On-Demand Elongation of Peptide Nanofibrils at Cellular Interfaces to Modulate Cell-Cell Interactions.

Natural cells can achieve specific cell-cell interactions by enriching nonspecific binding molecules on demand at intercellular contact faces, a pathway currently beyond synthetic capabilities. We are inspired to construct responsive peptide fibrils on cell surfaces, which elongate upon encountering target cells while maintaining a short length when contacting competing cells, as directed by a strand-displacement reaction arranged on target cell surfaces. With the display of ligands that bind to both target and competing cells, the contact-induced, region-selective fibril elongation selectively promotes host-target cell interactions via the accumulation of nonspecific ligands between matched cells. This approach is effective in guiding natural killer cells, the broad-spectrum effector lymphocytes, to eliminate specific cancer cells. In contrast to conventional methods relying on target cell-specific binding molecules for the desired cellular interactions, this dynamic scaffold-based approach would broaden the scope of cell combinations for manipulation and enhance the adjustability of cell behaviors for future applications.

Reassembly of Peptide Nanofibrils on Live Cell Surfaces Promotes Cell-Cell Interactions.

Nature regulates cellular interactions through the cell-surface molecules and plasma membranes. Despite advances in cell-surface engineering with diverse ligands and reactive groups, modulating cell-cell interactions through scaffolds of the cell-binding cues remains a challenging endeavor. Here, we assembled peptide nanofibrils on live cell surfaces to present the ligands that bind to the target cells. Surprisingly, with the same ligands, reducing the thermal stability of the nanofibrils promoted cellular interactions. Characterizations of the system revealed a thermally induced fibril disassembly and reassembly pathway that facilitated the complexation of the fibrils with the cells. Using the nanofibrils of varied stabilities, the cell-cell interaction was promoted to different extents with free-to-bound cell conversion ratios achieved at low (31%), medium (54%), and high (93%) levels. This study expands the toolbox to generate desired cell behaviors for applications in many areas and highlights the merits of thermally less stable nanoassemblies in designing functional materials.

Preserving Structurally Labile Peptide Nanosheets After Molecular Functionalization of the Self-Assembling Peptides.

A significant challenge in creating supramolecular materials is that conjugating molecular functionalities to building blocks often results in dissociation or undesired morphological transformation of their assemblies. Here we present a facile strategy to preserve structurally labile peptide assemblies after molecular modification of the self-assembling peptides. Sheet-forming peptides are designed to afford a staggered alignment with the segments bearing chemical modification sites protruding from the sheet surfaces. The staggered assembly allows for simultaneous separation of attached molecules from each other and from the underlying assembly motifs. Strikingly, using PEGs as the external molecules, PEG400 - and PEG700 -peptide conjugates directly self-associate into nanosheets with the PEG chains localized on the sheet surfaces. In contrast, the sheet formation based on in-register lateral packing of peptides does not recur upon the peptide PEGylation. This strategy allows for fabrication of densely modified assemblies with a variety of molecules, as demonstrated using biotin (hydrophobic molecule), c(RGDfK) (cyclic pentapeptide), and nucleic acid aptamer (negatively charged ssDNA). The staggered co-assembly also enables extended tunability of the amount/density of surface molecules, as exemplified by screening ligand-appended assemblies for cell targeting. This study paves the way for functionalization of historically challenging fragile assemblies while maintaining their overall morphology.

One-Pot Cyclization to Large Peptidomimetic Macrocycles by In Situ-Generated ß-Turn-Enforced Folding.

Macrocycles have been targets of extensive synthetic efforts for decades because of their potent molecular recognition and self-assembly capabilities. Yet, efficient syntheses of macrocyclic molecules via irreversible covalent bonds remain challenging. Here, we report an efficient approach to large peptidomimetic macrocycles by using the in situ-generated ß-turn structural motifs afforded in the amidothiourea moieties from the early steps of the reaction of 2 molecules of bilateral amino acid-based acylhydrazine with 2 molecules of diisothiocyanate. Four chiral and achiral peptidomimetic large macrocycles were successfully synthesized in high yields of 45-63% in a feasible one-pot reaction under sub-molar concentration conditions and were purified by simple filtration. X-ray crystallographic characterization of three macrocycles reveals an important feature that their four ß-turn structures, each maintained by four 10-membered intramolecular hydrogen bonds, alternatively network the four aromatic arms. This affords an interesting conformation switching mode upon anion binding. Binding of SO42- to 1L or 1D that contains 4 alanine residues (with the lowest steric hinderance among the macrocycles) leads to an inside-out structural change of the host macrocycle, as confirmed by the X-ray crystal structure of 1L-SO42- and 1D-SO42- complexes, accompanied by an inversion of the CD signals. On the basis of the strong sulfate affinity of the macrocycles, we succeeded in the removal of sulfate anions from water via a macrocycle-mediated liquid-liquid extraction method. Our synthetic protocol can be easily extended to other macrocycles of varying arms and/or chiral amino acid residues; thus, a variety of structurally and functionally diverse macrocycles are expected to be readily made.

Unequal Perylene Diimide Twins in a Quadruple Assembly.

Natural light-harvesting (LH) systems can divide identical dyes into unequal aggregate states, thereby achieving intelligent "allocation of labor". From a synthetic point of view, the construction of such kinds of unequal and integrated systems without the help of proteinaceous scaffolding is challenging. Here, we show that four octatetrayne-bridged ortho-perylene diimide (PDI) dyads (POPs) self-assemble into a quadruple assembly (POP)4 both in solution and in the solid state. The two identical PDI units in each POP are compartmentalized into weakly coupled PDIs (P520) and closely stacked PDIs (P550) in (POP)4 . The two extreme pools of PDI chromophores were unambiguously confirmed by single-crystal X-ray crystallography and NMR spectroscopy. To interpret the formation of the discrete quadruple assembly, we also developed a two-step cooperative model. Quantum-chemical calculations indicate the existence of multiple couplings within and across P520 and P550, which can satisfactorily describe the photophysical properties of the unequal quadruple assembly. This finding is expected to help advance the rational design of dye stacks to emulate functions of natural LH systems.

Design of Multicomponent Peptide Fibrils with Ordered and Programmable Compositional Patterns.

Advanced applications of biomacromolecular assemblies require a stringent degree of control over molecular arrangement, which is a challenge to current synthetic methods. Here we used a neighbor-controlled patterning strategy to build multicomponent peptide fibrils with an unprecedented capacity to manipulate local composition and peptide positions. Eight peptides were designed to have regulable nearest neighbors upon co-assembly, which, by simulation, afforded 412 different patterns within fibrils, with varied compositions and/or peptide positions. The fibrils with six prescribed patterns were experimentally constructed with high accuracy. The controlled patterning also applies to functionalities appended to the peptides, as exemplified by arranging carbohydrate ligands at nanoscale precision for protein recognition. This study offers a route to molecular editing of inner structures of peptide assemblies, prefiguring the uniqueness and richness of patterning-based material design.

Spontaneous Resolution of Helical Building Blocks through the Formation of Homochiral Helices in Two Dimensions.

Spontaneous resolution leading to conglomerate crystals remains a significant challenge. Here we propose the formation of orthogonal homochiral supramolecular helices in at least two dimensions to allow spontaneous resolution. We suggest the design rationale that the chiral species is made into helical building blocks to allow the helix formation. As a proof-of-concept, acetylalanine was made into a helical short azapeptide, its N-amidothiourea derivative containing a ß-turn structure, to which a halogen atom was further introduced at the phenylthiourea aromatic ring. The resultant folded species undergoes both intermolecular hydrogen and halogen bonding across the turn structure to form orthogonal intermolecular hydrogen-bonded and halogen-bonded supramolecular helices in two dimensions, and undergoes chiral resolution upon crystallization. Meanwhile, counterparts containing either an F-substituent with weak halogen bonding or no halogen atom crystallize as racemic compounds.

Self-Assembled Multivalent Ag-SR Coordination Polymers with Phosphatase-Like Activity.

We show herein the phosphatase-like catalytic activity of coordination polymers obtained after adding Ag+ -ions to thiols bearing hydrophobic alkyl chains terminated with a 1,4,7-triazacyclononane (TACN) group. The subsequent addition of Zn2+ -ions to the self-assembled polymers resulted in the formation of multivalent metal coordination polymers capable of catalysing the transphosphorylation of an RNA-model compound (2-hydroxypropyl-4-nitrophenyl phosphate, HPNPP) with high reactivity. Analysis of a series of metal ions showed that the highest catalytic activity was obtained when Ag+ -ions were used as the first metal ions to construct the backbone of the coordination polymer through interaction with the -SH group followed by Zn2+ -ions as the second metal ions complexed by the TACN-macrocycle. Furthermore, it was demonstrated that the catalytic activity could be modulated by changing the length of the hydrophobic alkyl chain.

Chiral Recognition by Flexible Coordination Polymers of Ag+ with a Cysteine-Based Chiral Thiol Ligand That Bears a Binding Site.

We report a new scheme for chiral recognition using coordination polymers of Ag+ with a chiral thiol ligand that contains a binding group. N-Benzoyl-l-cysteine ethyl ester equipped with a boronic acid group at the para position of the phenyl ring forms coordination polymers with Ag+ in alkaline aqueous solutions that exhibit excellent selectivity toward a d-glucose enantiomer over l-glucose, while the coordination polymers from the d-cysteine-based thiol ligand are specific for l-glucose. It is assumed that a conformation change occurs upon interaction of a saccharide molecule with the polymeric chain receptor, for which the next binding is promoted, leading to the highly effective chiral recognition, despite the flexible nature of the polymeric receptor.

Chalcogen bonding mediates the formation of supramolecular helices of azapeptides in crystals.

To explore whether chalcogen bonding was able to drive the formation of supramolecular helices, alanine-based azapeptides containing a ß-turn structure, with a thiophene group, respectively, incorporated in the N- or C-terminus, were employed as helical building blocks. While the former derivative formed a supramolecular M-helix via intermolecular SS chalcogen bonding in crystals, the latter formed P-helix via intermolecular SO chalcogen bonding.

Naphthodithiophene Diimide Based Chiral π-Conjugated Nanopillar Molecules.

The synthesis, structures, and properties of [4]cyclonaphthodithiophene diimides ([4]C-NDTIs) are described. NDTIs as important n-type building blocks were catenated in the α-positions of thiophene rings via an unusual electrochemical-oxidation-promoted macrocyclization route. The thiophene-thiophene junction in [4]C-NDTIs results in an ideal pillar shape. This interesting topology, along with appealing electronic and optical properties inherited from the NDTI units, endows the [4]C-NDTIs with both near-infrared (NIR) light absorptions, strong excitonic coupling, and tight encapsulation of C60 . Stable orientations of the NDTI units in the nanopillars lead to stable inherent chirality, which enables detailed circular dichroism studies on the impact of isomeric structures on π-conjugation. Remarkably, the [4]C-NDTIs maintain the strong π-π stacking abilities of NDTI units and thus adopt two-dimensional (2D) lattice arrays at the molecular level. These nanopillar molecules have great potential to mimic natural photosynthetic systems for the development of multifunctional organic materials.

Supramolecular Alternating Donor-Acceptor Assembly toward Intercalated Covalent Organic Frameworks.

Conventionally, z-direction modulation of two-dimensional covalent organic frameworks (2D-COFs) is difficult to achieve because they rely on spontaneous π-π interactions to form 3D architectures. Herein, we report a facile construction of a novel intercalated covalent organic framework (Intercalated-COF) by synchronizing operations of supramolecular donor-acceptor (D-A) interactions (A unit: 2,5,8,11-tetra(p-formylphenyl)-perylene diimide (PDI) 1; D unit: perylene 3, as intercalator) in the vertical directions, with polymerizations (by only reacting 1 with p-phenylenediamine 2) in the lateral directions. In this Intercalated-COF, the PDI-based covalent 2D layers are uniformly separated by perylene guest layers. This supramolecular strategy opens the possibility for z-direction modulation of 2D-COFs through "intercalating" various guest molecules and thus may contribute to the exploration of advanced applications of these porous and crystalline frameworks.

Turn Conformation of ß-Amino Acid-Based Short Peptides Promoted by an Amidothiourea Moiety at C-Terminus.

A C-terminal amidothiourea motif is shown to promote a ß-turn-like folded conformation in a series of ß-amino acid-based short peptides in both the solid state and solution phase by an intramolecular 11-membered ring hydrogen bond.

Balancing interactions in proline-based receptors for chiral recognition of l-/d-DOPA.

Proline based receptors (1-14) attached with phenylboronic acid and benzaldehyde binding groups at the N-/C- or C-/N-termini of the proline residue were created for chiral recognition of l-/d-DOPA, in an attempt to examine if balancing the two binding events would influence the recognition. By changing the positions of boronic acid and aldehyde groups substituted on the phenyl rings (1-4, 5-8) and the site at which phenylboronic acid and benzaldehyde moieties attached respectively to the N- and C-termini or C- and N-termini of the proline residue (1-4vs.5-8), and by introducing an electron-withdrawing fluorine atom in the phenyl ring of the weaker binder the benzaldehyde moiety (11vs.1, 14vs.5), we were able to show that a better balance of the two binding events does improve the chiral recognition. This finding can only be made with the current version of receptors that were equipped with two different binding groups. Together with the finding that the chiral recognition performance in mixed organic-aqueous solutions is tunable by varying the solvent composition, we have now arrived at a protocol for designing proline based receptors for extended applications in chiral recognition.

Design of Chiral Supramolecular Polymers Exhibiting a Negative Nonlinear Response.

Many synthetic and supramolecular chiral polymeric systems are known to exhibit the "majority rules effect" (MRE), a positive nonlinear response in which a small enantiomeric excess (ee) of the chiral building blocks leads to unproportionally large chiroptical signals near zero ee. In contrast, the opposite "racemate rules effect" (RRE), a negative nonlinear response in which the chiroptical signals are flat near zero ee, while giving large nonlinear chiroptical responses to ee at high values, has only been occasionally observed. The origin of this unusual ee dependence remains elusive largely because few systems have been established that exhibit this effect. Herein, we present a design approach that enables the development of chiral supramolecular polymers with a pronounced negative nonlinear response akin to RRE. This is achieved by in situ generating a bidentate inducer for supramolecular polymerization that exists in both meso- and homochiral forms upon reacting with chiral guests. The presence of the meso-inducer creates an aggregate structure that has a little response in the circular dichroism (CD) spectra as a function of ee at a particular wavelength, but a homochiral inducer gives large changes in response to ee at this wavelength. This allowed for an RRE-like response to be observed when the CD intensity of the supramolecular polymers was plotted against the ee of the chiral guests that generate the meso- and homochiral inducers without the necessity of the racemic guest preferentially being incorporated into the polymer.

Discrete π-Stacks from Self-Assembled Perylenediimide Analogues.

The formation of well-defined finite-sized aggregates represents an attractive goal in supramolecular chemistry. In particular, construction of discrete π-stacked dye assemblies remains a challenge. Reported here is the design and synthesis of a novel type of discrete π-stacked aggregate from two comparable perylenediimide (PDI) dyads (PEP and PBP). The criss-cross PEP-PBP dimers in solution and (PBP-PEP)-(PEP-PBP) tetramers in the solid state are well elucidated using single-crystal X-ray diffraction, dynamic light scattering, and diffusion-ordered NMR spectroscopy. Extensive π-π stacking between the PDI units of PEP and PBP as well as repulsive interactions of swallow-tailed alkyl substituents are responsible for the selective formation of discrete dimer and tetramer stacks. Our results reveal a new approach to preparing discrete π stacks that are appealing for making assemblies with well-defined optoelectronic properties.

Proline-Based Boronic Acid Receptors for Chiral Recognition of Glucose.

Chiral recognition remains a major challenge in the area of molecular receptor design. With this research, we set out to explore the use of proline-based receptors for chiral recognition. Importantly, the proline structure allows for the introduction of at least two different binding groups due to the availability of both an amine and carboxylic acid group. Here we report a proof-of-concept exploration into the chiral recognition of d/l-glucose as a model chiral species, which prefers to bind to at least two boronic acid groups. We evaluated several proline-based receptors incorporating two phenylboronic acid groups, respectively, at the N- and C-termini of the amino acid residue, via amide bonds. We confirmed that the receptors exhibited chiral recognition using CD, 1H NMR, and 19F NMR spectroscopy. Given the derivation diversity available, our strategy to use proline-based receptors for chiral recognition holds significant promise for extension to other chiral systems.

C-I···π Halogen Bonding Driven Supramolecular Helix of Bilateral N-Amidothioureas Bearing ß-Turns.

We report the first example of C-I···π halogen bonding driven supramolecular helix in highly dilute solution of micromolar concentration, using alanine based bilateral I-substituted N-amidothioureas that contain helical fragments, the ß-turn structures. The halogen bonding interactions afford head-to-tail linkages that help to propagate the helicity of the helical fragments. In support of this action of the halogen bonding, chiral amplification was observed in the supramolecular helix formed in acetonitrile solution. The present finding provides alternative tools in the design of self-assembling macromolecules.

Aggregation-Switching Strategy for Promoting Fluorescent Sensing of Biologically Relevant Species: A Simple Near-Infrared Cyanine Dye Highly Sensitive and Selective for ATP.

We report a strategy for enhanced performance of fluorescent sensing of biologically relevant species that often bind with natural receptors via multiple interactions. We propose making a fluorescent sensory molecule to form H-aggregates such that its emission is quenched leaving a low background, and upon binding to a biologically relevant species, the aggregate switches to another form in which the fluorescent species is better protected to afford a stronger emission signal. Meanwhile, the aggregated fluorescent dyes afford multiple interactions with the sensing species that require multiple binding sites. The lower background, stronger binding, and stronger signal would therefore lead to a much higher sensing performance, as improved selectivity would also result in along with the signal amplification. We thus designed a near-IR cyanine dye bearing two boronic acid groups (Cy-BA) for fluorescent sensing of ATP such that the boronic acid groups in the dye molecule bind to the cis-diol moiety in ATP. Introduction of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) below its critical aggregation concentration is key because Cy-BA molecules made into H-aggregates were practically nonfluorescent. Upon mixing with ATP, a dramatic enhancement in the fluorescence occurred because of the formation of ATP/Cy-BA/DTAB vesicles in which the fluorescent dye is well dispersed and protected. This sensing scheme, despite the dynamic nature of the boronic acid/cis-diol interaction, weakness of the electrostatic interactions among ATP/Cy-BA/DTAB, and poor selectivity of these interactions, allows for highly sensitive and selective detection of ATP in aqueous solution.

Recent advances in boronic acid-based optical chemosensors.

Reversible covalent binding of boronic acids with polyols and Lewis bases has facilitated the development of robust chemosensors for many biologically important species under physiologically or environmentally relevant conditions. This minireview covers selected examples of advances reported in this area from 2014 to 2016. While the discovery of new boron-containing binding motifs and identification of new analytical targets have expanded the ultility of boronic acid-based molecular recognition, unconventional sensing strategies such as exploitation of nanoscale self-assembly, multicomponent dynamic covalent assembly, and coupling boronate ester formation with a further chemical reaction have led to significantly improved sensor performance, enabling real-world applications in various areas such as cell biology and asymmetric catalysis.