Cooperative binding at lipid bilayer membrane surfaces

Reaction, Recognition, Relay: Anhydride Hydrolysis Reported by Conformationally Responsive Fluorinated Foldamers in Micelles

Natural membrane receptors are proteins that can report on changes in the concentration of external chemical messengers. Messenger binding to a receptor produces conformational changes that are relayed through the membrane into the cell; this information allows cells to adapt to changes in their environment. Artificial membrane receptors (R)-1 and (S)-1 are helical α-aminoisobutyric acid (Aib) foldamers that replicate key parts of this information relay. Solution-phase 19F NMR spectroscopy of zinc(II)-capped receptor 1, either in organic solvent or in membrane-mimetic micelles, showed messenger binding produced an enrichment of either left- or right-handed screw-sense; the chirality of the bound messenger was relayed to the other receptor terminus. Furthermore, in situ production of a chemical messenger in the external aqueous environment could be detected in real-time by a racemic mixture of receptor 1 in micelles. The hydrolysis of insoluble anhydrides produced carboxylate in the aqueous phase, which bound to the receptors and gave a distinct 19F NMR output from inside the hydrophobic region of the micelles.

Solvent effects in anion recognition

Anion recognition is pertinent to a range of environmental, medicinal and industrial applications. Recent progress in the field has relied on advances in synthetic host design to afford a broad range of potent recognition motifs and novel supramolecular structures capable of effective binding both in solution and at derived molecular films. However, performance in aqueous media remains a critical challenge. Understanding the effects of bulk and local solvent on anion recognition by host scaffolds is imperative if effective and selective detection in real-world media is to be viable. This Review seeks to provide a framework within which these effects can be considered both experimentally and theoretically. We highlight proposed models for solvation effects on anion binding and discuss approaches to retain strong anion binding in highly competitive (polar) solvents. The synthetic design principles for exploiting the aforementioned solvent effects are explored.

Recapitulating the Lateral Organization of Membrane Receptors at the Nanoscale

Many cell membrane functions emerge from the lateral presentation of membrane receptors. The link between the nanoscale organization of the receptors and ligand binding remains, however, mostly unclear. In this work, we applied surface molecular imprinting and utilized the phase behavior of lipid bilayers to create platforms that recapitulate the lateral organization of membrane receptors at the nanoscale. We used liposomes decorated with amphiphilic boronic acids that commonly serve as synthetic saccharide receptors and generated three lateral modes of receptor presentation─random distribution, nanoclustering, and receptor crowding─and studied their interaction with saccharides. In comparison to liposomes with randomly dispersed receptors, surface-imprinted liposomes resulted in more than a 5-fold increase in avidity. Quantifying the binding affinity and cooperativity proved that the boost was mediated by the formation of the nanoclusters rather than a local increase in the receptor concentration. In contrast, receptor crowding, despite the presence of increased local receptor concentrations, prevented multivalent oligosaccharide binding due to steric effects. The findings demonstrate the significance of nanometric aspects of receptor presentation and generation of multivalent ligands including artificial lectins for the sensitive and specific detection of glycans.

Significance of Receptor Mobility in Multivalent Binding on Lipid Membranes

Numerous key biological processes rely on the concept of multivalency, where ligands achieve stable binding only upon engaging multiple receptors. These processes, like viral entry or immune synapse formation, occur on the diffusive cellular membrane. One crucial, yet underexplored aspect of multivalent binding is the mobility of coupled receptors. Here, we discuss the consequences of mobility in multivalent processes from four perspectives: (I) The facilitation of receptor recruitment by the multivalent ligand due to their diffusivity prior to binding. (II) The effects of receptor preassembly, which allows their local accumulation. (III) The consequences of changes in mobility upon the formation of receptor/ligand complex. (IV) The changes in the diffusivity of lipid environment surrounding engaged receptors. We demonstrate how understanding mobility is essential for fully unravelling the principles of multivalent membrane processes, leading to further development in studies on receptor interactions, and guide the design of new generations of multivalent ligands.

Red-shifted tetra-ortho-halo-azobenzenes for photo-regulated transmembrane anion transport

Photo-responsive synthetic ion transporters are of interest as tools for studying transmembrane transport processes and have potential applications as targeted therapeutics, due to the possibility of spatiotemporal control and wavelength-dependent function. Here we report the synthesis of novel symmetric and non-symmetric red-shifted tetra-ortho-chloro- and tetra-ortho-fluoro azobenzenes, bearing pendant amine functionality. Functionalisation of the photo-switchable scaffolds with squaramide hydrogen bond donors enabled the preparation of a family of anion receptors, which act as photo-regulated transmembrane chloride transporters in response to green or red light. The subtle effects of chlorine/fluorine substitution, meta/para positioning of the anion receptors, and the use of more flexible linkers are explored. NMR titration experiments on the structurally diverse photo-switchable receptors reveal cooperative binding of chloride in the Z, but not E isomer, by the two squaramide binding sites. These results are supported by molecular dynamics simulations in explicit solvent and model membranes. We show that this intramolecular anion recognition leads to effective switching of transport activity in lipid bilayer membranes, in which optimal Z isomer activity is achieved using a combination of fluorine substitution and para-methylene spacer units.

Transmembrane signal transduction by cofactor transport

Information processing and cell signalling in biological systems relies on passing chemical signals across lipid bilayer membranes, but examples of synthetic systems that can achieve this process are rare. A synthetic transducer has been developed that triggers catalytic hydrolysis of an ester substrate inside lipid vesicles in response to addition of metal ions to the external vesicle solution. The output signal generated in the internal compartment of the vesicles is produced by binding of a metal ion cofactor to a head group on the transducer to form a catalytically competent complex. The mechanism of signal transduction is based on transport of the metal ion cofactor across the bilayer by the transducer, and the system can be reversibly switched between on and off states by adding cadmium(ii) and ethylene diamine tetracarboxylic acid input signals respectively. The transducer is also equipped with a hydrazide moiety, which allows modulation of activity through covalent conjugation with aldehydes. Conjugation with a sugar derivative abolished activity, because the resulting hydrazone is too polar to cross the bilayer, whereas conjugation with a pyridine derivative increased activity. Coupling transport with catalysis provides a straightforward mechanism for generating complex systems using simple components.

Synthetic transducers transport externally added metal ion cofactors across the lipid bilayer membrane of vesicles to trigger catalysis of ester hydrolysis in the inner compartment. Signal transduction activity is modulated by hydrazone formation.

Supramolecular chemistry in lipid bilayer membranes

Lipid bilayer membranes form compartments requisite for life. Interfacing supramolecular systems, including receptors, catalysts, signal transducers and ion transporters, enables the function of the membrane to be controlled in artificial and living cellular compartments. In this perspective, we take stock of the current state of the art of this rapidly expanding field, and discuss prospects for the future in both fundamental science and applications in biology and medicine.

How the biomimetic assembly of membrane receptors into multivalent domains is regulated by a small ligand

In living cells, communication requires the action of membrane receptors that are activated following very small environmental changes. A binary all-or-nothing behavior follows, making the organism extremely efficient at responding to specific stimuli. Using a minimal system composed of lipid vesicles, chemical models of a membrane receptor and their ligands, we show that bio-mimetic ON/OFF assembly of high avidity, multivalent domains is triggered by small temperature changes. Moreover, the intensity of the ON signal at the onset of the switch is modulated by the presence of small, weakly binding divalent ligands, reminiscent of the action of primary messengers in biological systems. Based on the analysis of spectroscopic data, we develop a mathematical model that rigorously describes the temperature-dependent switching of the membrane receptor assembly and ligand binding. From this we derive an equation that predicts the intensity of the modulation of the ON signal by the ligand-messenger as a function of the pairwise binding parameters, the number of binding sites that it features and the concentration. The behavior of our system, and the model derived, highlight the usefulness of weakly binding ligands in the regulation of membrane receptors and the pitfalls inherent to their binding promiscuity, such as non-specific binding to the membrane. Our model, and the equations derived from it, offer a valuable tool for the study of membrane receptors in both biological and biomimetic settings. The latter can be exploited to program membrane receptor avidity on sensing vesicles, create hierarchical protocell tissues or develop highly specific drug delivery vehicles.

General Method for Post-Synthetic Modification of Oligonucleotides Based on Oxidative Amination of 4-Thio-2'-deoxyuridine

Functionalized oligonucleotides (ONs) are widely applied as target binding molecules for biosensing and regulators for gene expression. Numerous efforts have been focused on developing facile methods for preparing these useful ONs carrying diverse modifications. Herein, we present a general method for postsynthetic modification of ONs via oxidative amination of 4-thio-2'-deoxyuridine (4SdU). 4SdU-containing ON can be derived by both alkyl and aromatic amines. Using this approach, ONs are successfully attached with alkyne/azide, biotin and dansylamide moieties, and these as-prepared ONs possess the expected biorthogonal reactivity, streptavidin affinity and fluorescent property, respectively. Furthermore, we also directly install fluorophores to the ON nucleobase based on oxidative amination of 4SdU, and these fluorophores exhibit distinct luminescence behaviors before and after conjugation. We believe our method will be a versatile strategy for constructing various functionalized ONs used in a wide range of nucleic acid applications.

Enhancing the selectivity of optical sensors using synthetic transmembrane ion transporters

Herein, we introduce a new method to optimize the properties of optical sensors, coined the transporter-liposome-fluorophore (TLF) approach. It is shown that this approach can greatly improve the selectivity of the sensor, increase the dynamic range and maintain the sensitivity of the original fluorophore.

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.

Switchable foldamer ion channels with antibacterial activity

Synthetic ion channels may have applications in treating channelopathies and as new classes of antibiotics, particularly if ion flow through the channels can be controlled. Here we describe triazole-capped octameric α-aminoisobutyric acid (Aib) foldamers that "switch on" ion channel activity in phospholipid bilayers upon copper(ii) chloride addition; activity is "switched off" upon copper(ii) extraction. X-ray crystallography showed that CuCl2 complexation gave chloro-bridged foldamer dimers, with hydrogen bonds between dimers producing channels within the crystal structure. These interactions suggest a pathway for foldamer self-assembly into membrane ion channels. The copper(ii)-foldamer complexes showed antibacterial activity against B. megaterium strain DSM319 that was similar to the peptaibol antibiotic alamethicin, but with 90% lower hemolytic activity.

Artificial Signal Transduction

Communication between and inside cells as well as their response to external stimuli relies on elaborated systems of signal transduction. They all require a directional transmission across membranes, often realized by primary messenger docking onto external receptor units and subsequent internalization of the signal in form of a released second messenger. This in turn starts a cascade of events which ultimately control all functions of the living cell. Although signal transduction is a fundamental biological process realized by supramolecular recognition and multiplication events with small molecules, chemists have just begun to invent artificial models which allow to study the underlying rules, and one day perhaps to rescue damaged transduction systems in nature. This review summarizes the exciting pioneering efforts of chemists to create simple models for the basic principles of signal transduction across a membrane. It starts with first attempts to establish molecular recognition events on liposomes with embedded receptor amphiphiles and moves on to simple transmembrane signaling across lipid bilayers. More elaborated systems step by step incorporate more elements of cell signaling, such as primary and secondary messenger or a useful cellular response such as cargo release.

Reversible photo-control over transmembrane anion transport using visible-light responsive supramolecular carriers

Supramolecular anion carriers responsive to visible light enable reversible two-colour photo-control over transmembrane anion transport.

Ion transport across lipid bilayer membranes in biology is controlled by membrane proteins, which in turn are regulated in response to chemical-, physical- and photo-stimuli. The design of synthetic supramolecular ion transporters able to be precisely controlled by external signals, in particular bio-compatible wavelengths of visible light, is key for achieving spatio-temporal control over function. Here we report two-colour responsive molecular photo-switches that act as supramolecular transmembrane anion carriers. Reversible switching of the photo-switch within the lipid bilayer membrane is achieved using biocompatible visible wavelengths of light, such that temporal control over transmembrane anion transport is achieved through alternating irradiation with red and blue light.

Supramolecular Chemistry in the Biomembrane

The combination of supramolecular functional systems with biomolecular chemistry has been a fruitful exercise for decades, leading to a greater understanding of biomolecules and to a great variety of applications, for example, in drug delivery and sensing. Within these developments, the phospholipid bilayer membrane, surrounding live cells, with all its functions has also intrigued supramolecular chemists. Herein, recent efforts from the supramolecular chemistry community to mimic natural functions of lipid membranes, such as sensing, molecular recognition, membrane fusion, signal transduction, and gated transport, are reviewed.

A Synthetic Vesicle-to-Vesicle Communication System

A molecular signal displayed on the external surface of one population of vesicles was used to trigger a catalytic process on the inside of a second population of vesicles. The key recognition event is the transfer of a protein (NeutrAvidin) bound to vesicles displaying desthiobiotin to vesicles displaying biotin. The desthiobiotin-protein complex was used to anchor a synthetic transducer in the outer leaflet of the vesicles, and when the protein was displaced, the transducer translocated across the bilayer to expose a catalytic headgroup to the internal vesicle solution. As a result, an ester substrate encapsulated on the inside of this second population of vesicles was hydrolyzed to give a fluorescence output signal. The protein has four binding sites, which leads to multivalent interactions with membrane-anchored ligands and very high binding affinities. Thus, biotin, which has a dissociation constant 3 orders of magnitude higher than desthiobiotin, did not displace the protein from the membrane-anchored transducer, and membrane-anchored biotin displayed on the surface of a second population of vesicles was required to generate an effective input signal.

Bis-pyrene probes of foldamer conformation in solution and in phospholipid bilayers

Exploring the detailed structural features of synthetic molecules in the membrane phase requires sensitive probes of conformation. Here we describe the design, synthesis and characterization of bis(pyrene) probes that report conformational changes in membrane-active dynamic foldamers. The probes were designed to distinguish between left-handed (M) and right-handed (P) screw-sense conformers of 3₁₀-helical α-aminoisobutyric acid (Aib) peptide foldamers, both in solution and in bilayer membranes. Several different bis(pyrene) probes were synthesized and ligated to the C-terminus of Aib tetramers that had different chiral residues at the N-terminus, residues that favored either an M or a P screw-sense in the 3₁₀-helix. The readily synthesized and conveniently incorporated N-acetyl-1,2-bis(pyren-1'-yl)ethylenediamine probe proved to have the best properties. In solution, changes in foldamer screw-sense induced substantial changes in the ratio of excimer/monomer fluorescence emission (E/M) for this reporter of conformation, with X-ray crystallography revealing that opposite screw-senses produce very different interpyrene distances in the reporter. In bilayers, this convenient and sensitive fluorescent reporter allowed, for the first time, an investigation of how the chirality of natural phospholipids affects foldamer conformation.

Length-Dependent Formation of Transmembrane Pores by 310-Helical α-Aminoisobutyric Acid Foldamers

The synthetic biology toolbox lacks extendable and conformationally controllable yet easy-to-synthesize building blocks that are long enough to span membranes. To meet this need, an iterative synthesis of α-aminoisobutyric acid (Aib) oligomers was used to create a library of homologous rigid-rod 3₁₀-helical foldamers, which have incrementally increasing lengths and functionalizable N- and C-termini. This library was used to probe the inter-relationship of foldamer length, self-association strength, and ionophoric ability, which is poorly understood. Although foldamer self-association in nonpolar chloroform increased with length, with a ∼14-fold increase in dimerization constant from Aib₆ to Aib₁₁, ionophoric activity in bilayers showed a stronger length dependence, with the observed rate constant for Aib₁₁ ∼70-fold greater than that of Aib₆. The strongest ionophoric activity was observed for foldamers with >10 Aib residues, which have end-to-end distances greater than the hydrophobic width of the bilayers used (∼2.8 nm); X-ray crystallography showed that Aib₁₁ is 2.93 nm long. These studies suggest that being long enough to span the membrane is more important for good ionophoric activity than strong self-association in the bilayer. Planar bilayer conductance measurements showed that Aib₁₁ and Aib₁₃, but not Aib₇, could form pores. This pore-forming behavior is strong evidence that Aib_m (m ≥ 10) building blocks can span bilayers.

Synthesis, spectroscopy, theoretical and biological studies of new gramine-steroids salts and conjugates

New gramine connections with bile acids (lithocholic, deoxycholic, cholic) and sterols (cholesterol, cholestanol) were synthesized. The structures of products were confirmed by spectral (NMR, FT-IR) analysis, mass spectrometry (ESI-MS) as well as PM5 semiempirical methods. Unexpectedly, the products of the reaction of gramine with cholesterol and cholestanol were symmetrical compounds consisting of two molecules of sterols connected by N(CH3)2 group. All new synthesized compounds interact in vitro with the human erythrocyte membrane and alter discoid erythrocyte shape inducing stomatocytosis or echinocytosis. Increase in the incorporation of the fluorescent dye merocyanine 540 (MC540) into the erythrocyte membrane indicates that new compounds at sublytic concentrations are capable of disturbing membrane phospholipids asymmetry and loosening the molecular packing of phospholipids in the bilayer. Gramine significantly decreases the membrane partitioning properties as well as haemolytic activity of lithocholic acid in its new salt. Moreover, both deoxycholic and cholic acids completely lost their membrane perturbing activities in the gramine salts. On the other hand, the capacity of new gramine-sterols connections to alter the erythrocyte membrane structure and its permeability is much higher in comparison with sterols alone. The dual effect of gramine on the bile acid and sterols cell membrane partitioning activity observed in our study should not be neglected in vivo.

Insights into the molecular basis of action of the AT1 antagonist losartan using a combined NMR spectroscopy and computational approach

The drug:membrane interactions for the antihypertensive AT1 antagonist losartan, the prototype of the sartans class, are studied herein using an integrated approach. The pharmacophore arrangement of the drug was revealed by rotating frame nuclear Overhauser effect spectroscopy (2D ROESY) NMR spectroscopy in three different environments, namely water, dimethyl sulfoxide (DMSO), and sodium dodecyl sulfate (SDS) micellar solutions mimicking conditions of biological transport fluids and membrane lipid bilayers. Drug association with micelles was monitored by diffusion ordered spectroscopy (2D DOSY) and drug:micelle intermolecular interactions were characterized by ROESY spectroscopy. The localisation of the drug in the micellar environment was investigated by introducing 5-doxyl and 16-doxyl stearic acids. The use of spin labels confirmed that losartan resides close to the micelle:water interface with the hydroxymethyl group and the tetrazole heterocyclic aromatic ring facing the polar surface with the potential to interact with SDS charged polar head groups in order to increase amphiphilic interactions. The spontaneous insertion, the diffusion pathway and the conformational features of losartan were monitored by Molecular Dynamics (MD) simulations in a modeled SDS micellar aggregate environment and a long exploratory MD run (580ns) in a phospholipid dipalmitoylphosphatidylcholine (DPPC) bilayer with the AT1 receptor embedded. MD simulations were in excellent agreement with experimental results and further revealed the molecular basis of losartan:membrane interactions in atomic-level detail. This applied integrated approach aims to explore the role of membranes in losartan's pathway towards the AT1 receptor.

Modulation of in-membrane receptor clustering upon binding of multivalent ligands

In living cells and biomimetic systems alike, multivalent ligands in solution can induce clustering of membrane receptors. The link between the receptor clustering and the ligand binding remains, however, poorly defined. Using minimalist divalent ligands, we develop a model that allows quantifying the modulation of receptor clustering by binding of ligands with any number of binding sites. The ligands, with weak binding affinity for the receptor and with binding sites held together by flexible linkers, lead to nearly quantitative clustering upon binding in a wide range of experimental conditions, showing that efficient modulation of receptor clustering does not require pre-organization or large binding affinities per binding site. Simulations show that, in the presence of ligands with five or more binding sites, an on/off clustering response follows a very small change in receptor density in the membrane, which is consistent with the highly cooperative behavior of multivalent biomolecular systems.

Density-dependent cooperative non-specific binding in solid-phase SELEX affinity selection

The non-specific binding of undesired ligands to a target is the primary factor limiting the enrichment of tight-binding ligands in affinity selection. Solution-phase non-specific affinity is determined by the free-energy of ligand binding to a single target. However, the solid-phase affinity might be higher if a ligand bound concurrently to multiple adjacent immobilized targets in a cooperative manner. Cooperativity could emerge in this case as a simple consequence of the relationship between the free energy of binding, localization entropy and the spatial distribution of the immobilized targets. We tested this hypothesis using a SELEX experimental design and found that non-specific RNA aptamer ligands can concurrently bind up to four bead-immobilized peptide targets, and that this can increase their effective binding affinity by two orders-of-magnitude. Binding curves were quantitatively explained by a new statistical mechanical model of density-dependent cooperative binding, which relates cooperative binding to both the target concentration and the target surface density on the immobilizing substrate. Target immobilization plays a key role in SELEX and other ligand enrichment methods, particularly in new multiplexed microfluidic purification devices, and these results have strong implications for optimizing their performance.

Zinc coordination to the bapbpy ligand in homogeneous solutions and at liposomes: zinc detection via fluorescence enhancement

In this work, the complexation of the bapbpy ligand to zinc dichloride is described (bapbpy = 6,6′-bis(2-aminopyridyl)-2,2′-bipyridine). The water-soluble, colorless complex [Zn(bapbpy)Cl]Cl·2H2O (compound 2·H2O) was synthesized; its X-ray crystal structure shows a mononuclear, pentacoordinated geometry with one chloride ligand in apical position. Upon excitation of its lowest-energy absorption band (375 nm) compound 2 shows intense emission (Φ = 0.50) at 418 nm in aqueous solution, and an excited state lifetime of 5 ns at room temperature. Photophysical measurements, DFT, and TD-DFT calculations prove that emission arises from vibronically coupled Ligand-to-Ligand Charge Transfer singlet excited states, characterized by electron density flowing from the lone pairs of the non-coordinated NH bridges to the π* orbitals of the pyridine rings. Monofunctionalization of the ligand with one long alkyl chain was realized to afford ligand 3, which can be inserted into dimyristoylphosphatidylglycerol (DMPG) or dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles. For negatively charged DMPG membranes the addition of a zinc salt to the vesicles leads to an enhancement of the fluorescence due to zinc coordination to the membrane-embedded tetrapyridyl ligand. No changes were observed for the zwitterionic DMPC lipids, where binding of the Zn ions does not take place. A modest binding constant was found (5 × 10(6) M(−1)) for the coordination of zinc cations to bapbpy-functionalized DMPG membranes, which allows for the detection of micromolar zinc concentrations in aqueous solution. The influence of chloride concentration and other transition metal ions on the zinc binding was evaluated, and the potential of liposome-supported metal chelators such as ligand 3 for zinc detection in biological media is discussed.

Enzyme-responsive liposomes modified adenoviral vectors for enhanced tumor cell transduction and reduced immunogenicity

Limitations of adenoviral (Ad) vectors for cancer gene therapy could be overcome by their combination with pharmaceutical technologies. Here we show that an enzyme-responsive liposomal formulation could significantly enhance the tumor cell transduction abilities and reduce the immunogenicity of Ad vectors. In the current research, the enzymatically cleavable PEG-lipids composed of a PEG/matrix metalloproteinase (MMP)-substrate peptide/cholesterol (PPC) were synthesized and characterized by (1)H NMR and TOF MS ES(+). The obtained MMP-cleavable lipids were inserted into the anionic liposomal Ad vectors (AL-Ad) by the post-insertion method. The results of in vitro infection assays indicated that the enzymatically cleavable formulation (PPC-AL-Ad) displayed a much higher gene expression than naked Ad5 and the non-cleavable PEG-lipid modified Ad vectors in tumor cells. More importantly, PPC-AL-Ad induces a lower production of neutralizing antibody and lower innate immune response, as well as significantly reduced liver toxicity in vivo. These findings suggest that PPC-AL-Ad is a promising system for gene delivery in tumor therapy.

Artificial signal transduction with primary and secondary messengers

The complete, entirely artificial, signal-transduction process was realized with a pair of tailored transmembrane units that were equipped with receptor- and reactive sites at both amphiphilic ends. Thus, docking of the primary messenger, transmission of the signal, and release of the secondary messenger could all be imitated in a single experimental setup. The system imitates the signaling principle of receptor tyrosine kinases and employs bisphosphonate head-groups for oligoamine-recognition and a pair of thiol nucleophiles and pyridine disulfide tail-groups for intravesicle S(N)2 displacement. This system operates in a unidirectional fashion, does not suffer from intervesicle competition, and is highly sensitive towards the lipid composition of the membrane and the nature of the primary messenger.

Self-assembled multivalency: dynamic ligand arrays for high-affinity binding

Multivalency is a powerful strategy for achieving high-affinity molecular recognition in biological systems. Recently, attention has begun to focus on using self-assembly rather than covalent scaffold synthesis to organize multiple ligands. This approach has a number of advantages, including ease of synthesis/assembly, tunability of nanostructure morphology and ligands, potential to incorporate multiple active units, and the responsive nature of self-assembly. We suggest that self-assembled multivalency is a strategy of fundamental importance in the design of synthetic nanosystems to intervene in biological pathways and has potential applications in nanomedicine.

Fluorescent cyclin-dependent kinase inhibitors block the proliferation of human breast cancer cells

Inhibitors of cyclin-dependent kinases (CDKs) are an emerging class of drugs for the treatment of cancers. CDK inhibitors are currently under evaluation in clinical trials as single agents and as sensitizers in combination with radiation therapy and chemotherapies. Drugs that target CDKs could have important inhibitory effects on cancer cell cycle progression, an extremely important mechanism in the control of cancer cell growth. Using rational drug design, we designed and synthesized fluorescent CDK inhibitors (VMY-1-101 and VMY-1-103) based on a purvalanol B scaffold. The new agents demonstrated more potent CDK inhibitory activity, enhanced induction of G2/M arrest and modest apoptosis as compared to purvalanol B. Intracellular imaging of the CDK inhibitor distribution was performed to reveal drug retention in the cytoplasm of treated breast cancer cells. In human breast cancer tissue, the compounds demonstrated increased binding as compared to the fluorophore. The new fluorescent CDK inhibitors showed undiminished activity in multidrug resistance (MDR) positive breast cancer cells, indicating that they are not a substrate for p-glycoprotein. Fluorescent CDK inhibitors offer potential as novel theranostic agents, combining therapeutic and diagnostic properties in the same molecule.

Pd(II)-mediated assembly of porphyrin channels in bilayer membranes

A membrane-spanning bis(meso-3-pyridyl) porphyrin 1 has been synthesized, embedded in EYPC vesicles, and upon Pd(II) addition has been shown to form ionophores that allow the passage of anionic 5/6-carboxyfluorescein through membranes. The geometric matching of bis(meso-3-pyridyl) porphyrin 1 and trans-Pd(II) was designed to give a cyclic porphyrin trimer [PdCl(2)(1)](3). However, solution-phase studies showed that PdCl(2)(PhCN)(2) cross linked 1 into linear oligomers at porphyrin concentrations above 10 mM, although the formation of cyclic species was inferred from studies at concentrations below 2 μM. Fluorescence titrations showed that embedding porphyrin 1 in bilayers greatly reduced its affinity for Pd(II), but the combination of porphyrin 1 and Pd(II) gave an ionophoric species that increased the rate of 5/6-carboxyfluorescein (5/6-CF) transit through the phospholipid bilayer 12-fold. A maximum in the 5/6-CF release rate was observed at a Pd(II) concentration of 4 μM, and the application of a solution-phase binding model to the membrane phase showed that this peak in ionophoric activity corresponded to the greatest extent of porphyrin oligomerization. Further studies suggested these Pd(II)/porphyrin oligomers transported 5/6-CF via a channel mechanism.

Mutual modulation between membrane-embedded receptor clustering and ligand binding in lipid membranes

Thanks largely to a cooperative chelate effect, clustered membrane-embedded proteins favourably bind to multivalent ligands in solution and, conversely, a multivalent receptor can induce the clustering of membrane-embedded proteins. Here, we use a chemical model to show that the binding of a monovalent ligand and the clustering of a membrane-embedded receptor are closely related processes that modulate each other without the contribution of any apparent multivalence effect. Clearly, the confinement of the receptor within the surface reveals cooperative effects between clustering and binding that are too weak to detect in bulk-solution systems. This work shows that for membrane-embedded receptors that undergo some degree of spontaneous clustering, analyses based on multivalence-mediated cooperativity are insufficient to describe fully the molecular recognition events induced by ligands in solution. Instead, a binding-clustering thermodynamic cycle is proposed for the analysis of the interaction of any kind of ligand with membrane-embedded receptors.

Fatty acylation enhances cellular localization of dansylated phenylalanines

Two series of fluorescent molecules were synthesized by acylation of dansyl ethylenediamine and phenylalanine dansyl ethylenediamine with one of either acetyl (C(2)), hexanyl (C(6)), cyclohexanecarbonyl (C(7)), myristyl (C(14)), or palmityl (C(16)) groups and examined for entry and localization in Chinese Hamster Ovary (CHO) cells in tissue culture. Gross total fluorescence retention and cellular microscopic fluorescence patterns were analyzed. In both series, molecules with myristyl or palmityl groups entered cells. Only in the phenylalanine series did hexyl and cyclohexanecarbonyl modification enable entry. Consistent with a mechanism of passive diffusion, entry of compounds into cells was neither energy dependent nor endocytosis linked. Acylated molecules were observed to localize in cytoplasm and not enter nuclei or associate with lipophilic plasma membranes.

Dynamic combinatorial chemistry at the phospholipid bilayer interface

Background

Molecular recognition at the environment provided by the phospholipid bilayer interface plays an important role in biology and is subject of intense investigation. Dynamic combinatorial chemistry is a powerful approach for exploring molecular recognition, but has thus far not been adapted for use in this special microenvironment.

Results

Thioester exchange was found to be a suitable reversible reaction to achieve rapid equilibration of dynamic combinatorial libraries at the egg phosphatidyl choline bilayer interface. Competing thioester hydrolysis can be minimised by judicial choice of the structure of the thioesters and the experimental conditions. Comparison of the library compositions in bulk solution with those in the presence of egg PC revealed that the latter show a bias towards the formation of library members rich in membrane-bound building blocks. This leads to a shift away from macrocyclic towards linear library members.

Conclusions

The methodology to perform dynamic combinatorial chemistry at the phospholipid bilayer interface has been developed. The spatial confinement of building blocks to the membrane interface can shift the ring-chain equilibrium in favour of chain-like compounds. These results imply that interfaces may be used as a platform to direct systems to the formation of (informational) polymers under conditions where small macrocycles would dominate in the absence of interfacial confinement.