Prodigiosins uncouple lysosomal vacuolar-type ATPase through promotion of H+/Cl- symport

Multistate Redox-Switchable Ion Transport Using Chalcogen-Bonding Anionophores

Synthetic supramolecular transmembrane anionophores have emerged as promising anticancer chemotherapeutics. However, key to their targeted application is achieving spatiotemporally controlled activity. Herein, we report a series of chalcogen-bonding diaryl tellurium-based transporters in which their anion binding potency and anionophoric activity are controlled through reversible redox cycling between Te oxidation states. This unprecedented in situ reversible multistate switching allows for switching between ON and OFF anion transport and is crucially achieved with biomimetic chemical redox couples.

Halide-selective, proton-coupled anion transport by phenylthiosemicarbazones

Phenylthiosemicarbazones (PTSCs) are proton-coupled anion transporters with pH-switchable behaviour known to be regulated by an imine protonation equilibrium. Previously, chloride/nitrate exchange by PTSCs was found to be inactive at pH 7.2 due to locking of the thiourea anion binding site by an intramolecular hydrogen bond, and switched ON upon imine protonation at pH 4.5. The rate-determining process of the pH switch, however, was not examined. We here develop a new series of PTSCs and demonstrate their conformational behaviour by X-ray crystallographic analysis and pH-switchable anion transport properties by liposomal assays. We report the surprising finding that the protonated PTSCs are extremely selective for halides over oxyanions in membrane transport. Owing to the high chloride over nitrate selectivity, the pH-dependent chloride/nitrate exchange of PTSCs originates from the rate-limiting nitrate transport process being inhibited at neutral pH.

Anion Recognition through Multivalent C-H Hydrogen Bonds: Anion-Induced Foldamer Formation and Transport across Phospholipid Membranes

A series of triazole-cyanostilbene receptors were designed and synthesized. The receptor binds with the anions through various CH···anion hydrogen bonding interactions, where strong binding was observed for SO₄^2- anions followed by Cl^-, Br^-, NO₃^-, and I^-, calculated from the ¹H NMR titration experiment. The NOESY NMR experiment of the receptor confirmed the formation of anion-induced folded conformation. The CH···anion hydrogen bonding interaction-mediated anion recognition and foldamer formation were further confirmed from geometry optimization studies of the anion-bound complex. The receptor transports Cl^- anions efficiently compared to SO₄^2- anions across the lipid bilayer membrane via a mobile carrier mechanism.

Highly Active Halogen Bonding and Chalcogen Bonding Chloride Transporters with Non-Protonophoric Activity

Synthetic anion transporters show much promise as potential anti-cancer agents and therapeutics for diseases associated with mis-regulation of protein anion channels. In such applications high activity and anion selectivity are crucial to overcome competing proton or hydroxide transport which dissipates cellular pH gradients. Here, highly active bidentate halogen bonding and chalcogen bonding anion carriers based on electron deficient iodo- and telluromethyl-triazole derivatives are reported. Anion transport experiments in lipid bilayer vesicles reveal record nanomolar chloride transport activity for the bidentate halogen bonding anion carrier, and remarkably high chloride over proton/hydroxide selectivity for the chalcogen bonding anionophore. Computational studies provide further insight into the role of sigma-hole mediated anion recognition and desolvation at the membrane interface. Comparison with hydrogen bonding analogues demonstrates the importance of employing sigma-hole donor motifs in synthetic anionophores for achieving both high transport activity and selectivity.

Biotechnological Activities and Applications of Bacterial Pigments Violacein and Prodigiosin

In this review, we discuss violacein and prodigiosin, two chromogenic bacterial secondary metabolites that have diverse biological activities. Although both compounds were "discovered" more than seven decades ago, interest into their biological applications has grown in the last two decades, particularly driven by their antimicrobial and anticancer properties. These topics will be discussed in the first half of this review. The latter half delves into the current efforts of groups to produce these two compounds. This includes in both their native bacterial hosts and heterogeneously in other bacterial hosts, including discussing some of the caveats related to the yields reported in the literature, and some of the synthetic biology techniques employed in this pursuit.

Engineering of stimuli-responsive lipid-bilayer membranes using supramolecular systems

The membrane proteins found in nature control many important cellular functions, including signal transduction and transmembrane ion transport, and these, in turn, are regulated by external stimuli, such as small molecules, membrane potential and light. Membrane proteins also find technological applications in fields ranging from optogenetics to synthetic biology. Synthetic supramolecular analogues have emerged as a complementary method to engineer functional membranes. This Review describes stimuli-responsive supramolecular systems developed for the control of ion transport, signal transduction and catalysis in lipid-bilayer-membrane systems. Recent advances towards achieving spatio-temporal control over activity in artificial and living cells are highlighted. Current challenges, the scope, limitations and future potential to exploit supramolecular systems for engineering stimuli-responsive lipid-bilayer membranes are discussed.

Transmembrane anion transport mediated by halogen bonding and hydrogen bonding triazole anionophores

Transmembrane ion transport by synthetic anionophores is typically achieved using polar hydrogen bonding anion receptors. Here we show that readily accessible halogen and hydrogen bonding 1,2,3-triazole derivatives can efficiently mediate anion transport across lipid bilayer membranes with unusual anti-Hofmeister selectivity. Importantly, the results demonstrate that the iodo-triazole systems exhibit the highest reported activity to date for halogen bonding anionophores, and enhanced transport efficiency relative to the hydrogen bonding analogues. In contrast, the analogous fluoro-triazole systems, which are unable to form intermolecular interactions with anions, are inactive. The halogen bonding anionophores also exhibit a remarkable intrinsic chloride over hydroxide selectivity, which is usually observed only in more complex anionophore designs, in contrast to the readily accessible acyclic systems reported here. This highlights the potential of iodo-triazoles as synthetically accessible and versatile motifs for developing more efficient anion transport systems. Computational studies provide further insight into the nature of the anion-triazole intermolecular interactions, examining the origins of the observed transport activity and selectivity of the systems, and revealing the role of enhanced charge delocalisation in the halogen bonding anion complexes.

Halogen and hydrogen bonding 1,2,3-triazole derivatives efficiently mediate anion transport across lipid bilayer membranes with unusual anion selectivity profiles.

A Glutathione Activatable Ion Channel Induces Apoptosis in Cancer Cells by Depleting Intracellular Glutathione Levels

Cancer cells use elevated glutathione (GSH) levels as an inner line of defense to evade apoptosis and develop drug resistance. In this study, we describe a novel 2,4-nitrobenzenesulfonyl (DNS) protected 2-hydroxyisophthalamide system that exploits GSH for its activation into free 2-hydroxyisophthalamide forming supramolecular M⁺ /Cl^- channels. Better permeation of the DNS protected compound into MCF-7 cells compared to the free 2-hydroxyisophthalamide and GSH-activatable ion transport resulted in higher cytotoxicity, which was associated with increased oxidative stress that further reduced the intracellular GSH levels and altered mitochondrial membrane permeability leading to the induction of the intrinsic apoptosis pathway. The GSH-activatable transport-mediated cell death was further validated in rat insulinoma cells (INS-1E); wherein the intracellular GSH levels showed a direct correlation to the resulting cytotoxicity. Lastly, the active compound was found to restrict the growth and proliferation of 3D spheroids of MCF-7 cells with efficiency similar to that of the anticancer drug doxorubicin.

Transmembrane H+/Cl− cotransport activity of bis(amido)imidazole receptors

Bis(amide) appended imidazole having a sickle-shaped trivalent hydrogen-bonding structure reported as a transmembrane H⁺/Cl⁻ symporter.

A step-wise synthetic approach is necessary to access γ-conjugates of folate: folate-conjugated prodigiosenes

Despite the vast literature that describes reacting folic acid with a pharmacophore, this route is ineffective in providing the correct regioisomer of the resulting conjugate. We herein present a step-wise route to the preparation of nine folate conjugates of the tripyrrolic prodigiosene skeleton. The strict requirement for step-wise construction of the folate core is demonstrated, so as to achieve conjugation at only the desired γ-carboxylic acid and thus maintain the α-carboxylic site for folate receptor (FRα) recognition. Linkages via ethylenediamine, polyethylene glycol and glutathione are demonstrated.

Despite the vast literature that describes reacting folic acid with a pharmacophore, this route is ineffective in providing the correct regioisomer of the resulting conjugate.

Anion-Transport Mechanism of a Triazole-Bearing Derivative of Prodigiosine: A Candidate for Cystic Fibrosis Therapy

Cystic fibrosis (CF) is a genetic lethal disease, originated from the defective function of the CFTR protein, a chloride and bicarbonate permeable transmembrane channel. CF mutations affect CFTR protein through a variety of molecular mechanisms which result in different functional defects. Current therapeutic approaches are targeted to specific groups of patients that share a common functional defect. We seek to develop an innovative therapeutic approach for the treatment of CF using anionophores, small molecules that facilitate the transmembrane transport of anions. We have characterized the anion transport mechanism of a synthetic molecule based on the structure of prodigiosine, a red pigment produced by bacteria. Anionophore-driven chloride efflux from large unilamellar vesicles is consistent with activity of an uniporter carrier that facilitates the transport of anions through lipid membranes down the electrochemical gradient. There are no evidences of transport coupling with protons. The selectivity sequence of the prodigiosin inspired EH160 ionophore is formate > acetate > nitrate > chloride > bicarbonate. Sulfate, phosphate, aspartate, isothionate, and gluconate are not significantly transported by these anionophores. Protonation at acidic pH is important for the transport capacity of the anionophore. This prodigiosin derived ionophore induces anion transport in living cells. Its low toxicity and capacity to transport chloride and bicarbonate, when applied at low concentration, constitute a promising starting point for the development of drug candidates for CF therapy.

Photo-induced anticancer activity and singlet oxygen production of prodigiosenes

The photo-induced cytotoxicity of prodigiosenes is reported. One prodigiosene represents a synthetic analogue of the natural product prodigiosin, and two are conjugated to molecules that target the estrogen receptor (ER). A comparison of incubation and irradiation frameworks for the three prodigiosenes is reported, with activity against ER- and ER+ lines explored. Furthermore, the ability of the three prodigiosenes to photosensitise the production of singlet oxygen is demonstrated, shedding mechanistic light onto possible photodynamic therapeutic effects of this class of tripyrroles.

Prodigiosenes conjugated to tamoxifen and estradiol

We report the synthesis of the first click-appended prodigiosene conjugates. Four prodigiosene conjugates of estradiol functionalised at the 7α-position were prepared, as were three prodigiosene conjugates of tamoxifen. The coupling between a prodigiosene and an 11-hydroxy estradiol derivative via an ether linkage was investigated, as was the 11- and 7-functionalisation of the estradiol core. The robustness of estradiol protecting groups was severely challenged by reactions typically used to equip such frameworks for 11- and 7-functionalisation. Specifically, and important to synthesis involving estradiol, TBS, TMS and THP are not useful protecting groups for the functionalisation of this core. When the chemical features of the therapeutic agent limit the choice of protecting group (in this case, prodigiosenes bearing aryl, NH, alkenyl and ester groups), click chemistry becomes an attractive synthetic strategy. The anti-cancer activity of the seven click prodigiosene conjugates was evaluated.

Prodigiosin R2, a new prodigiosin from the roseophilin producer Streptomyces griseoviridis 2464-S5

Roseophilin (2) is a unique prodigiosin-related compound produced by Streptomyces griseoviridis 2464-S5, and is characterized by a central furan ring and a bicyclic alkyl chain. During a search for biosynthetic intermediates of 2, a new metabolite designated prodigiosin R2 (1) was isolated from the culture of the roseophilin producer. The molecular formula of 1 was established as C₂₇H₃₅N₃O by high-resolution FAB-MS. The structure of 1 was determined by NMR spectroscopic analyses as a prodigiosin derivative with the same bicyclic alkyl chain as 2. Prodigiosin R2 (1) showed potent cytotoxicity against HeLa human cervical carcinoma cells and HT1080 human fibrosarcoma cells with IC₅₀s of 0.41 and 0.82 μM, respectively.

Indole-based perenosins as highly potent HCl transporters and potential anti-cancer agents

Prodigiosin is one of the most potent anion transporters in lipid bilayer membranes reported to date. Inspired by the structure of this natural product, we have recently designed and synthesised a new class of H⁺/Cl⁻ cotransporters named ‘perenosins’. Here we report a new library of indole-based perenosins and their anion transport properties. The new transporters demonstrated superior transmembrane transport efficiency when compared to other indole-based transporters, due to favourable encapsulating effects from the substituents on the perenosin backbone. Anion transport assays were used to determine the mechanism of chloride transport revealing that the compounds function as ‘strict’ HCl cotransporters. Cell viability studies showed that some compounds specifically trigger late-onset cell death after 72 h with a unique correlation to the position of alkyl chains on the perenosins. Further investigations of cell death mechanism showed a mixture of cell cycle arrest and apoptosis was responsible for the observed decrease in cell viability.

A synthetic ion transporter that disrupts autophagy and induces apoptosis by perturbing cellular chloride concentrations

Perturbations in cellular chloride concentrations can affect cellular pH and autophagy and lead to the onset of apoptosis. With this in mind, synthetic ion transporters have been used to disturb cellular ion homeostasis and thereby induce cell death; however, it is not clear whether synthetic ion transporters can also be used to disrupt autophagy. Here, we show that squaramide-based ion transporters enhance the transport of chloride anions in liposomal models and promote sodium chloride influx into the cytosol. Liposomal and cellular transport activity of the squaramides is shown to correlate with cell death activity, which is attributed to caspase-dependent apoptosis. One ion transporter was also shown to cause additional changes in lysosomal pH, which leads to impairment of lysosomal enzyme activity and disruption of autophagic processes. This disruption is independent of the initiation of apoptosis by the ion transporter. This study provides the first experimental evidence that synthetic ion transporters can disrupt both autophagy and induce apoptosis.

Pyrrole N-H Anion Complexes

Synthetic pyrrole-based anion receptors date back to the 1990s. They have been extensively developed in the context of macrocyclic systems as expanded porphyrins and calixpyrroles, and related systems. The chemistry of open-chain pyrrolic systems is, in many respects, no less venerable. It also has more direct analogy to naturally occurring pyrrole-based anion binding motifs. However, it has not been the subject of a comprehensive review. Presented herein is a summary of efforts devoted to the creation of de novo pyrrole-based receptors, as well as the anion recognition chemistry of naturally occurring pyrrolic systems as prodigiosins and their synthetic analogues.

A synthetic ion transporter that disrupts autophagy and induces apoptosis by perturbing cellular chloride concentrations

Perturbations in cellular chloride concentrations can affect cellular pH and autophagy and lead to the onset of apoptosis. With this in mind, synthetic ion transporters have been used to disturb cellular ion homeostasis and thereby induce cell death; however, it is not clear whether synthetic ion transporters can also be used to disrupt autophagy. Here, we show that squaramide-based ion transporters enhance the transport of chloride anions in liposomal models and promote sodium chloride influx into the cytosol. Liposomal and cellular transport activity of the squaramides is shown to correlate with cell death activity, which is attributed to caspase-dependent apoptosis. One ion transporter was also shown to cause additional changes in lysosomal pH, which leads to impairment of lysosomal enzyme activity and disruption of autophagic processes. This disruption is independent of the initiation of apoptosis by the ion transporter. This study provides the first experimental evidence that synthetic ion transporters can disrupt both autophagy and induce apoptosis.

Optical properties of prodigiosin and obatoclax: action spectroscopy and theoretical calculations

Prodiginine molecules (prodigiosin and obatoclax) are well-known pH-chromic dyes with promising anti-tumor properties. They present multiple tautomeric and rotameric forms. The protonation state and the structure of such flexible ligands in interaction with a protein are crucial to understand and to model the protein's biological activities. The determination of the protonation state via UV/vis absorption is possible if the ligand spectra of the neutral and protonated states are sufficiently different, and also if we can eliminate other factors potentially impacting the spectrum. Upon measuring the absorption spectra of the ligand in solution, varying solvents and pH values, we have determined that the optical properties of prodigiosin and obatoclax depend on the protonation state and not on the solvent permittivity constant. In parallel, action spectroscopy (using tunable lasers coupled to ion traps) in the gas phase of protonated and sodiated prodigiosin and obatoclax molecules has been performed to evaluate the sensitivity of the charge and the conformational state to their optical properties free of solvent. The spectra are interpreted using computational simulations of molecular structures and electronic excitations. The excitation energies are only slightly sensitive to various isomerizations, and may be used to distinguish between protonated and deprotonated states, even in the presence of a sodium counter-ion.

pH-Regulated Nonelectrogenic Anion Transport by Phenylthiosemicarbazones

Gated ion transport across biological membranes is an intrinsic process regulated by protein channels. Synthetic anion carriers (anionophores) have potential applications in biological research; however, previously reported examples are mostly nonspecific, capable of mediating both electrogenic and electroneutral (nonelectrogenic) transport processes. Here we show the transmembrane Cl(-) transport studies of synthetic phenylthiosemicarbazones mimicking the function of acid-sensing (proton-gated) ion channels. These anionophores have remarkable pH-switchable transport properties with up to 640-fold increase in transport efficacy on going from pH 7.2 to 4.0. This "gated" process is triggered by protonation of the imino nitrogen and concomitant conformational change of the anion-binding thiourea moiety from anti to syn. By using a combination of two cationophore-coupled transport assays, with either monensin or valinomycin, we have elucidated the fundamental transport mechanism of phenylthiosemicarbazones which is shown to be nonelectrogenic, inseparable H(+)/Cl(-) cotransport. This study demonstrates the first examples of pH-switchable nonelectrogenic anion transporters.

Effects of five membered aromatic heterocycles at the meso-position on the electronic properties of 3-pyrrolyl BODIPY

The synthesis, structures and properties of 3-pyrrolyl BODIPYs containing five membered aromatic heterocycles such as pyrrole, thiophene and furan at the meso-position are described.

Racemization barriers of atropisomeric 3,3′-bipyrroles: an experimental study with theoretical verification

The activation barrier of racemization was determined for atropisomeric 3,3′-bipyrroles and they are found to be configurationally stable.

Perenosins: a new class of anion transporter with anti-cancer activity

A new class of anion transporter named ‘perenosins’ consisting of a pyrrole linked through an imine to either an indole, benzimidazole or indazole is reported.

Synthesis and Structure-Activity Relationships of Tambjamines and B-Ring Functionalized Prodiginines as Potent Antimalarials

Synthesis and antimalarial activity of 94 novel bipyrrole tambjamines (TAs) and a library of B-ring functionalized tripyrrole prodiginines (PGs) against a panel of Plasmodium falciparum strains are described. The activity and structure-activity relationships demonstrate that the ring-C of PGs can be replaced by an alkylamine, providing for TAs with retained/enhanced potency. Furthermore, ring-B of PGs/TAs can be substituted with short alkyl substitutions at either 4-position (replacement of OMe) or 3- and 4-positions without impacting potency. Eight representative TAs and two PGs have been evaluated for antimalarial activity against multidrug-resistant P. yoelii in mice in the dose range of 5-100 mg/kg × 4 days by oral administration. The KAR425 TA offered greater efficacy than previously observed for any PG, providing 100% protection to malaria-infected mice until day 28 at doses of 25 and 50 mg/kg × 4 days, and was also curative in this model in a single oral dose (80 mg/kg). This study presents the first account of antimalarial activity in tambjamines.

Synthesis of Cycloprodigiosin Identifies the Natural Isolate as a Scalemic Mixture

The enantiomers of the natural product cycloprodigiosin were prepared using an expedient five-step synthetic sequence that takes advantage of a Schöllkopf–Barton–Zard (SBZ) pyrrole annulation with a chiral isocyanoacetate and a nitrocyclohexene derivative. Using chiral HPLC and X-ray crystallographic analyses of the synthetically prepared material and natural isolate (isolated from the marine bacterium Pseudoalteromonas rubra), naturally occurring cycloprodigiosin was determined to be a scalemic mixture occurring in an enantiomeric ratio of 83:17 (R)/(S) at C4′.

Highly effective yet simple transmembrane anion transporters based upon ortho-phenylenediamine bis-ureas

Simple, highly fluorinated receptors are shown to function as highly effective transmembrane anion antiporters with the most active transporters rivalling the transport efficacy of natural anion transporter prodigiosin for bicarbonate.

Prodigiosin alkaloids: recent advancements in total synthesis and their biological potential

The present review article is focused on the medicinal potential and total synthesis of prodigiosins witnessed in the last decade. The aim will be to provide an inspiration to the marvels and pit falls of constructing the polypyrrole heterocycles with in the complex systems.

Synthetic anion transporters that bear a terminal ethynyl group

Non-pyrrolic synthetic anion transporters without cytotoxicity are capable of transporting the chloride anion through membranes.

Stereospecific synthesis of 23-hydroxyundecylprodiginines and analogues and conversion to antimalarial premarineosins via a Rieske oxygenase catalyzed bicyclization

Facile and highly efficient synthetic routes for the synthesis of (S)- and (R)-23-hydroxyundecylprodiginines ((23S)-2, and (23R)-2), 23-ketoundecylprodiginine (3), and deuterium-labeled 23-hydroxyundecylprodiginine ([23-d]-2) have been developed. We demonstrated a novel Rieske oxygenase MarG catalyzed stereoselective bicyclization of (23S)-2 to premarineosin A (4), a key step in the tailoring process of the biosynthesis of marineosins, using a marG heterologous expression system. The synthesis of various A-C-ring functionalized prodiginines 32-41 was achieved to investigate the substrate promiscuity of MarG. The two analogues 32 and 33 exhibit antimalarial and cytotoxic activities stronger than those of the marineosin intermediate 2, against Plasmodium falciparum strains (CQ(S)-D6, CQ(R)-Dd2, and 7G8) and hepatocellular HepG2 cancer cell line, respectively. Feeding of 34-36 to Streptomyces venezuelae expressing marG led to production of novel premarineosins, paving a way for the production of marineosin analogues via a combinatorial synthetic/biosynthetic approach. This study presents the first example of oxidative bicyclization mediated by a Rieske oxygenase.

Tris-thiourea tripodal-based molecules as chloride transmembrane transporters: insights from molecular dynamics simulations

The interaction of six tripodal synthetic chloride transmembrane transporters with a POPC bilayer was investigated by means of molecular dynamics simulations using the general Amber force field (GAFF) for the transporters and the LIPID11 force field for phospholipids. These transporters are structurally simple molecules, based on the tris(2-aminoethyl)amine scaffold, containing three thiourea binding units coupled with three n-butyl (1), phenyl (2), fluorophenyl (3), pentafluorophenyl (4), trifluoromethylphenyl (5), or bis(trifluoromethyl)phenyl (6) substituents. The passive diffusion of 1-6⊃ Cl(-) was evaluated with the complexes initially positioned either in the water phase or inside the bilayer. In the first scenario the chloride is released in the water solution before the synthetic molecules achieve the water-lipid interface and permeate the membrane. In the latter one, only when the chloride complex reaches the interface is the anion released to the water phase, with the transporter losing the initial ggg tripodal shape. Independently of the transporter used in the membrane system, the bilayer structure is preserved and the synthetic molecules interact with the POPC molecules at the phosphate headgroup level, via N-H···O hydrogen bonds. Overall, the molecular dynamics simulations' results indicate that the small tripodal molecules in this series have a low impact on the bilayer and are able to diffuse with chloride inside the lipid environment. Indeed, these are essential conditions for these molecules to promote the transmembrane transport as anion carriers, in agreement with experimental efflux data.

Synthetic prodigiosenes and the influence of C-ring substitution on DNA cleavage, transmembrane chloride transport and basicity

Analogues of the tripyrrolic natural product prodigiosin bearing an additional methyl and a carbonyl group at the C-ring were synthesised and evaluated. In vitro anticancer activity screening (NCI) and the study of modes of action (copper-mediated cleavage of double-stranded DNA and transmembrane transport of chloride anions) showed that the presence of the methyl group is not detrimental to activity. Furthermore, although the presence of an ester conjugated to the prodigiosene C-ring seems to decrease both pK(a) and chloride transport efficiency compared to the natural product, these analogues still exhibit a high rate of chloride transport. All analogues exhibit good in vitro anticancer activity and reduced toxicity compared to the natural product: compare an acute systemic toxicity of 100 mg kg(-1) in mice vs. 4 mg kg(-1) for prodigiosin, pointing towards a larger therapeutic window than for the natural product.