Chloride anion transporters inhibit growth of methicillin-resistant Staphylococcus aureus (MRSA) in vitro.

A series of aminopyrrolic receptors were tested as anion transporters using POPC liposome model membranes. Many were found to be effective Cl(-) transporters and to inhibit clinical strains of Staphylococcus aureus growth in vitro. The best transporters proved effective against the methicillin-resistant Staphylococcus aureus (MRSA) strains, Mu50 and HP1173. Tris-thiourea tren-based chloride transporters were also shown to inhibit the growth of S. aureus in vitro.

Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes: Cooperativity and Mechanism of Motion.

The existence of two rings in [3]pseudorotaxanes presents opportunities for those rings to undergo double switching and cooperative mechanical coupling. To investigate this capability, we identified a new strategy for bringing two rings into contact with each other and conducted mechanistic studies to reveal their kinetic cooperativity. A redox-active tetrazine ligand bearing two binding sites was selected to allow for two mobile copper(I) macrocycle ring moieties to come together. To realize this switching modality, ligands were screened against their ability to serve as stations on which the rings are initially parked, ultimately identifying 5,5'-dimethyl-2,2'-bipyridine. The kinetics of switching a macrocycle in a single-site [2]pseudorotaxane between bipyridine and single-site tetrazine stations were examined using electrochemistry. The forward movement was rate-limited by the bimolecular reaction between reduced tetrazine and bipyridine [2]pseudorotaxane. Two bipyridines were then used with a double-site tetrazine to verify double switching of two rings. Our results indicated stepwise movements, with the first ring moving 4 times more frequently (faster) than the second. While this behavior is indicative of anticooperative kinetics, positive thermodynamic cooperativity sets the two rings in motion even though just one tetrazine is reduced with one electron. Double switching in this [3]pseudorotaxane uniquely demonstrates how a series of independent thermodynamic states and kinetic paths govern an apparently simple mechanical motion.

Synthetic aminopyrrolic receptors have apoptosis inducing activity.

We report two synthetic aminopyrrolic compounds that induce apoptotic cell death. These compounds have been previously shown to act as receptors for mannosides. The extent of receptor-induced cell death is greater in cells expressing a high level of high-mannose oligosaccharides than in cells producing lower levels of high-mannose glycans. The ability of synthetic receptors to induce cell death is attenuated in the presence of external mannosides. The present results provide support for the suggestion that the observed cell death reflects an ability of the receptors to bind mannose displayed on the cell surface. Signaling pathway studies indicate that the synthetic receptors of the present study promote JNK activation, induce Bax translocation to the mitochondria, and cause cytochrome c release from the mitochondria into the cytosol, thus promoting caspase-dependent apoptosis. Such effects are also observed in cells treated with mannose-binding ConA. The present results thus serve to highlight what may be an attractive new approach to triggering apoptosis via modes of action that differ from those normally used to promote apoptosis.

Synthetic ion transporters can induce apoptosis by facilitating chloride anion transport into cells.

Anion transporters based on small molecules have received attention as therapeutic agents because of their potential to disrupt cellular ion homeostasis. However, a direct correlation between a change in cellular chloride anion concentration and cytotoxicity has not been established for synthetic ion carriers. Here we show that two pyridine diamide-strapped calix[4]pyrroles induce coupled chloride anion and sodium cation transport in both liposomal models and cells, and promote cell death by increasing intracellular chloride and sodium ion concentrations. Removing either ion from the extracellular media or blocking natural sodium channels with amiloride prevents this effect. Cell experiments show that the ion transporters induce the sodium chloride influx, which leads to an increased concentration of reactive oxygen species, release of cytochrome c from the mitochondria and apoptosis via caspase activation. However, they do not activate the caspase-independent apoptotic pathway associated with the apoptosis-inducing factor. Ion transporters, therefore, represent an attractive approach for regulating cellular processes that are normally controlled tightly by homeostasis.

Mechanistic evaluation of motion in redox-driven rotaxanes reveals longer linkers hasten forward escapes and hinder backward translations.

Mechanistic understanding of the translational movements in molecular switches is essential for designing machine-like prototypes capable of following set pathways of motion. To this end, we demonstrated that increasing the station-to-station distance will speed up the linear movements forward and slow down the movements backward in a homologous series of bistable rotaxanes. Four redox-active rotaxanes, which drove a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) mobile ring between a tetrathiafulvalene (TTF) station and an oxyphenylene station, were synthesized with only variations to the lengths of the glycol linker connecting the two stations (n = 5, 8, 11, and 23 atoms). We undertook the first mechanistic study of the full cycle of motion in this class of molecular switch using cyclic voltammetry. The kinetics parameters (k, ΔG(‡)) of switching were determined at different temperatures to provide activation enthalpies (ΔH(‡)) and entropies (ΔS(‡)). Longer glycol linkers led to modest increases in the forward escape (t(1/2) = 60 to <7 ms). The rate-limiting step involves movement of the tetracationic CBPQT(4+) ring away from the singly oxidized TTF(+) unit by overcoming one of the thiomethyl (SMe) speed bumps before proceeding on to the secondary oxyphenylene station. Upon reduction, however, the return translational movement of the CBPQT(4+) ring from the oxyphenylene station back to the neutral TTF station was slowed considerably by the longer linkers (t(1/2) = 1.4 to >69 s); though not because of a diffusive walk. The reduced rate of motion backward depended on folded structures that were only present with longer linkers.

A stereodynamic and redox-switchable encapsulation-complex containing a copper ion held by a tris-quinolinyl basket.

We investigated the coordination of Cu(I)/Cu(II) ions to chiral basket (S(3))-1. The results of both experimental and computational studies suggest the formation of a copper redox-switchable system capable of entrapping CH(3)CN.

Quantification of the π-π interactions that govern tertiary structure in donor-acceptor [2]pseudorotaxanes.

Flexibility in pseudorotaxanes and interlocked molecules that rely on interactions between π-donor-acceptor subunits provides access to folded structures reminiscent of the tertiary structure of proteins. While they have been described before, only now have we been able to quantify one such tertiary structure by making use of pseudorotaxanes designed for the purpose. Here, the enhanced stability of a pseudorotaxane inside a folded structure is measured to be ΔG = ca. 0.5 kcal mol(-1). The tertiary structure is stabilized by a charge-transfer interaction between a tetrathiafulvalene-based π-donor that can situate alongside a π-accepting paraquat-based macrocycle by folding of a flexible linker. At room temperature, it was estimated that 70% of the pseudorotaxanes examined here exist in their folded state. This quantitative information is critical for the creation of interlocked molecular machines that have predictable energetics and structures and for revealing a complexity approaching biological molecules.

A tristable [2]pseudo[2]rotaxane.

A strategy towards increasing the lifetime of the metastable state of a [2]rotaxane incorporating tetrathiafulvalene, 1,5-dioxynaphthalene and bipyridinium (BIPY(2+)) is presented. Incorporation of BIPY(2+) served multiple roles as an electrostatic barrier to relaxation, a supramolecular recognition site for bis-1,5-dioxynaphthalene[38]crown-10 macrocycle, and upon reduction a recognition site for the mechanically bonded cyclobis(paraquat-p-phenylene) ring.

Bilability is defined when one electron is used to switch between concerted and stepwise pathways in Cu(I)-based bistable [2/3]pseudorotaxanes.

Supramolecular switches operate as simple machines by using a stimulus to turn stations off and on, generating thermodynamic differences that define bistability and enable motion. What has not been previously investigated, yet is required to gain further control over molecular movements for complex operations, is an understanding of how the same stimulus can also switch pathways off and on, thus, defining the kinetic property of bilability. To address this challenge, the mechanisms of the forward and return reactions of redox-switchable Cu(I)-based [2/3]pseudorotaxanes have been quantitatively characterized utilizing mechanistic cyclic voltammetry and employing a series of isosteric bis-bidentate ligands. First, the bistability of the switch is retained across the series of ligands: Reduction of the ligand drives the reaction forward where a [2]pseudorotaxane switches into a reduced [3]pseudorotaxane and reoxidation drives the switching cycle back to the beginning. Second, the switch is bilabile with the forward reaction following an association-activated interchange pathway (concerted), whereas the reverse reaction follows a different dissociation-based dethreading pathway (stepwise). The forward reaction is more sensitive to denticity (bidentate tetrazinyl ligand, k(2) = 12,000 M(-1) s(-1), versus the monodentate pyrazinyl ligand, k(2) = 1500 M(-1) s(-1)) than to electronics (k(2) = 12,000 M(-1) s(-1) for methyl and trifluoromethyl substituents). The rate of return with the pyrazinyl ligand is k(1) = 50 s(-1). Consequently, both the mechanism and the thermodynamics of switching are stimuli dependent; they change with the oxidation state of the ligand. These findings have implications for the future design of molecular motors, which can be built from systems displaying allosterically coupled bistability and bilability.

Reduction of a redox-active ligand drives switching in a Cu(I) pseudorotaxane by a bimolecular mechanism.

The reduction of a redox-active ligand is shown to drive reversible switching of a Cu(I) [2]pseudorotaxane ([2]PR(+)) into the reduced [3]pseudorotaxane ([3]PR(+)) by a bimolecular mechanism. The unreduced pseudorotaxanes [2]PR(+) and [3]PR(2+) are initially self-assembled from the binucleating ligand, 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine (Me(2)BPTZ), and a preformed copper-macrocycle moiety (Cu-M(+)) based on 1,10-phenanthroline. X-ray crystallography revealed a syn geometry of the [3]PR(2+). The UV-vis-NIR spectra show low-energy metal-to-ligand charge-transfer transitions that red shift from 808 nm for [2]PR(+) to 1088 nm for [3]PR(2+). Quantitative analysis of the UV-vis-NIR titration shows the stepwise formation constants to be K(1) = 8.9 x 10(8) M(-1) and K(2) = 3.1 x 10(6) M(-1), indicative of negative cooperativity. The cyclic voltammetry (CV) and coulometry of Me(2)BPTZ, [2]PR(+), and [3]PR(2+) shows the one-electron reductions at E(1/2) = -0.96, -0.65, and -0.285 V, respectively, to be stabilized in a stepwise manner by each Cu(+) ion. CVs of [2]PR(+) show changes with scan rate consistent with an EC mechanism of supramolecular disproportionation after reduction: [2]PR(0) + [2]PR(+) = [3]PR(+) + Me(2)BPTZ(0) (K(D)*, k(d)). UV-vis-NIR spectroelectrochemistry was used to confirm the 1:1 product stoichiometry for [3]PR(+):Me(2)BPTZ. The driving force (DeltaG(D)* = -5.1 kcal mol(-1)) for the reaction is based on the enhanced stability of the reduced [3]PR(+) over reduced [2]PR(0) by 365 mV (8.4 kcal mol(-1)). Digital simulations of the CVs are consistent with a bimolecular pathway (k(d) = 12 000 s(-1) M(-1)). Confirmation of the mechanism provides a basis to extend this new switching modality to molecular machines.