Nanomechanical Recognition of N-Methylammonium Salts

Solvent-responsive cavitand lanthanum complex

A new, solvent responsive tetra-phosphonate cavitand lanthanum complex forms a dimer in acetonitrile, interconverts into a monomeric complex in acetone and is disassembled in methanol.

Probing the Structural Determinants of Amino Acid Recognition: X-Ray Studies of Crystalline Ditopic Host-Guest Complexes of the Positively Charged Amino Acids, Arg, Lys, and His with a Cavitand Molecule

Crystallization of tetraphosphonate cavitand Tiiii[H, CH₃, CH₃] in the presence of positively charged amino acids, namely arginine, lysine, or histidine, afforded host-guest complex structures. The X-ray structure determination revealed that in all three structures, the fully protonated form of the amino acid is ditopically complexed by two tetraphosphonate cavitand molecules. Guanidinium, ammonium, and imidazolium cationic groups of the amino acid side chain are hosted in the cavity of a phosphonate receptor, and are held in place by specific hydrogen bonding interactions with the P=O groups of the cavitand molecule. In all three structures, the positively charged α-ammonium groups form H-bonds with the P=O groups, and with a water molecule hosted in the cavity of a second tetraphosphonate molecule. Furthermore, water-assisted dimerization was observed for the cavitand/histidine ditopic complex. In this 4:2 supramolecular complex, a bridged water molecule is held by two carboxylic acid groups of the dimerized amino acid. The structural information obtained on the geometrical constrains necessary for the possible encapsulation of the amino acids are important for the rational design of devices for analytical and medical applications.

Probing lysine mono-methylation in histone H3 tail peptides with an abiotic receptor coupled to a non-plasmonic resonator

Binder and effector molecules that allow studying and manipulating epigenetic processes are of biological relevance and pose severe technical challenges. We report the first example of a synthetic receptor able to recognize mono-methylated lysines in a histone H3 tail peptide, which has relevant functions in epigenetic regulation. Recognition is robust and specific regardless of the position and the number of mono-methylated lysines along the polypeptide chain. The peptide is first captured in solution by a tetraphosphonate cavitand (Tiiii) that selectively binds its Lys-NMe⁺ moieties. Separation from solution and detection of the peptide-Tiiii complexes is then enabled in one single step by an all dielectric SiO₂-TiO₂ core-shell resonator (T-rex), which captures the complex and operates fully reproducible signal transduction by non-plasmonic surface enhanced Raman scattering (SERS) without degrading the complex. The realized abiotic probe is able to distinguish multiple mono-methylated peptides from the single mono-methylated ones.

An electrochemiluminescence-supramolecular approach to sarcosine detection for early diagnosis of prostate cancer

Monitoring Prostate Cancer (PCa) biomarkers is an efficient way to diagnosis this disease early, since it improves the therapeutic success rate and suppresses PCa patient mortality: for this reason a powerful analytical technique such as electrochemiluminescence (ECL) is already used for this application, but its widespread usability is still hampered by the high cost of commercial ECL equipment. We describe an innovative approach for the selective and sensitive detection of the PCa biomarker sarcosine, obtained by a synergistic ECL-supramolecular approach, in which the free base form of sarcosine acts as co-reagent in a Ru(bpy)3(2+)-ECL process. We used magnetic micro-beads decorated with a supramolecular tetraphosphonate cavitand (Tiiii) for the selective capture of sarcosine hydrochloride in a complex matrix like urine. Sarcosine determination was then obtained with ECL measurements thanks to the complexation properties of Tiiii, with a protocol involving simple pH changes - to drive the capture-release process of sarcosine from the receptor - and magnetic micro-bead technology. With this approach we were able to measure sarcosine in the μM to mM window, a concentration range that encompasses the diagnostic urinary value of sarcosine in healthy subjects and PCa patients, respectively. These results indicate how this ECL-supramolecular approach is extremely promising for the detection of sarcosine and for PCa diagnosis and monitoring, and for the development of portable and more affordable devices.

The Origin of Selectivity in the Complexation of N-Methyl Amino Acids by Tetraphosphonate Cavitands

We report on the eligibility of tetraphosphonate resorcinarene cavitands for the molecular recognition of amino acids. We determined the crystal structure of 13 complexes of the tetraphosphonate cavitand Tiiii[H, CH3, CH3] with amino acids. (1)H NMR and (31)P NMR experiments and ITC analysis were performed to probe the binding between cavitand Tiiii[C3H7, CH3, C2H5] or the water-soluble counterpart Tiiii[C3H6Py(+)Cl(-), CH3, C2H5] and a selection of representative amino acids. The reported studies and results allowed us (i) to highlight the noncovalent interactions involved in the binding event in each case; (ii) to investigate the ability of tetraphosphonate cavitand receptors to discriminate between the different amino acids; (iii) to calculate the Ka values of the different complexes formed and evaluate the thermodynamic parameters of the complexation process, dissecting the entropic and enthalpic contributions; and (iv) to determine the solvent influence on the complexation selectivity. By moving from methanol to water, the complexation changed from entropy driven to entropy opposed, leading to a drop of almost three orders in the magnitude of the Ka. However, this reduction in binding affinity is associated with a dramatic increase in selectivity, since in aqueous solutions only N-methylated amino acids are effectively recognized. The thermodynamic profile of the binding does not change in PBS solution. The pivotal role played by cation-π interactions is demonstrated by the linear correlation found between the log Ka in methanol solution and the depth of (+)N-CH3 cavity inclusion in the molecular structures. These findings are relevant for the potential use of phosphonate cavitands as synthetic receptors for the detection of epigenetic modifications of histones in physiological media.

Applications of isothermal titration calorimetry - the research and technical developments from 2011 to 2015

Isothermal titration calorimetry is a widely used biophysical technique for studying the formation or dissociation of molecular complexes. Over the last 5 years, much work has been published on the interpretation of isothermal titration calorimetry (ITC) data for single binding and multiple binding sites. As over 80% of ITC papers are on macromolecules of biological origin, this interpretation is challenging. Some researchers have attempted to link the thermodynamics constants to events at the molecular level. This review highlights work carried out using binding sites characterized using x-ray crystallography techniques that allow speculation about individual bond formation and the displacement of individual water molecules during ligand binding and link these events to the thermodynamic constants for binding. The review also considers research conducted with synthetic binding partners where specific binding events like anion-π and π-π interactions were studied. The revival of assays that enable both thermodynamic and kinetic information to be collected from ITC data is highlighted. Lastly, published criticism of ITC research from a physical chemistry perspective is appraised and practical advice provided for researchers unfamiliar with thermodynamics and its interpretation. Copyright © 2016 John Wiley & Sons, Ltd.

A tetrasulfate-resorcin[6]arene cavitand as the host for organic ammonium guests

A resorcin[6]arene cavitand bearing sulfate bridges is herein reported. The host is able to interact with ammonium guests through the sulfate bridges.

The effect of number and position of P=O/P=S bridging units on cavitand selectivity toward methyl ammonium salts

The present work reports the synthesis and complexation properties of five mixed bridge P=O/P=S cavitands toward N,N-methyl butyl ammonium chloride (1) as prototype guest. The influence of number and position of P=O and P=S groups on the affinity of phosphonate cavitands toward 1 is assessed via ITC titrations in DCE as solvent. Comparison of the resulting Kass values, the enthalpic and entropic contributions to the overall binding with those of the parent tetraphosphonate Tiiii and tetrathiophosphonate TSiiii cavitands allows one to single out the simultaneous dual H-bond between the cavitand and the salt as the major player in complexation.

Micro- and nano-structure based oligonucleotide sensors

This paper presents a review of micro- and nano-structure based oligonucleotide detection and quantification techniques. The characteristics of such devices make them very attractive for Point-of-Care or On-Site-Testing biosensing applications. Their small scale means that they can be robust and portable, their compatibility with modern CMOS electronics means that they can easily be incorporated into hand-held devices and their suitability for mass production means that, out of the different approaches to oligonucleotide detection, they are the most suitable for commercialisation. This review discusses the advantages of micro- and nano-structure based sensors and covers the various oligonucleotide detection techniques that have been developed to date. These include: Bulk Acoustic Wave and Surface Acoustic Wave devices, micro- and nano-cantilever sensors, gene Field Effect Transistors, and nanowire and nanopore based sensors. Oligonucleotide immobilisation techniques are also discussed.

Structural study of a small molecule receptor bound to dimethyllysine in lysozyme

Lysine is a ubiquitous residue on protein surfaces. Post translational modifications of lysine, including methylation to the mono-, di- or trimethylated amine result in chemical and structural alterations that have major consequences for protein interactions and signalling pathways. Small molecules that bind to methylated lysines are potential tools to modify such pathways. To make progress in this direction, detailed structural data of ligands in complex with methylated lysine is required. Here, we report a crystal structure of p-sulfonatocalix[4]arene (sclx₄) bound to methylated lysozyme in which the lysine residues were chemically modified from Lys-NH₃⁺ to Lys-NH(Me₂)⁺. Of the six possible dimethyllysine sites, sclx₄ selected Lys116-Me₂ and the dimethylamino substituent was deeply buried in the calixarene cavity. This complex confirms the tendency for Lys-Me₂ residues to form cation-π interactions, which have been shown to be important in protein recognition of histone tails bearing methylated lysines. Supporting data from NMR spectroscopy and MD simulations confirm the selectivity for Lys116-Me₂ in solution. The structure presented here may serve as a stepping stone to the development of new biochemical reagents that target methylated lysines.

pH-responsive host–guest polymerization and blending

pH-responsive supramolecular polymerization and polymer blending between complementary host and guest macromolecules are driven by tetraphosphonate cavitand/N-methyl ammonium complexation.

Reversible photo-responsive vesicle based on the complexation between an azobenzene containing molecule and α-cyclodextrin

Reversible spherical vesicles were constructed by the self-assembly of an amphiphilic molecule (Azo-Cl). With equimolar α-cyclodextrin (α-CD), the vesicles formation and destruction could be controlled by alternating UV-vis irradiation.

Isothermal titration calorimetry study of a bistable supramolecular system: reversible complexation of cryptand[2.2.2] with potassium ions

Isothermal titration calorimetry (ITC) is used to investigate the thermodynamics of the complexation of potassium ions by 1,10-diaza-4,7,13,16,21,24-hexaoxabicyclo[8.8.8]hexacosane (cryptand[2.2.2]) in aqueous solution. By changing the pH of the solution it was possible to trigger the reversible complexation/decomplexation of the cryptand in consecutive in situ experiments and to assess for the first time the use of ITC to monitor the thermodynamics of a bistable system.

Cavitand-grafted silicon microcantilevers as a universal probe for illicit and designer drugs in water

The direct, clean, and unbiased transduction of molecular recognition into a readable and reproducible response is the biggest challenge associated to the use of synthetic receptors in sensing. All possible solutions demand the mastering of molecular recognition at the solid-liquid interface as prerequisite. The socially relevant issue of screening amine-based illicit and designer drugs is addressed by nanomechanical recognition at the silicon-water interface. The methylamino moieties of different drugs are all first recognized by a single cavitand receptor through a synergistic set of weak interactions. The peculiar recognition ability of the cavitand is then transferred with high fidelity and robustness on silicon microcantilevers and harnessed to realize a nanomechanical device for label-free detection of these drugs in water.

Protein assembly mediated by sulfonatocalix[4]arene

The binding of anionic p-sulfonatocalix[4]arene to cationic lysozyme results in self assembly and the formation of protein tetramer chains, as revealed by X-ray crystallography.

Supramolecular self-assemblies as functional nanomaterials

In this review, we survey the diversity of structures and functions which are encountered in advanced self-assembled nanomaterials. We highlight their flourishing implementations in three active domains of applications: biomedical sciences, information technologies, and environmental sciences. Our main objective is to provide the reader with a concise and straightforward entry to this broad field by selecting the most recent and important research articles, supported by some more comprehensive reviews to introduce each topic. Overall, this compilation illustrates how, based on the rules of supramolecular chemistry, the bottom-up approach to design functional objects at the nanoscale is currently producing highly sophisticated materials oriented towards a growing number of applications with high societal impact.

Development of robust and standardized cantilever sensors based on biotin/NeutrAvidin coupling for antibody detection

A cantilever-based protein biosensor has been developed providing a customizable multilayer platform for the detection of antibodies. It consists of a biotin-terminated PEG layer pre-functionalized on the gold-coated cantilever surface, onto which NeutrAvidin is adsorbed through biotin/NeutrAvidin specific binding. NeutrAvidin is used as a bridge layer between the biotin-coated surface and the biotinylated biomolecules, such as biotinylated bovine serum albumin (biotinylated BSA), forming a multilayer sensor for direct antibody capture. The cantilever biosensor has been successfully applied to the detection of mouse anti-BSA (m-IgG) and sheep anti-BSA(s-IgG) antibodies. As expected, the average differential surface stress signals of about 5.7 ± 0.8 × 10⁻³ N/m are very similar for BSA/m-IgG and BSA/s-IgG binding, i.e., they are independent of the origin of the antibody. A statistic evaluation of 112 response curves confirms that the multilayer protein cantilever biosensor shows high reproducibility. As a control test, a biotinylated maltose binding protein was used for detecting specificity of IgG, the result shows a signal of bBSA layer in response to antibody is 5.8 × 10⁻³ N/m compared to bMBP. The pre-functionalized biotin/PEG cantilever surface is found to show a long shelf-life of at least 40 days and retains its responsivity of above 70% of the signal when stored in PBS buffer at 4 °C. The protein cantilever biosensor represents a rapid, label-free, sensitive and reliable detection technique for a real-time protein assay.

Supramolecular sensing with phosphonate cavitands

Molecular recognition is a recurrent theme in chemical sensing because of the importance of selectivity for sensor performances. The popularity of molecular recognition in chemical sensing has resulted from the progress made in mastering weak interactions, which has enabled the design of synthetic receptors according to the analyte to be detected. However, the availability of a large pool of modular synthetic receptors so far has not had a significant impact on sensors used in the real world. This technological gap has emerged because of the difficulties in transferring the intrinsic molecular recognition properties of a given receptor from solution to interfaces and in finding high fidelity transduction modes for the recognition event. This Account focuses on the ways to overcome these two bottlenecks, and we recount our recent efforts to produce highly selective supramolecular sensors using phosphonate cavitands as receptors. Through two examples, we present an overview of the different operating strategies that are implemented depending on whether the interface is vapor-solid or liquid-solid. First we describe the selective detection of short chain aliphatic alcohols in the vapor phase. In this example, we solved a key issue common to all sensors for organic vapors: the dissection of the specific interaction (between cavitand and the alcohol) from ubiquitous nonspecific dispersion interactions (between the analytes and interferents in the solid layer). We removed responses resulting from the nonspecific interactions of the analytes with interferents by directly connecting the recognition event at the interface to the transduction mechanism (photoinduced charge transfer). The second example addresses the specific detection of sarcosine in urine. Recent research has suggested that sarcosine can serve as reliable biomarker of the aggressive forms of prostate cancer. Tetraphosphonate cavitands can complex N-methyl ammonium salts with impressive selectivity in solution, and we used this property as a starting point. The sensor implementation requires that we first graft the cavitand onto silicon and gold surfaces as monolayers. The exclusive recognition of sarcosine by these supramolecular sensors originates from their operation in aqueous environments, where synergistic multiple interactions with the phosphonate cavitand are possible only for N-methyl ammonium derivatives. We couple that selectivity with detection modes that probe the strength of the complexation either directly (microcantilever) or via exchange with molecules that have comparable affinity for the cavity (fluorescence dye displacement).

Enantiopure supramolecular cages: synthesis and chiral recognition properties

Enantiopure compounds are ubiquitous in the chemical sciences and present a particular interest in the field of molecular recognition and host-guest systems. Indeed, chiral molecular receptors are at the basis of numerous biological recognition processes and have important implications in biochemistry or pharmacology. Chemists have been investigating this field for several decades, which has led to the development of the synthesis of chiral hosts, their enantiomeric differentiation, and the studies of their recognition properties towards important and bio-relevant chiral guest substrates. The design of molecular cages is a rather difficult task that is even more demanding when enantiopure molecules are required. In this review we chose to present the main families of synthetic organic supramolecular cages that have been developed, whose structures contain stereogenic centers or present an inherent chirality, giving rise to chiral supramolecular cages. Particular attention is given to obtaining enantiopure compounds. Their recognition properties are also underlined. A last important aspect of the review is to present how chiroptical spectroscopies can be used to characterize the recognition phenomena displayed by supramolecular cages.

Cavitand-functionalized SWCNTs for N-methylammonium detection

Single-walled carbon nanotubes (SWCNTs) have been functionalized with highly selective tetraphosphonate cavitand receptors. The binding of charged N-methylammonium species to the functionalized SWCNTs was analyzed by X-ray photoelectron spectroscopy and confirmed by (31)P MAS NMR spectroscopy. The cavitand-functionalized SWCNTs were shown to function as chemiresistive sensory materials for the detection of sarcosine and its ethyl ester hydrochloride in water with high selectivity at concentrations as low as 0.02 mM. Exposure to sarcosine and its derivative resulted in an increased conductance, in contrast to a decreased conductance response observed for potential interferents such as the structurally related glycine ethyl ester hydrochloride.