Helical Receptors Research Articles

Complexation-driven assembly of imine-linked helical receptors showing adaptive folding and temperature-dependent guest selection

The development of synthetic receptors capable of selectively binding guests with diverse structures and multiple functional groups poses a significant challenge. Here, we present the efficient assembly of foldamer-based receptors for monosaccharides, utilising the principles of complexation-induced equilibrium shifting and adaptive folding. Diimine 4 can be quantitatively assembled from smaller components when d-galactose is added as a guest among monosaccharides we examined. During this assembly, dual complexation-induced equilibrium shifts toward both the formation of diimine 4 and the conversion of d-galactose into α-d-galactofuranose are observed. Diimine 6 is quantitatively assembled in the presence of two different guests, methyl β-d-glucopyranoside and methyl β-d-galactopyranoside, resulting in the formation of two dimeric complexes: (6-MP)2⊃(methyl β-d-glucopyranoside)2 and (6-MM)2⊃(methyl β-d-galactopyranoside∙2H2O)2, respectively. These two complexes exhibit distinct folding structures with domain-swapping cavities depending on the bound guest and temperature. Interestingly, (6-MM)2⊃(methyl β-d-galactopyranoside∙2H2O)2 is exclusively formed at lower temperatures, while (6-MP)2⊃(methyl β-d-glucopyranoside)2 is only formed at higher temperatures.

Subtle Modification of Imine-linked Helical Receptors to Significantly Alter their Binding Affinities and Selectivities for Chiral Guests.

Aromatic helical receptors P-1 and P-2 were slightly modified by aerobic oxidation to afford new receptors P-7 and P-8 with right-handed helical cavities. This subtle modification induced significant changes in the binding properties for chiral guests. Specifically, P-1 was reported to bind d-tartaric acid (Ka =35500 M-1 ), used as a template, much strongly than l-tartaric acid (326 M-1 ). In contrast, its modified receptor P-7 exhibited significantly reduced affinities for d-tartaric acid (3600 M-1 ) and l-tartaric acid (125 M-1 ). More dramatic changes in the affinities and selectivities were observed for P-2 and P-8 upon binding of polyol guests. P-2 was determined to selectively bind d-sorbitol (52000 M-1 ) over analogous guests, but P-8 showed no binding selectivity: d-sorbitol (1890 M-1 ), l-sorbitol (3330 M-1 ), d-arabitol (959 M-1 ), l-arabitol (4970 M-1 ) and xylitol (4960 M-1 ) in 5% (v/v) DMSO/CH2 Cl2 at 25±1 °C. These results clearly demonstrate that even subtle post-modifications of synthetic receptors may significantly alter their binding affinities and selectivities, in particular for guests of long and flexible chains.

Ground- and excited-state dynamic control of an anion receptor by hydrostatic pressure.

Stimulus-responsive supramolecular architectures have become an attractive alternative to conventional ones for many applications in sensing, drug-delivery and switchable memory systems. Herein, we used an anion receptor (H: host) as a hydrostatic-pressure-manipulatable fluorescence foldamer and halide anions as chiral (binaphthylammonium) and achiral (tetrabutylammonium) ion pairs (SS or RR·X and TBA·X; X = Cl, Br), and then investigated their (chir)optical properties and molecular recognition behavior under hydrostatic pressures. The conformational changes and optical properties of H in various organic solvents were revealed by UV/vis absorption and fluorescence spectra and fluorescence lifetimes upon hydrostatic pressurization. The anion-recognition abilities of H upon interactions with SS or RR·X and TBA·X at different pressure ranges were determined by hydrostatic-pressure spectroscopy to quantitatively afford the binding constant (Kanion) and apparent reaction volume changes . The results obtained indicate that hydrostatic pressure as well as solvation plays significant roles in the dynamic control of the present supramolecular system in the ground and excited states. This work will provide a new guideline for further developing hydrostatic-pressure-responsive foldamers and supramolecular materials.

Template‐Directed Quantitative One‐Pot Synthesis of Homochiral Helical Receptors Enabling Enantioselective Binding

AbstractTemplate‐directed synthesis and dynamic covalent chemistry were implemented to achieve quantitative one‐pot syntheses of homochiral helical cavities inside aromatic foldamers. One‐handed helical receptors P‐1, M‐1, P‐2 and M‐2 were assembled from their precursors in the presence of appropriate templates (d‐ and l‐tartaric acid, and d‐ and l‐sorbitol, respectively) via three sequential steps in one pot: imine‐linked chain elongation, template‐induced folding and [4+2] cycloaddition between helical turns. These helical receptors were proven to enantioselectively bind chiral guests used as the templates, and the differences between the association constants of enantiomeric guests were up to more than two orders of magnitude. The structures and binding modes of the receptors were fully characterized by single‐crystal X‐ray crystallography and 1H NMR spectroscopy.

Template-Directed Quantitative One-Pot Synthesis of Homochiral Helical Receptors Enabling Enantioselective Binding.

Template-directed synthesis and dynamic covalent chemistry were implemented to achieve quantitative one-pot syntheses of homochiral helical cavities inside aromatic foldamers. One-handed helical receptors P-1, M-1, P-2 and M-2 were assembled from their precursors in the presence of appropriate templates (d- and l-tartaric acid, and d- and l-sorbitol, respectively) via three sequential steps in one pot: imine-linked chain elongation, template-induced folding and [4+2] cycloaddition between helical turns. These helical receptors were proven to enantioselectively bind chiral guests used as the templates, and the differences between the association constants of enantiomeric guests were up to more than two orders of magnitude. The structures and binding modes of the receptors were fully characterized by single-crystal X-ray crystallography and 1 H NMR spectroscopy.

Strategies for identifying dynamic regions in protein complexes: Flexibility changes accompany methylation in chemotaxis receptor signaling states

Bacterial chemoreceptors are organized in arrays composed of helical receptors arranged as trimers of dimers, coupled to a histidine kinase CheA and a coupling protein CheW. Ligand binding to the external domain inhibits the kinase activity, leading to a change in the swimming behavior. Adaptation to an ongoing stimulus involves reversible methylation and demethylation of specific glutamate residues. However, the exact mechanism of signal propagation through the helical receptor to the histidine kinase remains elusive. Dynamics of the receptor cytoplasmic domain is thought to play an important role in the signal transduction, and current models propose inverse dynamic changes in different regions of the receptor. We hypothesize that the adaptational modification (methylation) controls the dynamics by stabilizing a partially ordered domain, which in turn modulates the binding of the kinase, CheA. We investigated the difference in dynamics between the methylated and unmethylated states of the chemoreceptor using solid-state NMR. The unmethylated receptor (CF4E) shows increased flexibility relative to the methylated mimic (CF4Q). Methylation helix 1 (MH1) has been shown to be flexible in the methylated mimic receptor. Our analysis indicates that in addition to MH1, methylation helix 2 also becomes flexible in the unmethylated receptor. In addition, we have demonstrated that both states of the receptor have a rigid region and segments with intermediate timescale dynamics. The strategies used in this study for identifying dynamic regions are applicable to a broad class of proteins and protein complexes with intrinsic disorder and dynamics spanning multiple timescales.

Oriented Membrane Protein Reconstitution into Tethered Lipid Membranes for AFM Force Spectroscopy

Membrane proteins act as a central interface between the extracellular environment and the intracellular response and as such represent one of the most important classes of drug targets. The characterization of the molecular properties of integral membrane proteins, such as topology and interdomain interaction, is key to a fundamental understanding of their function. Atomic force microscopy (AFM) and force spectroscopy have the intrinsic capabilities of investigating these properties in a near-native setting. However, atomic force spectroscopy of membrane proteins is traditionally carried out in a crystalline setup. Alternatively, model membrane systems, such as tethered bilayer membranes, have been developed for surface-dependent techniques. While these setups can provide a more native environment, data analysis may be complicated by the normally found statistical orientation of the reconstituted protein in the model membrane. We have developed a model membrane system that enables the study of membrane proteins in a defined orientation by single-molecule force spectroscopy. Our approach is demonstrated using cell-free expressed bacteriorhodopsin coupled to a quartz glass surface in a defined orientation through a protein anchor and reconstituted inside an artificial membrane system. This approach offers an effective way to study membrane proteins in a planar lipid bilayer. It can be easily transferred to all membrane proteins that possess a suitable tag and can be reconstituted into a lipid bilayer. In this respect, we anticipate that this technique may contribute important information on structure, topology, and intra- and intermolecular interactions of other seven-transmembrane helical receptors.

Probing G Protein-Coupled Receptor—Ligand Interactions with Targeted Photoactivatable Cross-Linkers

It has been 50 years since F. H. Westheimer and colleagues reported the first use of a photoactivatable cross-linking reagent to study the active site of chymotrypsin. In studies of seven transmembrane helical receptors, also known as G protein-coupled receptors (GPCRs), recent simultaneous advances in structural biology, molecular dynamics simulations, and amber codon suppression methods have allowed the development of a targeted photo-cross-linking strategy to probe receptor-ligand interactions in cell membranes. We review here recent advances in targeted photo-cross-linking of GPCR-ligand complexes in the context of extensive earlier work that primarily relied upon the use of ligand analogues with photoactivatable constituents.

ChemInform Abstract: Molecular Recognition: From Solution Science to Nano/Materials Technology

AbstractReview: 319 refs.

Photoresponsive receptors for binding and releasing anions

Photoresponsive receptors allow users to modulate when and where molecular hosts can bind and release their targets. This capability was pioneered in biochemistry, trained on cations, and is only now emerging in the recognition chemistry of anions. This mini review outlines the five instances where anion receptors have been manipulated by light. Cylindrical, acyclic, and helical receptors have been integrated with azobenzenes, stiff‐stilbenes, and dithienylethenes to generate modest‐to‐moderate changes in the binding affinities for various halides and polyatomic anions. The central design requirement involves choosing the appropriate photoswitch such that the two photostates can define two different organizations of the receptor, thus, enabling one state to bind better than the other. This challenge underscores the importance of analyzing the possible conformations, complexes, and stoichiometries that may be accessible by host and guest as a means to identify structures that will successfully elicit a switching response. Copyright © 2012 John Wiley & Sons, Ltd.

Molecular recognition: from solution science to nano/materials technology

In the 25 years since its Nobel Prize in chemistry, supramolecular chemistry based on molecular recognition has been paid much attention in scientific and technological fields. Nanotechnology and the related areas seek breakthrough methods of nanofabrication based on rational organization through assembly of constituent molecules. Advanced biochemistry, medical applications, and environmental and energy technologies also depend on the importance of specific interactions between molecules. In those current fields, molecular recognition is now being re-evaluated. In this review, we re-examine current trends in molecular recognition from the viewpoint of the surrounding media, that is (i) the solution phase for development of basic science and molecular design advances; (ii) at nano/materials interfaces for emerging technologies and applications. The first section of this review includes molecular recognition frontiers, receptor design based on combinatorial approaches, organic capsule receptors, metallo-capsule receptors, helical receptors, dendrimer receptors, and the future design of receptor architectures. The following section summarizes topics related to molecular recognition at interfaces including fundamentals of molecular recognition, sensing and detection, structure formation, molecular machines, molecular recognition involving polymers and related materials, and molecular recognition processes in nanostructured materials.

The allosteric vestibule of a seven transmembrane helical receptor controls G-protein coupling

Seven transmembrane helical receptors (7TMRs) modulate cell function via different types of G proteins, often in a ligand-specific manner. Class A 7TMRs harbour allosteric vestibules in the entrance of their ligand-binding cavities, which are in the focus of current drug discovery. However, their biological function remains enigmatic. Here we present a new strategy for probing and manipulating conformational transitions in the allosteric vestibule of label-free 7TMRs using the M2 acetylcholine receptor as a paradigm. We designed dualsteric agonists as 'tailor-made' chemical probes to trigger graded receptor activation from the acetylcholine-binding site while simultaneously restricting spatial flexibility of the receptor's allosteric vestibule. Our findings reveal for the first time that a 7TMR's allosteric vestibule controls the extent of receptor movement to govern a hierarchical order of G-protein coupling. This is a new concept assigning a biological role to the allosteric vestibule for controlling fidelity of 7TMR signalling.

Anion-induced contraction of helical receptors

Uptake of a tetrafluoroborate anion into the centre of a dicopper bis-salicylaldoxime complex causes a major contraction of the cavity and an increase in helicity.

Comprehensive features of natural and in vitro selected GNRA tetraloop-binding receptors

Specific recognitions of GNRA tetraloops by small helical receptors are among the most widespread long-range packing interactions in large ribozymes. However, in contrast to GYRA and GAAA tetraloops, very few GNRA/receptor interactions have yet been identified to involve GGAA tetraloops in nature. A novel in vitro selection scheme based on a rigid self-assembling tectoRNA scaffold designed for isolation of intermolecular interactions with A-minor motifs has yielded new GGAA tetraloop-binding receptors with affinity in the nanomolar range. One of the selected receptors is a novel 12 nt RNA motif, (CCUGUG … AUCUGG), that recognizes GGAA tetraloop hairpin with a remarkable specificity and affinity. Its physical and chemical characteristics are comparable to those of the well-studied ‘11nt’ GAAA tetraloop receptor motif. A second less specific motif (CCCAGCCC … GAUAGGG) binds GGRA tetraloops and appears to be related to group IC3 tetraloop receptors. Mutational, thermodynamic and comparative structural analysis suggests that natural and in vitro selected GNRA receptors can essentially be grouped in two major classes of GNRA binders. New insights about the evolution, recognition and structural modularity of GNRA and A-minor RNA–RNA interactions are proposed.

Approaches to the Molecular Modeling of 7‐Transmembrane Helical Receptors

7-Transmembrane helical receptors (7TMs) represent the single most important class of target for drug therapy; therefore, a great deal of effort has gone into computational studies of their structures. Historically, these were based on low resolution electron diffraction data, together with the use of computational methods such as multiple sequence alignments, distance geometry, and molecular mechanics calculations. In the year 2000 the situation changed when the first crystal structure of a 7TM, was published. It was then possible to use the homology modeling techniques to generate more accurate models of these proteins. This unit reviews the modeling of 7TMs and describes in detail how homology modeling can be used to build a structure of the 5-HT2a receptor. Special attention is given to the initial sequence alignment, the most important step in the process. Use of automatic alignment programs often produces incorrect results, and manual intervention is necessary before proceeding further.