and AVLA showed better quenching the PL fluorescence. The identification of potential extracts as source of functional components contributing to nutrition and fat regulation can be improved through such study.

Mutational analysis to explore long-range allosteric couplings involved in a pentameric channel receptor pre-activation and activation

Pentameric ligand-gated ion channels (pLGICs) mediate chemical signaling through a succession of allosteric transitions that are yet not completely understood as intermediate states remain poorly characterized by structural approaches. In a previous study on the prototypic bacterial proton-gated channel GLIC, we generated several fluorescent sensors of the protein conformation that report a fast transition to a pre-active state, which precedes the slower process of activation with pore opening. Here, we explored the phenotype of a series of allosteric mutations, using simultaneous steady-state fluorescence and electrophysiological measurements over a broad pH range. Our data, fitted to a three-state Monod-Wyman-Changeux model, show that mutations at the subunit interface in the extracellular domain (ECD) principally alter pre-activation, while mutations in the lower ECD and in the transmembrane domain principally alter activation. We also show that propofol alters both transitions. Data are discussed in the framework of transition pathways generated by normal mode analysis (iModFit). It further supports that pre-activation involves major quaternary compaction of the ECD, and suggests that activation involves principally a reorganization of a ‘central gating region’ involving a contraction of the ECD β-sandwich and the tilt of the channel lining M2 helix.

Promoting Room Temperature Phosphorescence through Electron Transfer from Carbon Dots to Promethazine

Room temperature phosphorescence (RTP) as a fascinating phenomenon shows great potential toward multiple applications. Howbeit, it is challengeable to improve the phosphorescence efficiency of carbon dots (CDs) owing to their short lifetime. Herein, we proposed a facile, rapid, and gram-scale strategy to synthesize the cross-linked carbon dots (named N-CDs) with both bright blue fluorescence and green RTP emissions. To be specific, the polymer of polyethylenimine (PEI) served as the cross-linking agent and carbon source, during which process phosphoric acid accelerated the formation of the compact carbon core within 30 s. Subsequently, the cross-linked carbon dots with the rigid network formed a small singlet-triplet energy splitting (ΔE_ST) of 0.490 eV, thus exhibiting a long RTP lifetime of 429.880 ms while coated on the filter paper through the hydrogen bonds. Taking advantage of the double luminescence, we successfully achieved the dual-channel detection of promethazine by N-CDs. The fluorescence of N-CDs was obviously quenched by promethazine through the electron-transfer process, displaying the linear range from 0.4 to 8 mM. Significantly, the electron transfer (ET) from carbon dots to promethazine boosted their phosphorescence efficiency and prolonged the lifetime to 565.190 ms, and the enhanced phosphorescence facilitated the sensitive recognition of promethazine with the concentration range of 1-3000 μM. Meanwhile, the possible autofluorescence interference from biological samples could be avoided through this RTP assaying mode, providing the more accurate results. Also, their RTP and fluorescence endowed the current N-CDs with the ability of dual-signal painting and imaging. This strategy may broaden the new approaches to produce the long-lifetime and high-efficiency RTP material toward the sensing purpose.

Interaction between Curcumin and β-Casein: Multi-Spectroscopic and Molecular Dynamics Simulation Methods

Effect of temperature and pH on the interaction of curcumin with β-casein was explored by fluorescence spectroscopy, ultraviolet-visible spectroscopy and molecular dynamics simulation. The spectroscopic results showed that curcumin could bind to β-casein to form a complex which was driven mainly by electrostatic interaction. The intrinsic fluorescence of β-casein was quenched by curcumin through static quenching mechanism. The binding constants of curcumin to β-casein were 6.48 × 10⁴ L/mol (298 K), 6.17 × 10⁴ L/mol (305 K) and 5.73 × 10⁴ L/mol (312 K) at pH 2.0, which was greater than that (3.98 × 10⁴ L/mol at 298 K, 3.90 × 10⁴ L/mol at 305 K and 3.41 × 10⁴ L/mol at 312 K) at pH 7.4. Molecular docking study showed that binding energy of β-casein-curcumin complex at pH 2.0 (−7.53 kcal/mol) was lower than that at pH 7.4 (−7.01 kcal/mol). The molecular dynamics simulation study showed that the binding energy (−131.07 kJ/mol) of β-casein-curcumin complex was relatively low at pH 2.0 and 298 K. α-Helix content in β-casein was decreased and random coil content was increased in the presence of curcumin. These results can promote a deep understanding of interaction between curcumin and β-casein and provide a reference for improving the bioavailability of curcumin.