Evaluation of a novel pyridinium cation-linked styryl-based boronate probe for the detection of selected inflammation-related oxidants

Reactive oxygen and nitrogen species (RONS) are a range of chemical individuals produced by living cells that contribute to the proper functioning of organisms. Cells under oxidative and nitrative stress show excessive production of RONS (including hydrogen peroxide, H2O2, hypochlorous acid, HOCl, and peroxynitrite, ONOO-) which may result in a damage proteins, lipids, and genetic material. Thus, the development of probes for in vivo detection of such oxidants is an active area of research, focusing on molecular redox sensors, including boronate-caged fluorophores. Here, we report a boronate-based styryl probe with a cationic pyridinium moiety (BANEP+) for the fluorescent detection of selected biological oxidants in vitro and in vivo. We compare the chemical reactivity of the BANEP+ probe toward H2O2, HOCl, and ONOO- and examine the influence of the major intracellular non-enzymatic antioxidant molecule, glutathione (GSH). We demonstrate that, at the physiologically relevant GSH concentration, the BANEP+ probe is efficiently oxidized by peroxynitrite, forming its phenolic derivative HNEP+. GSH does not affect the fluorescence properties of the BANEP+ and HNEP+ dyes. Finally, we report the identification of a novel type of molecular marker, with the boronate moiety replaced by the iodine atom, formed from the probe in the presence of HOCl and iodide anion. We conclude that the reported chemical reactivity and structural features of the BANEP+ probe may be a basis for the development of new red fluorescent probes for in vitro and in vivo detection of ONOO-.

Illuminating the monoamine transporters: Fluorescently labelled ligands to study dopamine, serotonin and norepinephrine transporters

Fluorescence microscopy has revolutionized the visualization of physiological processes in live-cell systems. With the recent innovations in super resolution microscopy, these events can be examined with high precision and accuracy. The development of fluorescently labelled small molecules has provided a significant advance in understanding the physiological relevance of targeted proteins that can now be visualized at the cellular level. One set of physiologically important target proteins are the monoamine transporters (MATs) that play an instrumental role in maintaining monoamine signalling homeostasis. Understanding the mechanisms underlying their regulation and dysregulation is fundamental to treating several neuropsychiatric conditions such as attention deficit hyperactivity disorder (ADHD), anxiety, depression and substance use disorders. Herein, we describe the rationale behind the small molecule design of fluorescently labelled ligands (FLL) either as MAT substrates or inhibitors as well as their applications to advance our understanding of this class of transporters in health and disease.

Hybridization-Sensitive Fluorescent Probes for DNA and RNA by a Modular "Click" Approach

Fluorescent DNA probes were prepared in a modular approach using the “click” post-synthetic modification strategy. The new glycol-based module and DNA building block place just two carbons between the phosphodiester bridges and anchor the dye by an additional alkyne group. This creates a stereocenter in the middle of this artificial nucleoside substitute. Both enantiomers and a variety of photostable cyanine–styryl dyes as well as thiazole orange derivatives were screened as “clicked” conjugates in different surrounding DNA sequences. The combination of the (S)-configured DNA anchor and the cyanylated cyanine–styryl dye shows the highest fluorescence light-up effect of 9.2 and a brightness of approximately 11,000 M^–1 cm^–1. This hybridization sensitivity and fluorescence readout were further developed utilizing electron transfer and energy transfer processes. The combination of the hybridization-sensitive DNA building block with the nucleotide of 5-nitroindole as an electron acceptor and a quencher increases the light-up effect to 20 with the DNA target and to 15 with the RNA target. The fluorescence readout could significantly be enhanced to values between 50 and 360 by the use of energy transfer to a second DNA probe with commercially available dyes, like Cy3.5, Cy5, and Atto590, as energy acceptors at the 5′-end. The latter binary probes shift the fluorescent readout from the range of 500–550 nm to the range of 610–670 nm. The optical properties make these fluorescent DNA probes potentially useful for RNA imaging. Due to the strong light-up effect, they will not require washing procedures and will thus be suitable for live-cell imaging.

Nonlinear Dependence on Na+ Ions for the Binding Dynamics of Cucurbit[6]uril with the trans-1-Methyl-4-(4-hydroxystyryl)pyridinium Cation

The binding dynamics of the trans-1-methyl-4-(4-hydroxystyryl)pyridinium cation (HSP⁺) to cucurbit[6]uril (CB[6]) in the presence of Na⁺ cations were studied to establish the effect of the relative concentrations of the system's components (HSP⁺, CB[6], and Na⁺) on these dynamics. The formation of the HSP⁺@CB[6] complex was temporally uncoupled from the photoisomerization of trans-HSP⁺, while a nonlinear effect of the Na⁺ cation concentration on the HSP⁺@CB[6] dynamics was observed. This nonlinearity is a consequence of Na⁺ having the opposite effect on the association and dissociation rate constants for the HSP⁺@CB[6] complex, creating a conceptual framework for using such nonlinearities to control multistep reactions in cucurbit[n]uril chemistry.

A two-photon ratiometric fluorescent probe for highly selective sensing of mitochondrial cysteine in live cells

We report herein a two-photon ratiometric fluorescent probe (DNEPI) for mitochondrial cysteine (Cys) detection on the basis of a merocyanine (compound 1) as the two-photon fluorophore and a 2,4-dinitrobenzensulfonyl (DNBS) unit as the biothiol reaction site. Upon reaction with Cys in DMSO/PBS (1/1, v/v), DNEPI showed a distinct ratiometric fluorescence emission characteristic (F_{583 nm}/F_{485 nm}) linearly proportional to Cys concentrations over the range of 2-10 μM, which was attribute to the enhanced intramolecular charge transfer (ICT) effect by cleavage of the sulfonic acid ester bond of DNEPI to release compound 1. More importantly, the probe could detect Cys with a fast response time (within 2 min) and the detection limit was quantitatively calculated as 0.29 μM. Furthermore, DNEPI not only exhibited high selectivity toward Cys over other similar biothiols, including homocysteine (Hcy) and glutathione (GSH), but also displayed significant mitochondrial-targeting ability, which were favorable for mitochondrial Cys-selective imaging. Subsequently, application of DNEPI to Cys imaging in live cells was successfully achieved by two-photon fluorescence microscopy, suggesting that the probe proposed here could be used to monitor mitochondrial Cys concentration changes in live cells with negligible interference from other biological thiols.

Toward Serotonin Fluorescent False Neurotransmitters: Development of Fluorescent Dual Serotonin and Vesicular Monoamine Transporter Substrates for Visualizing Serotonin Neurons

Ongoing efforts in our laboratories focus on design of optical reporters known as fluorescent false neurotransmitters (FFNs) that enable the visualization of uptake into, packaging within, and release from individual monoaminergic neurons and presynaptic sites in the brain. Here, we introduce the molecular probe FFN246 as an expansion of the FFN platform to the serotonergic system. Combining the acridone fluorophore with the ethylamine recognition element of serotonin, we identified FFN54 and FFN246 as substrates for both the serotonin transporter and the vesicular monoamine transporter 2 (VMAT2). A systematic structure-activity study revealed the basic structural chemotype of aminoalkyl acridones required for serotonin transporter (SERT) activity and enabled lowering the background labeling of these probes while maintaining SERT activity, which proved essential for obtaining sufficient signal in the brain tissue (FFN246). We demonstrate the utility of FFN246 for direct examination of SERT activity and SERT inhibitors in 96-well cell culture assays, as well as specific labeling of serotonergic neurons of the dorsal raphe nucleus in the living tissue of acute mouse brain slices. While we found only minor FFN246 accumulation in serotonergic axons in murine brain tissue, FFN246 effectively traces serotonin uptake and packaging in the soma of serotonergic neurons with improved photophysical properties and loading parameters compared to known serotonin-based fluorescent tracers.

Dimalononitrile-containing probe based on aggregation-enhanced emission features for the multi-mode fluorescence detection of volatile amines

A novel multi-mode probe consisting of a hexaphenyl-1,3-butadiene derivative, 2,2′-((((1Z,3Z)-1,2,3,4-tetraphenylbuta-1,3-diene-1,4-diyl)bis(4,1-phenylene))bis(methanylylidene))dimalononitrile (ZZ–HPB–CN), with typical aggregation-enhanced emission (AEE) features was easily prepared for the highly sensitive and rapid detection of amine vapors. The ZZ–HPB–CN sensor, which was prepared by simply depositing ZZ–HPB–CN on filter paper, could detect low concentration vapors of volatile amines using fluorescence, ultraviolet and naked-eye detection. The limit of detection of the sensor was as low as 1 ppb for the fluorescence detection. The color change of the sensor caused by 1–10 ppm amine vapors could be observed under UV light or with the naked eye. The high sensitivity, quick response and easy operation of the probe give it great potential for real-life applications.

Binding-induced, turn-on fluorescence of the EGFR/ERBB kinase inhibitor, lapatinib

We report the photophysical properties, binding-induced turn-on emission, and fluorescence imaging of the cellular uptake and distribution of lapatinib, an EGFR/ERBB inhibitor. Lapatinib, a type II, i.e. inactive state, inhibitor that targets the ATP binding pocket of the EGFR family of receptor tyrosine kinases. DFT calculations predict that the 6-furanylquinazoline core of lapatinib should exhibit an excited state with charge transfer character and an S0 to S1 transition energy of 3.4 eV. Absorption confirms an optical transition in the near UV to violet, while fluorescence spectroscopy shows that photoemission is highly sensitive to solvent polarity. The hydrophobicity of lapatinib leads to fluorescent aggregates in solution, however, binding to the lipid-carrier protein, BSA or to the kinase domain of ERBB2, produces spectroscopically distinct photoemission. Confocal fluorescence microscopy imaging of lapatinib uptake in ERBB2-overexpressing MCF7 and BT474 cells reveals pools of intracellular inhibitor with emission profiles consistent with aggregated lapatinib.

A Novel Styryldehydropyridocolinium Homodimer: Synthesis and Fluorescence Properties Upon Interaction with DNA

A novel homodimer of the styryldehydropyridocolinium dye (TPTP) has been synthesized and characterized. Free TPTP exhibited low fluorescence quantum yield and large Stokes shift (over 160 nm) in water. However, it showed a significant fluorescence turn-on effect upon intercalation into DNA base pairs. Meanwhile, the fluorescence intensity of the intercalated structures formed by TPTP and DNA decreased quickly upon addition of deoxyribonuclease I, indicating that the dye can be used to monitor deoxyribonuclease I activity and DNA hydrolysis. Electrophoresis analysis revealed that the dye had intercalative binding to DNA and can potentially be used for DNA staining in electrophoresis. Thus, the innate nature of large Stokes shift and excellent fluorescence turn on effect upon interaction with DNA endue the dye with a wide range of applications.

Detection of amines with extended distyrylbenzenes by strip assays

We herein describe the synthesis and property evaluation of three novel aldehyde-substituted pentameric phenylenevinylenes carrying branched oligo(ethylene glycol) (swallowtail, Sw) substituents. The targets were synthesized by a combination of Heck coupling and Wittig or Horner reactions of suitable precursor modules. If the pentameric phenylenevinylene carries only two of these Sw substituents, it is no longer water-soluble. When six of the Sw substituents are attached, regardless of their position, the pentameric phenylenevinylenes are well water-soluble. The dialdehydes were investigated with respect to their amine-sensing capabilities both in water as well as in the solid state, sprayed onto thin layer chromatography (TLC) plates (alox, silica gel, reversed phase silica gel). The recognition of amine vapors using the sprayed-on phenylenevinylene dialdehydes is superb and allows the identification of different amines on regular silica TLC plates via color changes, analyzed by a statistical tool, the multivariate analysis of variance (MANOVA) protocol.

APP+, a fluorescent analogue of the neurotoxin MPP+, is a marker of catecholamine neurons in brain tissue, but not a fluorescent false neurotransmitter

We have previously introduced fluorescent false neurotransmitters (FFNs) as optical reporters that enable visualization of individual dopaminergic presynaptic terminals and their activity in the brain. In this context, we examined the fluorescent pyridinium dye 4-(4-dimethylamino)phenyl-1-methylpyridinium (APP+), a fluorescent analogue of the dopaminergic neurotoxin MPP+, in acute mouse brain tissue. APP+ is a substrate for the dopamine transporter (DAT), norepinephrine transporter (NET), and serotonin transporter (SERT), and as such represented a candidate for the development of new FFN probes. Here we report that APP+ labels cell bodies of catecholaminergic neurons in the midbrain in a DAT- and NET-dependent manner, as well as fine dopaminergic axonal processes in the dorsal striatum. APP+ destaining from presynaptic terminals in the dorsal striatum was also examined under the conditions inducing depolarization and exocytotic neurotransmitter release. Application of KCl led to a small but significant degree of destaining (approximately 15% compared to control), which stands in contrast to a nearly complete destaining of the new generation FFN agent, FFN102. Electrical stimulation of brain slices at 10 Hz afforded no significant change in the APP+ signal. These results indicate that the majority of the APP+ signal in axonal processes originates from labeled organelles including mitochondria, whereas only a minor component of the APP+ signal represents the releasable synaptic vesicular pool. These results also show that APP+ may serve as a useful probe for identifying catecholaminergic innervations in the brain, although it is a poor candidate for the development of FFNs.

Probing the functional limits of the norepinephrine transporter with self-reporting, fluorescent stilbazolium dimers

A series of stilbazolium dimers were synthesized and investigated as sterically demanding ligands targeting the norepinephrine transporter (NET). The environmentally sensitive fluorescence of the dyes enables their use as self-reporting ligands; binding to and displacement from NET can be monitored by fluorescence microscopy.

Fluorescent reporters of monoamine transporter distribution and function

Serotonin is a monoamine serving as a chemical messenger in diverse brain regions, as well as in blood and various other organs. We synthesized six ethylamine functionalized fluorophores as fluorescent probes for serotonin. The one with best spectral properties and aqueous solubility, 6-amino-2-(2-aminoethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione, was studied in detail both in vivo and in vitro. It was shown to act as a ligand for serotonin transporter (SERT) without acute cerebral or cardiovascular toxicity or adverse effects. Fluorescent serotonin analogs can be used for direct visualization of SERT distribution and activity in live tissue.