Low-dimensional compounds containing bioactive ligands. Part XVII: Synthesis, structural, spectral and biological properties of hybrid organic-inorganic complexes based on [PdCl4]2- with derivatives of 8-hydroxyquinolinium.

In this study, four hybrid organic-inorganic compounds (8-H2Q)2[PdCl4] (1), (H2ClQ)2[PdCl4] (2), (H2NQ)2[PdCl4] (3) and (H2MeQ)2[PdCl4]·2H2O (4) (where 8-H2Q = 8-hydroxyquinolinium, H2ClQ = 5-chloro-8-hydroxyquinolinium, H2NQ = 5-nitro-8-hydroxyquinolinium and H2MeQ = 2-methyl-8-hydroxyquinolinium) were synthesized through organic cation modulation. Single-crystal X-ray structure analysis of compounds 1 and 3 indicates that their structures are planar and consist of [PdCl4]2- anions and 8-H2Q or H2NQ cations, respectively. Both ionic components are held together through ionic interactions and hydrogen bonds forming infinite chains linked through π-π interactions to form 2D structures. Furthermore, NMR spectroscopy, UV-Vis spectroscopy, elemental analysis, and FT-IR spectroscopy were used to explore the synthesized compounds. The DNA interaction, antimicrobial activity, antiproliferative activity, and radical scavenging effect of the compounds were evaluated. The hybrid compounds and their free ligands can interact with the calf thymus DNA via an intercalation mode involving the insertion of the aromatic chromophore between the base pairs of DNA; compound 1 has the highest binding affinity. Moreover, they have high antimicrobial efficacy against the tested 14 strains of microorganisms with minimum inhibitory concentration values ranging from <1.95 to 250 µg/mL. The antiproliferative activity of the compounds was investigated against three different cancer cell lines, and their selectivity was verified on mesenchymal stem cells. Compounds 1 and 2 displayed selective and high cytotoxicity against human lung and breast cancer cells and showed moderate cytotoxicity against colon cancer cells. Accordingly, they might be auspicious candidates for future pharmacological investigations in lung and breast cancer research.

Photochemical synthesis, intercalation with DNA and antitumor evaluation in vitro of benzo[d]thiazolo[3,2-a]quinolin-10-ium derivatives.

A new anticancer benzo[d]thiazolo[3,2-a]quinolin-10-ium derivatives were synthesized and characterized. Anticancer evaluation in vitro against four cancer cell lines including adenocarcinomic human alveolar basal epithelial cells (A549), hepatocellular carcinoma (HepG2), prostate cancer (PC3) and breast cancer (MCF7) indicated that some of prepared compounds shows higher selectivity in comparison with doxorubicin. DNA interaction studies by optical, CD, NMR spectroscopies showed the high affinity of benzothiazole ligands towards the dsDNA. The ligand-DNA interaction occurs through the intercalation of benzo[d]thiazolo[3,2-a]quinolin-10-ium derivatives with nucleic acid. The investigation of formed ligand - DNA complexes by docking and molecular dynamic calculations was applied for analysis of the relationship between structure and anticancer activity. The results suggested that benzo[d]thiazolo[3,2-a]quinolin-10-ium derivatives might serve as a novel scaffold for the future development to new antitumor agents.

Efficient Regioselective Synthesis of Novel Water-Soluble 2H,3H-[1,4]thiazino[2,3,4-ij]quinolin-4-ium Derivatives by Annulation Reactions of 8-quinolinesulfenyl Halides.

Regioselective synthesis of novel 2H,3H-[1,4]thiazino[2,3,4-ij]quinolin-4-ium derivatives has been developed by annulation reactions of 8-quinolinesulfenyl halides with vinyl chalcogenides (vinyl ethers, divinyl sulfide, divinyl selenide and phenyl vinyl sulfide) and tetravinyl silane. The novel reagent 8-quinolinesulfenyl bromide was used in the annulation reactions. The influence of the substrate structure and the nature of heteroatoms on the direction of the reactions and on product yields has been studied. The opposite regiochemistry was observed in the reactions with vinyl chalcogenides and tetravinyl silane. The obtained condensed heterocycles are novel water-soluble functionalized compounds with promising biological activity.

Pyridinium-2-carbaldoximes with quinolinium carboxamide moiety are simultaneous reactivators of acetylcholinesterase and butyrylcholinesterase inhibited by nerve agent surrogates.

The pyridinium-2-carbaldoximes with quinolinium carboxamide moiety were designed and synthesised as cholinesterase reactivators. The prepared compounds showed intermediate-to-high inhibition of both cholinesterases when compared to standard oximes. Their reactivation ability was evaluated in vitro on human recombinant acetylcholinesterase (hrAChE) and human recombinant butyrylcholinesterase (hrBChE) inhibited by nerve agent surrogates (NIMP, NEMP, and NEDPA) or paraoxon. In the reactivation screening, one compound was able to reactivate hrAChE inhibited by all used organophosphates and two novel compounds were able to reactivate NIMP/NEMP-hrBChE. The reactivation kinetics revealed compound 11 that proved to be excellent reactivator of paraoxon-hrAChE better to obidoxime and showed increased reactivation of NIMP/NEMP-hrBChE, although worse to obidoxime. The molecular interactions of studied reactivators were further identified by in silico calculations. Molecular modelling results revealed the importance of creation of the pre-reactivation complex that could lead to better reactivation of both cholinesterases together with reducing particular interactions for lower intrinsic inhibition by the oxime.

Real-time imaging of alkaline phosphatase activity of diabetes in mice via a near-infrared fluorescent probe.

A novel water-soluble near-infrared fluorescent probe named QX-P with simple synthesis is developed. QX-P has high sensitivity and selectivity to ALP. Moreover, the probe can not only visualize ALP activity in four cell lines, but also real-time image ALP activity during the diagnosis and treatment of diabetes in mice.

Activatable fluorescent probe based on aggregation-induced emission for detecting hypoxia-related pathological conditions.

Hypoxia, as a condition in which a region of body has low oxygen tension, is closely related to a variety of pathological conditions, and in many diseases local hypoxia occurs that would further increase the severity of diseases. Hence the extent of hypoxia could reflect the related pathological conditions and diseases, and the detection of hypoxia is of great significance. In hypoxia, the elevated level of nitroreductase (NTR) usually occurs, which could serve as a biomarker for hypoxia and thus the related diseases. Herein, an activatable fluorescent probe TPAQS-NO2 based on aggregation-induced emission (AIE) was designed for hypoxia detection via responding to NTR. The probe consists of an electron acceptor quinolinium and an electron donor triphenylamine group. The activated probe shows a large Stokes shift (186 nm). The probe TPAQS-NO2 was successfully used for detecting the early-stage and the advanced-stage tumors via NTR detection in 4T1 tumor-bearing mouse model. Furthermore, the probe TPAQS-NO2 was applied for detecting NTR in the cerebral ischemia (CIS) mouse model. The probe could offer an effective approach for detecting hypoxia-related pathological conditions.

Design, Synthesis, and Dual Evaluation of Quinoline and Quinolinium Iodide Salt Derivatives as Potential Anticancer and Antibacterial Agents.

A series of novel quinoline and quinolinium iodide derivatives were designed and synthesized to discover potential anticancer and antibacterial agents. With regard to anticancer properties, in vitro cytotoxicities against three human cancer cell lines (A-549, HeLa and SGC-7901) were evaluated. The antibacterial properties against two strains, Escherichia coli (ATCC 29213) and Staphylococcus aureus (ATCC 8739), along with minimum inhibitory concentration (MIC) values were evaluated. The target alkyliodine substituted compounds exhibited significant antitumor and antibacterial activity, of which compound 8-((4-(benzyloxy)phenyl)amino)-7-(ethoxycarbonyl)-5-propyl-[1,3]dioxolo[4,5-g]quinolin-5-ium (12) was found to be the most potent derivative with IC50 values of 4.45±0.88, 4.74±0.42, 14.54±1.96, and 32.12±3.66 against A-549, HeLa, SGC-7901, and L-02 cells, respectively, stronger than the positive controls 5-FU and MTX. Furthermore, compound 12 had the most potent bacterial inhibitory activity. The MIC of this compound against both E. coli and S. aureus was 3.125 nmol ⋅ mL-1 , which was smaller than that against the reference agents amoxicillin and ciprofloxacin.

Multi-state amine sensing by electron transfers in a BODIPY probe.

Amines are ubiquitous in the chemical industry and are present in a wide range of biological processes, motivating the development of amine-sensitive sensors. There are many turn-on amine sensors, however there are no examples of turn-on sensors that utilize the amine's ability to react by single electron transfer (SET). We investigated a new turn-on amine probe with a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore. BODIPY fluorescence is first preprogrammed into an off state by internal photoinduced electron transfer (PET) to an electron-deficient quinolinium ring, resulting in fluorescence quenching. At low concentrations of aliphatic amine (0 to 10 mM), this PET pathway is shut down by external SET from the amine to the photoexcited charge-transfer state of the probe and the fluorescence is turned on. At high concentrations of amine (50 mM to 1 M), we observed collisional quenching of the BODIPY fluorescence. The probe is selective for aliphatic amines over aromatic amines, and aliphatic thiols or alcohols. The three molecular processes modulate the BODIPY fluorescence in a multi-mechanistic way with two of them producing a direct response to amine concentrations. The totality of the three molecular processes produced the first example of a multi-state and dose-responsive amine sensor.

A fluorescent probe based on aggregation-induced emission for hydrogen sulfide-specific assaying in food and biological systems.

A fluorescent probe based on a triphenylamine benzopyridine platform for hydrogen sulfide (H2S) assaying has been designed and synthesized. As a result of the H2S-triggered cleavage reaction, the disappearance of the quenching effect of dinitrophenyl and the increased hydrophobicity in a poor solvent lead to the aggregation-induced emission (AIE) effect; consequently an obvious 'turn-on' fluorescence signal can be observed in this process. The probe TPANF features high selectivity towards H2S, low detection limit (0.17 µM), and good photostability and biocompatibility. Moreover, it has been successfully utilized to monitor H2S in food samples to distinguish the extent of food deterioration and to identify the H2S concentration variation in living cells. In addition, endogenous H2S in HCT-116 xenograft tumor tissues was imaged by using this probe. The approach could provide useful insight for the development of other activatable AIE-based probes that are potentially helpful for specific assaying in food chemistry and biological systems.

Antibacterial evaluation and mode of action study of BIMQ, a novel bacterial cell division inhibitor.

The worldwide spreading of antibiotic resistant bacteria is currently an extremely serious health risk and therefore to develop new antibiotics is an urgent need. In the present study, the antibacterial activity of a new indolyl quinolinium compound and its underline mechanism were investigated. The compound shows an outstanding antibacterial activity against the tested Gram-positive bacteria. The MIC values are in the range of 1-4 µg/mL. The elongation of B. subtilis cells indicates that the compound can inhibit cell division effectively. In addition, the biochemical studies prove that the compound is able to disrupt FtsZ polymerization effectively through a stimulatory mechanism. Furthermore, the compound can delay the development of drug resistance mutants.

Tuning the selectivity of N-alkylated styrylquinolinium dyes for sensing of G-quadruplex DNA.

Selective and sensitive detection of G-quadruplex DNA structures is an important issue and attracts extensive interest. To this end, numerous small molecular fluorescent probes have been designed. Here, we present a series of N-alkylated styrylquinolinium dyes named Ls-1, Ls-2 and Ls-3 with varying side groups at the chain end. We found that these dyes exhibited different binding behaviors to DNAs, and Ls-2 with a sulfonato group at the chain end displayed sensitivity and selectivity to G-quadruplex DNA structures in vitro. The characteristics of this dye and its interaction with G-quadruplex DNA were comprehensively investigated by means of UV-vis spectrophotometry, fluorescence, circular dichroism and molecular docking. Furthermore, confocal fluorescence images and MTT assays indicated dye Ls-2 could pass through membrane and enter the living HepG2 cells with low cytotoxicity.

Dicyanoisophorone-Based Near-Infrared-Emission Fluorescent Probe for Detecting NAD(P)H in Living Cells and in Vivo.

NADH and NADPH are ubiquitous coenzymes in all living cells that play vital roles in numerous redox reactions in cellular energy metabolism. To accurately detect the distribution and dynamic changes of NAD(P)H under physiological conditions is essential for understanding their biological functions and pathological roles. In this work, we developed a near-infrared (NIR)-emission fluorescent small-molecule probe (DCI-MQ) composed of a dicyanoisophorone chromophore conjugated to a quinolinium moiety for in vivo NAD(P)H detection. DCI-MQ has the advantages of high water solubility, a rapid response, extraordinary selectivity, great sensitivity (a detection limit of 12 nM), low cytotoxicity, and NIR emission (660 nm) in response to NAD(P)H. Moreover, the probe DCI-MQ was successfully applied for the detection and imaging of endogenous NAD(P)H in both living cells and tumor-bearing mice, which provides an effective tool for the study of NAD(P)H-related physiological and pathological processes.

Genetic Engineering of an Artificial Metalloenzyme for Transfer Hydrogenation of a Self-Immolative Substrate in Escherichia coli's Periplasm.

Artificial metalloenzymes (ArMs), which combine an abiotic metal cofactor with a protein scaffold, catalyze various synthetically useful transformations. To complement the natural enzymes' repertoire, effective optimization protocols to improve ArM's performance are required. Here we report on our efforts to optimize the activity of an artificial transfer hydrogenase (ATHase) using Escherichia coli whole cells. For this purpose, we rely on a self-immolative quinolinium substrate which, upon reduction, releases fluorescent umbelliferone, thus allowing efficient screening. Introduction of a loop in the immediate proximity of the Ir-cofactor afforded an ArM with up to 5-fold increase in transfer hydrogenation activity compared to the wild-type ATHase using purified mutants.

A Comparative Study on High Selectivities of Human Telomeric Dimeric G-Quadruplexes by Dimeric G-Quadruplex Binders.

Three new polyether-tethered bisquinolinium dimers (3 a-c) were synthesized, and their binding affinities, selectivities, and thermal stabilization towards dimeric G-quadruplex DNA (G2T1) in human telomeric regions were studied. The bisquinolinium dimer with a medium-length polyether linker (3 b) showed 30-425-fold higher binding affinity and selectivity towards antiparallel G2T1 than towards monomeric quadruplexes, which included human telomeric monomeric G-quadruplexes (G1), c-kit 1, c-kit 2, and c-myc. In addition, compound 3 b induced the formation of quadruplexes and displayed the highest level of thermal stabilization (ΔTm >28.1 °C) among all reported multimeric G-quadruplex binders. Compound 3 b also displayed a higher selectivity towards antiparallel G2T1 than monomer 360 A and bisquinolinium dimers 3 a and c. In contrast with our recent research on the analogous berberine dimer 1 b and dinickel-salphen complex 2 c, polyether linkers and their monomeric G-quadruplex binders in these dimeric G-quadruplex binders play a crucial role in regulating the binding affinities, selectivities, and thermal stabilization towards G2T1. More interestingly, these dimeric G-quadruplex compounds bind through end-stacking with the two adjacent G-quadruplex units in G2T1, and they showed high selectivity towards antiparallel G2T1 rather than mixed-type G2T1. In addition, compound 3 b, which displayed high selectivity towards antiparallel G2T1, showed strong telomerase inhibition and potent anticancer activities against HeLa and MCF-7 cells.

Lead optimization-hit expansion of new asymmetrical pyridinium/quinolinium compounds as choline kinase α1 inhibitors.

AIM: Choline kinase α inhibitors represent one of the newest classes of cytotoxic drugs for cancer treatment, since aberrant choline metabolism is a characteristic shared by many human cancers. RESULTS: Here, we present a new class of asymmetrical pyridinium/quinolinium derivatives developed and designed based on drug optimization. CONCLUSION: Among all compounds described here, compound 8, bearing a 7-chloro-4N-methyl-p-chloroaniline quinolinium moiety, exhibited the greatest inhibitory activity at the enzyme (IC50 = 0.29 µM) and antiproliferative activity in cellular assays (GI50 = 0.29-0.92 µM). Specifically, compound 8 strongly induces a cell-cycle arrest in G1 phase, but it does not significantly induce apoptosis while causing senescence in the MDA-MB-231 cell line.

Responsive Fluorescence Probe for Selective and Sensitive Detection of Hypochlorous Acid in Live Cells and Animals.

The development of effective bioanalytical methods for rapid, sensitive and specific detection of HOCl in vitro and in vivo plays a key role for better understanding the roles of this molecule in normal and diseased conditions, but remains challenging due to the highly reactive nature of HOCl and the complicated biological conditions. In this work, a new fluorescence probe, PQI, was developed for monitoring of the HOCl level in biological samples. PQI was easily synthesized by a one-step condensation reaction. Upon addition of HOCl, significant changes in the absorption spectra and the color of the solution were noticed, facilitating the "naked eye" detection of HOCl in PBS buffer. The fluorescence of PQI was found to be significantly increased within a few seconds, leading to "OFF-ON" fluorescence response towards HOCl. The sensing mechanism, oxidation of thioether by HOCl, was confirmed by HRMS titration analysis. PQI features a large Stokes shift, high sensitivity and selectivity, and rapid fluorescence response towards HOCl. Quantitative detection of HOCl in single live cells was demonstrated through fluorescence imaging and flow cytometry analysis. PQI was then successfully used in visualisation of HOCl in live zebrafish and nude mice.

Rational design of carbamate-based dual binding site and central AChE inhibitors by a "biooxidisable" prodrug approach: Synthesis, in vitro evaluation and docking studies.

Herein, we report a new class of dual binding site AChE inhibitor 4 designed to exert a central cholinergic activation thanks to a redox-activation step of a prodrug precursor 3. Starting from potent pseudo-irreversible quinolinium salts AChE inhibitors 2 previously reported, a new set of diversely substituted quinolinium salts 2a-p was prepared and assayed for their inhibitory activity against AChE. Structure-activity relationship (SAR) analysis of 2a-p coupled with molecular docking studies allowed us to determine which position of the quinolinium scaffold may be considered to anchor the phtalimide fragment presumed to interact with the peripheral anionic site (PAS). In addition, molecular docking provided insight on the linker length required to connect both quinolinium and phatlimide moieties without disrupting the crucial role of quinolinium salt moiety within the catalytic active site (CAS); namely placing the carbamate in the correct position to trigger carbamylation of the active-site serine hydroxyl. Based on this rational design, the putative dual binding site inhibitor 4 and its prodrug 3 were synthesized and subsequently evaluated in vitro against AChE. Pleasingly, whereas compound 4 showed to be a highly potent inhibitor of AChE (IC50 = 6 nM) and binds to AChE-PAS to the same extent as donepezil, its prodrug 3 revealed to be inactive (IC50 > 10 µM). These preliminary results constitute one of the few examples of carbamate-based dual binding site AChE inhibitors.

A Unified Strategy for the Syntheses of the Isoquinolinium Alkaloids Berberine, Coptisine, and Jatrorrhizine.

Total syntheses of the antibacterial alkaloids berberine, coptisine, and jatrorrhizine have been achieved in four steps through a unified route. The key step of this strategy is an efficient intramolecular Friedel-Crafts alkoxyalkylation which, following oxidation, establishes the isoquinolinium core of these natural products. Herein, the design and development of this synthetic strategy, which has enabled the shortest and most efficient syntheses of these alkaloids reported to date, is described.

Topology-Selective, Fluorescent "Light-Up" Probes for G-Quadruplex DNA Based on Photoinduced Electron Transfer.

Six novel probes were prepared by covalent attachment of a G4-DNA ligand (bis(quinolinium) pyridodicarboxamide; PDC) to various coumarin or pyrene fluorophores. In the absence of DNA, the fluorescence of all probes is quenched due to intramolecular photoinduced electron transfer (PET), as evidenced by photophysical and electrochemical studies, molecular modeling, and DFT calculations. All probes demonstrate similarly high thermal stabilization of various G4-DNA substrates belonging to different folding topologies, as assessed by fluorescence melting experiments; however, their fluorimetric response is strongly heterogeneous with respect to the structures of the probes and G4-DNA targets. Thus, the probes containing the 7-diethylaminocoumarin fluorophore demonstrate significant fluorescence enhancement in the presence of G4-DNA, with the strongest "light-up" response (20- to 180-fold) observed for antiparallel G4 structures as well as for hybrid G4 structures, formed by the variants of human telomeric sequence and capable of a conformation change to the antiparallel isoform. These results shed light on the influence of the linker and electronic properties of fluorophores on the efficiency of G4-DNA "light-up" probes operating via PET.

Synthesis and Antimicrobial Activity of Sulfur Derivatives of Quinolinium Salts.

A novel method for cleavage of the dithiine ring in 5,12-(dimethyl)-thioqinantrenium bis-chloride 1 "via" reaction with sodium hydrosulfide leads to 1-methyl-3-mercaptoquinoline-4(1H)-thione 2. Further transformation of thiol and thione functions of compound 2 leads to a series of sulfide and disulfide derivatives of quinolinium salts 4 and 6. 1-Methyl-4-chloro-3-benzylthioquinoline chloride 8 was obtained by N-alkylating 4-chloro-3-benzylthioquinoline using dimethyl sulfate. Antimicrobial activity of the obtained compounds was investigated using six Gram-positive and six Gram-negative bacterial strains, as well as Candida albicans yeast. Greater activity was demonstrated towards Gram-positive strains. MIC values for compounds and with benzylthio 4d and benzoylthio 4f substituents in 3-quinoline position were found to be in the 0.5-1 µg/mL range, at a level similar to that of ciprofloxacin (reference). Compounds 4d and 4f also demonstrated interesting antifungal properties (MIC = 1).