Sequence-Selective Binding of Oligopeptides in Water through Hydrophobic Coding

Nanoparticles that Distinguish Chemical and Supramolecular Contexts of Lysine for Single-Site Functionalization of Protein

Lysine is one of the most abundant residues on the surface of proteins and its site-selective functionalization is extremely challenging. The existing methods of functionalization rely on differential reactivities of lysine on a protein, making it impossible to label less reactive lysines selectively. We here report polymeric nanoparticles that mimic enzymes involved in the posttranslational modifications of proteins that distinguish the chemical and supramolecular contexts of a lysine and deliver the labeling reagent precisely to its ε amino group. The nanoparticles are prepared through molecular imprinting of cross-linkable surfactant micelles, plus an in situ, on-micelle derivatization of the peptide template prior to the imprinting. The procedures encode the polymeric nanoparticles with all the supramolecular information needed for sequence identification and precise labeling, allowing single-site functionalization of a predetermined lysine on the target protein in a mixture.

Molecularly imprinted nanoparticles reveal regulatory scaffolding features in Pyk2 tyrosine kinase

Pyk2 is a multi-domain non-receptor tyrosine kinase that serves dual roles as a signaling enzyme and scaffold. Pyk2 activation involves a multi-stage cascade of conformational rearrangements and protein interactions initiated by autophosphorylation of a linker site. Linker phosphorylation recruits Src kinase, and Src-mediated phosphorylation of the Pyk2 activation loop confers full activation. The regulation and accessibility of the initial Pyk2 autophosphorylation site remains unclear. We employed peptide-binding molecularly imprinted nanoparticles (MINPs) to probe the regulatory conformations controlling Pyk2 activation. MINPs differentiating local structure and phosphorylation state revealed that the Pyk2 autophosphorylation site is protected in the autoinhibited state. Activity profiling of Pyk2 variants implicated FERM and linker residues responsible for constraining the autophosphorylation site. MINPs targeting each Src docking site disrupt the higher-order kinase interactions critical for activation complex maturation. Ultimately, MINPs targeting key regulatory motifs establish a useful toolkit for probing successive activational stages in the higher-order Pyk2 signaling complex.

Rational Design and Synthesis of an Artificial Enzyme for SN2 Reactions through Micellar Imprinting

The rational design of catalysts with enzyme-like properties is an elusive goal of chemists despite tremendous interest. Molecular imprinting inside surfactant micelles, followed by postmodification, creates a tailored active site in a water-soluble polymeric "artificial enzyme" for the benzylation of 4-nitrophenol. The reaction happens under neutral conditions with excellent substrate selectivity. Similar to many enzymes, electrostatics play vital roles in catalysis and can be tuned through different bases introduced into the active site.

Carbonic anhydrase mimics with rationally designed active sites for fine-tuned catalytic activity and selectivity in ester hydrolysis

Numerous hydrolytic enzymes utilize zinc as a cofactor for catalysis. We here report water-soluble polymeric nanoparticles with zinc ions in active sites and a nearby base as a mimic of carbonic anhydrase (CA). Their pKa of 6.3-6.4 for zinc-bound water is lower than the 6.8-7.3 value for natural enzymes, which allows the catalyst to hydrolyze nonactivated alkyl esters under neutral conditions-a long sought-after goal for artificial esterases. The size and shape of the active site can be rationally tuned through a template used in molecular imprinting. Subtle structural changes in the template, including shifting an ethyl group by one C-N bond and removal of a methylene group, correlate directly with catalytic activity. A catalyst can be made to be highly specific or have broad substrate specificity through modular synthesis of templates.

Acid-Functionalized Artificial Enzymes with Tunable Selectivity for Vinyl Ether Hydrolysis

Enzymes often employ catalytic groups with a medium or low intrinsic activity for highly challenging catalytic tasks. In this work, we report nanoparticle catalysts with accurately positioned carboxylic acids through either a covalent or noncovalent imprinting technique. The rationally designed active site allows the catalysis to be highly selective or quite unselective with respect to the substrate. With the proper catalyst, the hydrolysis proceeds smoothly in neutral water or even a slightly basic solution at room temperature.

Nanoparticle Scanners for the Identification of Key Sequences Involved in the Assembly and Disassembly of β-Amyloid Peptides

The aggregation of β-amyloid peptides (Aβ), implied in the development and progression of Alzheimer's disease, is driven by a complex set of intramolecular and intermolecular interactions involving both hydrophobic and polar residues. The key residues responsible for the forward assembling process may be different from those that should be targeted to disassemble already formed aggregates. Molecularly imprinted nanoparticle (MINP) receptors are reported in this work to strongly and selectively bind specific segments of Aβ₄₀. Combined fluorescence spectroscopy, atomic force microscopy (AFM) imaging, and circular dichroism (CD) spectroscopy indicate that binding residues 21-30 near the loop region is most effective at inhibiting the aggregation of monomeric Aβ₄₀, but residues 11-20 that include the internal β strand closer to the N-terminal represent the best target for disaggregating already formed aggregates in the polymerization phase. Once the aggregation proceeds to the saturation phase, binding residues 1-10 has the largest effect on the disaggregation, likely because of the accessibility of these amino acids relative to others to the MINP receptors.

Synthesis of a magnetic micelle molecularly imprinted polymers to selective adsorption of rutin from Sophora japonica

Rutin is a naturally active compound with biological and medical value. The traditional extraction and separation method not only destroys the structure and activity of rutin, but results in a low extraction rate. In this work, the magnetic micellar molecularly imprinted polymer of rutin with a selective recognition function, i.e., RMMMIP was synthesized from 4 to Vinylphenylboron acid and 4-Vinylpyridine as functional monomer, derivatives of cholic acid as amphiphilic molecules. The internal hydrophobic and external hydrophilic characteristics of micelle was used to weaken the solvation of rutin and strengthen the non-covalent interaction between functional monomer and rutin. Fe₃O₄, as the core, endowed the composite materials with good magnetic responsiveness and was easy to separate solid from liquid. Then its structure and adsorption were studied, adsorbing capacity and recognition specific factor of RMMMIP are 11.9 mg·g^-1 and 3.55 respectively. RMMMIP was used for the separation of rutin from crude extracts of Sophora japonica Linn and showed a better selective adsorption capacity than quercetin, naringin and cyanidin-3-O-glucose. It indicated that RMMMIP as a specific adsorbent had the potential to be a practical way to purify rutin from rutin crude extracts in the future.

Molecularly imprinted materials for glycan recognition and processing

Carbohydrates are the most abundant organic molecules on Earth and glycosylation is the most common posttranslational modification of proteins. Glycans are involved in a plethora of biological processes including cell adhesion, bacterial and viral infection, inflammation, and cancer development. Coincidently, glycosides were some of the earliest molecules imprinted and have been instrumental in the development of covalent molecular imprinting technology. This perspective illustrates recently developed molecularly imprinted materials for glycan binding and processing. Novel imprinting techniques and postmodification led to development of synthetic glycan-binding materials capable of competing with natural lectins in affinity and artificial glycosidases for selective hydrolysis of complex glycans. These materials are expected to significantly advance glycochemistry, glycobiology, and related areas such as biomass conversion.

Dynamic Tuning in Synthetic Glycosidase for Selective Hydrolysis of Alkyl and Aryl Glycosides

Enzymes use sophisticated conformational control to optimize the dynamics of their protein framework for efficient catalysis. Although it is difficult to employ a similar strategy to improve catalysis in a synthetic enzyme, we here report that modulation of the dynamics of the substrate in the active site is readily achievable in a complex between a molecularly imprinted nanoparticle and its acid cofactor, through tuning of the size and shape of the imprinted site. As the alkyl glucoside substrate is bound with increasing strength and held in a more tightly fitted pocket, the acid-catalyzed glycan hydrolysis becomes more difficult. A larger, wider active site, although less able to bind the substrate, affords a higher catalytic activity, likely due to easier alignment of the substrate and the acid cofactor for a general acid catalysis. The substrate selectivity is controlled by both the tightness of the aglycon-binding site and the orientation of the glycan-binding boroxole group.

Site-Selective Catalytic Epoxidation of Alkene with Tunable, Atomic Precision by Molecularly Imprinted Artificial Epoxidases

Distinction of chemical functionality by their local chemical environment is a skill mastered by enzymes, evident from the selective synthesis, cleavage, and transformation of peptides, nucleic acids, and polysaccharides that abound with the same type of functional groups. In contrast, synthetic catalysts are generally better at differentiating functional groups based on their electronic and steric properties. Here we report artificial epoxidases prepared through molecular imprinting of surface-core doubly cross-linked micelles, followed by efficient functionalization of the imprinted site in the micellar core via photoaffinity labeling. The size and shape of the active sites are tuned by the modularly synthesized templates, with the oxygen-delivering peroxy acid group positioned accurately. These catalysts are used in epoxidation of alkene in water with hydrogen peroxide under mild conditions, without any additional additives. Most importantly, atomic precision is achieved in the catalysis and enables alkenes to be distinguished that differ in the position of the carbon-carbon double bond by a single carbon.

Molecular recognition of enzymes and modulation of enzymatic activity by nanoparticle conformational sensors

Regulation of enzyme activity is key to dynamic processes in biology but is difficult to achieve with synthetic systems. We here report molecularly imprinted nanoparticles with strong binding for the N- and C-terminal peptides on lysozyme. Binding affinity for the enzyme correlated with conformational flexibility of the peptides in the protein structure. Significantly, binding at the C-terminus of lysozyme enhanced the performance of the enzyme at elevated temperatures and that at the N-terminus lowered the enzyme activity. These nanoparticles, when clicked onto magnetic nanoparticles, could also be used to fish out the protein of interest from a mixture in a single step.

Epitope-imprinted polymers: Design principles of synthetic binding partners for natural biomacromolecules

Molecular imprinting (MI) has been explored as an increasingly viable tool for molecular recognition in various fields. However, imprinting of biologically relevant molecules like proteins is severely hampered by several problems. Inspired by natural antibodies, the use of epitopes as imprinting templates has been explored to circumvent those limitations, offering lower costs and greater versatility. Here, we review the latest innovations in this technology, as well as different applications where MI polymers (MIPs) have been used to target biomolecules of interest. We discuss the several steps in MI, from the choice of epitope and functional monomers to the different production methods and possible applications. We also critically explore how MIP performance can be assessed by various parameters. Last, we present perspectives on future breakthroughs and advances, offering insights into how MI techniques can be expanded to new fields such as tissue engineering.

Substrate Protection in Controlled Enzymatic Transformation of Peptides and Proteins

Proteins are involved in practically every single biological process. The many enzymes involved in their synthesis, cleavage, and posttranslational modification (PTM) carry out highly specific tasks with no usage of protecting groups. Yet, the chemists' strategy of protection/deprotection potentially can be highly useful, for example, when a specific biochemical reaction catalyzed by a broad-specificity enzyme needs to be inhibited, during infection of cells by enveloped viruses, in the invasion and spread of cancer cells, and upon mechanistic investigation of signal-transduction pathways. Doing so requires highly specific binding of peptide substrates in aqueous solution with biologically competitive affinities. Recent development of peptide-imprinted cross-linked micelles allows such protection and affords previously impossible ways of manipulating peptides and proteins in enzymatic transformations.

Sequence-selective recognition of cationic amphipathic tripeptides with similar structures in aqueous solutions by cucurbit[7]uril

Sequence-selective recognition of cationic amphipathic peptides by synthetic receptors is significant to biological applications, but it is still a great challenging task. Here we first study the binding characteristics of receptor cucurbit[7]uril (CB[7]) to the smallest aromatic tripeptides X1GG (X1 = tryptophan (W), phenylalanine (F), and tyrosine (Y)) and basic tripeptides X2GG (X2 = arginine (R), lysine (K), and histidine (H)) by molecular dynamics simulations. The study indicates that the sidechains of aromatic X1 residues can be encapsulated into the CB[7] cavity, while the sidechains of basic X2 residues prefer to locate at the CB[7] portal. Based on that, we consider hydrophobic aromatic residues as the N-terminus, the smallest glycine (G) as the 2nd-residue and basic residues as the C-terminus, and design nine tripeptides X1GX2 (X1 = F, Y, W and X2 = H, K, R). We found that there is a great influence of the C-terminal basic residue of X1GX2 on binding with CB[7] due to the introduction of a new binding site between CB[7] and the sidechain of the C-terminal residue. Interestingly, CB[7] can differentiate WGR and WGK with similar structures efficiently because of their eight orders of magnitude difference in the association constant (Ka). Besides, for WGR, YGR, and YGK with a nanomolar binding affinity (Ka > 109 M-1), on reversing the sequence order of the 2nd-residue and 3rd-residue, their Ka reduces by about at least 1000-fold, implying the sequence dependence of CB[7] on recognizing these tripeptides. These results predict the potential applications of CB[7] in recognizing cationic amphipathic peptides.

Tandem Aldol Reaction from Acetal Mixtures by an Artificial Enzyme with Site-Isolated Acid and Base Functionalities

Site-isolation of catalysts can enable incompatible catalysts such as acid and base to be used in one pot for enhanced efficiency and other benefits. Although many synthetic platforms have been reported for this purpose, they generally do not possess the exquisite selectivity of site-isolated enzymes in nature. Here we report water-soluble protein-sized nanoparticles with site-isolated acids in the core and amines on the surface. The catalysts were made through molecular imprinting of cross-linked micelles, followed by facile one-step photoaffinity labeling of the imprinted binding site. With a tunable, substrate-specific active site, the bifunctional artificial enzyme catalyzed highly selective tandem cross aldol reaction between acetone and mixtures of isomeric aryl acetals. It could also transform a less reactive substrate over a more reactive one.

Sequence-Selective Protection of Peptides from Proteolysis

Proteolysis of proteins and peptides is involved in the infection of cells by enveloped viruses and also in the invasion and spread of cancer cells. Shutting down broad‐specificity proteases, however, is problematic because normal functions by these proteases will be affected. Herein, nanoparticle receptors were prepared from molecular imprinting for complex biological peptides. Their strong and selective binding enabled them to protect their targeted sequences from proteolysis in aqueous solution at stoichiometric amounts. Generality of the method was demonstrated by the protection of hydrophobic and hydrophilic peptides from different proteases, selective protection of a segment of a long peptide, and selective protection of a targeted peptide in a mixture. Most interestingly, two receptors targeting different parts of a long peptide could work in cooperation to protect the overall sequence, highlighting the versatility of the method.

Selective Hydrolysis of Aryl Esters under Acidic and Neutral Conditions by a Synthetic Aspartic Protease Mimic

Aspartic proteases use a pair of carboxylic acids to activate water molecules for nucleophilic attack. Here we report a nanoparticle catalyst with a similar catalytic motif capable of generating a hydroxide ion in its active site even under acidic reaction conditions. The synthetic enzyme accelerated the hydrolysis of para-nitrophenyl acetate (PNPA) by 91,000 times and could also hydrolyze nonactivated aryl esters at pH 7. The distance between the two acids and, in particular, the flexibility of the catalytic groups in the active site controlled the catalytic efficiency. The synthetic enzyme readily detected the addition of a single methyl on the acyl group of the substrate, as well as the substitution pattern on the phenyl ring.

Temperature-Dependent Low-Frequency Vibrations of Thiamine Crystal Containing Hydrated Ions

Low-frequency vibrations of crystalline molecules are very sensitive to the local environment in which the molecules, for example, hydrated ions captured in crystals, find themselves. We present low-temperature X-ray crystallographic measurements on the harvested thiamine crystal containing hydrated ions and its temperature-dependent terahertz spectra and synchrotron infrared microspectra. It is found from the X-ray structure that the hydrated ions and hydration water are in a similar environment to liquid, although those are captured in crystals. The vibrationally resolved THz spectra of two states in the present organic crystals containing hydrated ions are well explained by the difference in the hydrogen-bonded pattern. Peak assignments were performed based on highly accurate first-principles calculations incorporating relativistic effects and dispersion corrections. The temperature dependences are observed for the vibrations around the chloride ions and hydration water due to the loose binding of chloride ions, the bond elongation with increasing temperature, and the cleavage of weak hydrogen bonds.

Synthetic Glycosidase Distinguishing Glycan and Glycosidic Linkage in Its Catalytic Hydrolysis

Selective hydrolysis of carbohydrates is vital to the processing of these molecules in biology but has rarely been achieved with synthetic catalysts. The challenge is especially difficult because the catalyst needs to distinguish the inversion of a single hydroxyl and the α or β glycosidic bonds that join monosaccharide building blocks. Here we report synthetic glycosidase prepared through molecular imprinting within a cross-linked micelle. The nanoparticle catalyst resembles natural enzymes in dimension, water-solubility, and a hydrophilic/hydrophobic surface-core topology. Its boronic acid-functionalized active site binds its targeted glycoside substrate and an acid cofactor simultaneously, with the acidic group in close proximity to the exocyclic glycosidic oxygen. The hydrophobically anchored acid cofactor is tunable in acidity and causes selective cleavage of the targeted glycoside in mildly acidic water. Selectivity for both the glycan and the α/β glycosidic bond can be rationally designed through the molecular imprinting process.

pH-Controlled Nanoparticle Catalysts for Highly Selective Tandem Henry Reaction from Mixtures

Nature has a remarkable ability to perform selective transformation of complex biological mixtures into desired products using enzymatic catalysts. We report the preparation of nanoparticle catalysts through molecular imprinting within cross-linked micelles. These catalysts were highly selective for their targeted substrates and could selectively hydrolyze less reactive acetals over more reactive ones even under basic conditions. Their catalytic activity and selectivity were tunable through rational postmodification of the active site. These properties enabled the nanoparticle catalysts to produce the desired β-nitro alcohol from a four-component acetal mixture in a tandem deprotection/Henry reaction that required incompatible acidic and basic catalysts in the two steps.

Synthetic glycosidases for the precise hydrolysis of oligosaccharides and polysaccharides

Glycosidases are an important class of enzymes for performing the selective hydrolysis of glycans. Although glycans can be hydrolyzed in principle by acidic water, hydrolysis with high selectivity using nonenzymatic catalysts is an unachieved goal. Molecular imprinting in cross-linked micelles afforded water-soluble polymeric nanoparticles with a sugar-binding boroxole in the imprinted site. Post-modification installed an acidic group near the oxygen of the targeted glycosidic bond, with the acidity and distance of the acid varied systematically. The resulting synthetic glycosidase hydrolyzed oligosaccharides and polysaccharides in a highly controlled fashion simply in hot water. These catalysts not only broke down amylose with similar selectivities to those of natural enzymes, but they also could be designed to possess selectivity not available with biocatalysts. Substrate selectivity was mainly determined by the sugar residues bound within the active site, including their spatial orientations. Separation of the product was accomplished through in situ dialysis, and the catalysts left behind could be used multiple times with no signs of degradation. This work illustrates a general method to construct synthetic glycosidases from readily available building blocks via self-assembly, covalent capture, and post-modification. In addition, controlled, precise, one-step hydrolysis is an attractive way to prepare complex glycans from naturally available carbohydrate sources.

Molecularly Imprinted Micelles for Fluorescent Sensing of Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used over-the-counter drugs and their uncontrolled disposal is a significant environmental concern. Although their fluorescent sensing is a desirable method of detection for its sensitivity and simplicity, the structural similarity of the drugs makes the design of selective sensors highly challenging. A thiourea-based fluorescent functional monomer was identified in this work to enable highly efficient synthesis of molecularly imprinted nanoparticle (MINP) sensors for NSAIDs such as Indomethacin or Tolmetin. Micromolar binding affinities were obtained in aqueous solution, with binding selectivities comparable to those reported for polyclonal antibodies. The detection limit was ~50 ng/mL in aqueous solution, and common carboxylic acids such as acetic acid, benzoic acid, and citric acid showed negligible interference.

Synthetic lectins for selective binding of glycoproteins in water

Although synthetic mimics of lectins can be extremely useful in biological and biomedical research, molecular recognition of carbohydrates has been hampered by their strong solvation in water and subtle structural differences among analogues. Molecularly imprinted nanoparticle receptors were prepared with glycans directly cleaved from glycoproteins. Functionalized with boroxole groups in the binding sites, these water-soluble synthetic lectins bound the parent glycoproteins selectively in water with an association constant of Ka = 104-105 M-1. The strong binding enabled the receptors to protect the targeted glycans from enzymatic cleavage. When clicked onto magnetic nanoparticles, the receptors enabled facile isolation of glycoproteins from a mixture.

Selective Binding of Complex Glycans and Glycoproteins in Water by Molecularly Imprinted Nanoparticles

Synthetic receptors to recognize biological glycans are in great need for modern glycoscience and technology, but their design and synthesis have been a daunting challenge due to strong solvation of carbohydrates in water and structural complexity of the guest. Molecular imprinting in surfactant micelles with amide cross-linkers provides a convenient one-pot method to prepare nanoparticle receptors for glycosides, glycans, and glycoproteins, taking advantage of hydrogen-bonding interactions near the surfactant/water interface. Biologically competitive micromolar binding affinities were obtained in water and subtle structural differences of glycans could be distinguished.

Cross-linked small-molecule capsules with excitation wavelength-dependent photoluminescence and high loading capacity: design, synthesis and application in imaging-guided drug delivery

The cross-linked small-molecule micelles (cSMs) have found applications in many fields but their low loading capacity and non-fluorescence property hindered their further development. Herein, water-soluble organic nanoparticles were applied as templates to "stretch" the hydrophobic core of cSMs and photo-cross-linking was employed to supply photoluminescence. The resulting cross-linked small-molecule capsules (cSCs) not only reserve the superior properties of cSMs of accurate monomer, easy functionalization and robust stability, but also achieve high drug loading capacity and excitation wavelength-dependent fluorescence, where the drug loading contents (DLCs) for various hydrophobic drugs were more than 30-fold higher than that of cSMs, and the maximum quantum yield could be as high as 12.0%. Featuring these superiorities, the cSCs hold promising potential in many fields and an example of doxorubicin-loaded cSCs (DOX@cSCs) for multichannel imaging-guided drug delivery is shown in this work.

Selective Binding of Dopamine and Epinephrine in Water by Molecularly Imprinted Fluorescent Receptors

Catecholamines play important roles in biology but their structural similarity makes it challenging to construct synthetic receptors with selective binding. A combination of covalent and noncovalent binding groups in the hydrophobic core of water-soluble nanoparticles enabled them to recognize dopamine and epinephrine with an association constant (Ka ) of 3-4×104  M-1 in water, an order of magnitude higher than those of previously reported synthetic hosts. In addition, minute structural changes among analogues were detected including the addition or removal of a single hydroxyl or methyl group.

Self-Assembly and Molecular Recognition in Water: Tubular Stacking and Guest-Templated Discrete Assembly of Water-Soluble, Shape-Persistent Macrocycles

Supramolecular chemistry in aqueous media is an area with great fundamental and practical significance. To examine the role of multiple noncovalent interactions in controlled assembling and binding behavior in water, the self-association of five water-soluble hexakis(m-phenylene ethynylene) (m-PE) macrocycles, along with the molecular recognition behavior of the resultant assemblies, is investigated with UV-vis, fluorescence, CD, and NMR spectroscopy, mass spectrometry, and computational studies. In contrast to their different extents of self-aggregation in organic solvents, all five macrocycles remain aggregated in water at concentrations down to the micromolar (μM) range. CD spectroscopy reveals that 1-F₆ and 1-H₆, two macrocycles carrying chiral side chains and capable of H-bonded self-association, assemble into tubular stacks. The tubular stacks serve as supramolecular hosts in water, as exemplified by the interaction of macrocycles 1-H₆ and 2-H₆ and guests G1 through G4, each having a rod-like oligo(p-phenylene ethynylene) (p-PE) segment flanked by two hydrophilic chains. Fluorescence and ¹H NMR spectroscopy revealed the formation of kinetically stable, discrete assemblies upon mixing 2-H₆ and a guest. The binding stoichiometry, determined with fluorescence, ¹H NMR, and ESI-MS, reveals that the discrete assemblies are novel pseudorotaxanes, each containing a pair of identical guest molecules encased by a tubular stack. The two guest molecules define the number of macrocyclic molecules that comprise the host, which curbs the "infinite" stack growth, resulting in a tubular stack with a cylindrical pore tailoring the length of the p-PE segment of the bound guests. Each complex is stabilized by the action of multiple noncovalent forces including aromatic stacking, side-chain H-bonding, and van der Waals interactions. Thus, the interplay of multiple noncovalent forces aligns the molecules of macrocycles 1 and 2 into tubular stacks with cylindrical inner pores that, upon binding rod-like guests, lead to tight, discrete, and well-ordered tubular assemblies that are unprecedented in water.

Molecularly Imprinted Nanoparticles for Biomedical Applications

Molecularly imprinted polymers (MIPs) are synthetic receptors with tailor-made recognition sites for target molecules. Their high affinity and selectivity, excellent stability, easy preparation, and low cost make them promising substitutes to biological receptors in many applications where molecular recognition is important. In particular, spherical MIP nanoparticles (or nanoMIPs) with diameters typically below 200 nm have drawn great attention because of their high surface-area-to-volume ratio, easy removal of templates, rapid binding kinetics, good dispersion and handling ability, undemanding functionalization and surface modification, and their high compatibility with various nanodevices and in vivo biomedical applications. Recent years have witnessed significant progress made in the preparation of advanced functional nanoMIPs, which has eventually led to the rapid expansion of the MIP applications from the traditional separation and catalysis fields to the burgeoning biomedical areas. Here, a comprehensive overview of key recent advances made in the preparation of nanoMIPs and their important biomedical applications (including immunoassays, drug delivery, bioimaging, and biomimetic nanomedicine) is presented. The pros and cons of each synthetic strategy for nanoMIPs and their biomedical applications are discussed and the present challenges and future perspectives of the biomedical applications of nanoMIPs are also highlighted.

Dandelion flower-like micelles

Dandelion flower-like micelles (DFMs) were prepared by self-assembly of polycaprolactone (PCL) functionalized surface cross-linked micelles (SCMs). Upon reductive stimuli, the SCMs can be released from the DFMs by non-Brownian motion at an average speed of 19.09 μm s-1. Similar to the property of dandelion flowers dispersing their seeds over a long distance, the DFMs demonstrated enhanced multicellular tumor spheroid (MTS) penetration, a useful property in the treatment of many diseases including cancer, infection-of-biofilm diseases and ocular problems.

Integrating ionic liquids with molecular imprinting technology for biorecognition and biosensing: A review

As promising alternatives to natural receptors, artificial molecularly imprinted polymers (MIPs) have received great attention in biotechnology. Nevertheless, some bottlenecks limit their further development, including low adsorption capacity, poor recognition efficiency, slow response, and insipid aqueous compatibility. Ionic liquids (ILs) show the features of tailored structures and properties, high conductivity, good solubility, and excellent stability. Because of these advantages, they have found intensive use in MIPs by remedying the latter's shortcomings. In this review, we summarize the integration of ILs and MIPs for biorecognition and biosensing. The versatile roles of ILs in improving the performance of MIPs are firstly summarized, including serving as solvents, porogens, functional monomers, organic surface modifiers, dummy templates, and cross-linkers. Then, specific applications of IL-based MIPs in peptide recognition, protein sensing, and food safety analysis are discussed. Finally, future trends and challenges for the design and development of IL-based MIPs and their applications in the biorecognition and biosensing are proposed.

Multivalent Ultrasensitive Interfacing of Supramolecular 1D Nanoplatforms

Multivalent display on linear platforms is used by many biomolecular systems to effectively interact with their corresponding binding partners in a dose-responsive and ultrasensitive manner appropriate to the biological system at hand. Synthetic supramolecular multivalent displays offer a matching approach for the modular and bottom-up construction and systematic study of dynamic 1D materials. Fundamental studies into multivalent interactions between such linear, 1D materials have been lacking because of the absence of appropriate modular nanoplatforms. In this work we interfaced two synthetic multivalent linear nanoplatforms based on a dynamic supramolecular polymer, formed by hybrid discotic-oligonucleotide monomers, and a series of complementary DNA-duplex-based multivalent ligands, also with appended short oligonucleotides. The combination of these two multivalent nanoplatforms provides for the first time entry to study multivalent effects in dynamic 1D systems, of relevance for the conceptual understanding of multivalency in biology and for the generation of novel multivalent biomaterials. Together the two nanoscaffolds provide easy access to libraries of multivalent ligands with tunable affinities. The DNA scaffold allows for exact control over valency and spatial ligand distribution, and the discotic supramolecular polymer allows for dynamic adaptation and control over receptor density. The interaction between the two nanoplatforms was studied as a function of ligand interaction strength, valency, and density. Usage of the enhancement parameter β allowed quantification of the effects of ligand valency and affinity. The results reveal a generalized principle of additive binding increments. Receptor density is shown to be crucially and nonlinearly correlated to complex formation, leading to ultrasensitive responses. The results reveal that, not unlike biomolecular signaling, high density multivalent display of receptors is crucial for functionally increased affinities.

Zwitterionic Molecularly Imprinted Cross-Linked Micelles for Alkaloid Recognition in Water

Molecular imprinting within surface/core doubly cross-linked micelles afforded water-soluble nanoparticle receptors for their template molecules. Extremely strong imprinting effects were consistently observed, with the imprinting factor >100:1 in comparison to nonimprinted nanoparticles prepared without the templates. The ionic nature of the cross-linkable surfactant strongly impacted the imprinting and binding process. Imprinted receptors prepared with a zwitterionic cross-linkable surfactant (4) outperformed a similar cationic one (1) when the template was zwitterionic or cationic and preferred their templates over structural analogues regardless of their ionic characteristics. Electrostatic interactions, however, dominated the receptors made with the cationic surfactant. The same micellar imprinting applied to simple as well as complex alkaloids. Imprinted receptors from 4 were also shown to categorize their alkaloid guests according to their structural similarity.

Effects of nano-confinement and conformational mobility on molecular imprinting of cross-linked micelles

Molecular imprinting is a facile method to create guest-complementary binding sites in a cross-linked polymeric network. When performed within cross-linked micelles, the resulting molecularly imprinted nanoparticles (MINPs) exhibited an extraordinary ability to distinguish subtle structural changes in the guest, including the shift of a hydrophilic or hydrophobic group by 1 carbon and addition of a single methylene/methyl group. A high surface-cross-linking density prior to core-cross-linking was key to the high-fidelity imprinting, enhancing both the binding affinity of the imprinted micelle for the template and selectivity among structural analogues. Whereas the imprinted site closely complemented the hydrophilic surface anchoring group and rigid hydrophobic aromatic core, it was expanded significantly for a conformationally mobile small group (i.e., methoxy).

Chiral Gating for Size- and Shape-Selective Asymmetric Catalysis

A poor or mediocre stereoselectivity is a key roadblock for a chiral catalyst to find practical adoptions. We report a facile method to create a tunable chiral space near a chiral catalyst to augment its selectivity. The space was created rationally through templated polymerization within cross-linked micelles, using readily available amino acid derivatives. It provided gated entrance of reactants to the catalyst, enabling a mediocre prolinamide to catalyze aldol condensation in water with excellent yields and ee, in a size- and shape-selective manner.

General Method for Peptide Recognition in Water through Bioinspired Complementarity

A general method for peptide recognition has been elusive despite decades of research. Strong binding and selectivity among closely related peptides are necessary for biological applications but have been difficult to achieve with synthetic receptors. With inspiration from highly specific protein-protein and protein-ligand interactions, protein-sized, water-soluble imprinted nanoparticles were prepared via templated polymerization of peptides within cross-linked micelles. Combination of hydrophobic and polar interactions afforded micromolar to submicromolar binding affinities for selected tripeptides. A "golden pair" of functional monomers was identified to enhance both the affinity and selectivity of binding, and enabled differentiation of subtly different sequences including single-point variation of lysine by arginine and insertion of a single glycine at the N- or C-terminus. Biological peptides (β-amyloid peptides) afforded even stronger binding (tens of nanomolar) due to a larger number of complementary interactions between the host and the guest, opening doors to a wide range of biological applications.

Molecularly Imprinted Materials for Selective Biological Recognition

Molecular imprinting is an approach of generating imprinting cavities in polymer structures that are compatible with the target molecules. The cavities have memory for shape and chemical recognition, similar to the recognition mechanism of antigen-antibody in organisms. Their structures are also called biomimetic receptors or synthetic receptors. Owing to the excellent selectivity and unique structural predictability of molecularly imprinted materials (MIMs), practical MIMs have become a rapidly evolving research area providing key factors for understanding separation, recognition, and regenerative properties toward biological small molecules to biomacromolecules, even cell and microorganism. In this review, the characteristics, morphologies, and applicability of currently popular carrier materials for molecular imprinting, especially the fundamental role of hydrogels, porous materials, hierarchical nanoparticles, and 2D materials in the separation and recognition of biological templates are discussed. Moreover, through a series of case studies, emphasis is given on introducing imprinting strategies for biological templates with different molecular scales. In particular, the differences and connections between small molecular imprinting (bulk imprinting, "dummy" template imprinting, etc.), large molecular imprinting (surface imprinting, interfacial imprinting, etc.), and cell imprinting strategies are demonstrated in detail. Finally, future research directions are provided.

Synthetic nanoparticles for selective hydrolysis of bacterial autoinducers in quorum sensing

N-acyl homoserine lactones (AHLs) are signal molecules used by a large number of gram-negative bacteria in quorum sensing and their hydrolysis is known to inhibit biofilm formation. Micellar imprinting of AHL-like templates with catalytic functional monomers yielded water-soluble nanoparticles with AHL-shaped active site and nearby catalytic groups. Either Lewis acidic zinc ions or nucleophilic pyridyl ligands could be introduced through this strategy, yielding artificial enzymes for the hydrolysis of AHLs in a substrate-selective fashion.

Binding-promoted chemical reaction in the nanospace of a binding site: effects of environmental constriction

Chemical reactions in a confined nanospace can be very different from those in solution. Imine formation between molecular amines and an aldehyde inside a molecularly imprinted receptor was promoted strongly by the binding. Although how well the amine fit in the binding pocket and its electronic nature both influenced the reaction, the freedom of movement for the amine was the most important factor determining the binding-normalized reactivity.

Tuning surface-cross-linking of molecularly imprinted cross-linked micelles for molecular recognition in water

Molecular recognition in water is an important challenge in supramolecular chemistry. Surface-core double cross-linking of template-containing surfactant micelles by the click reaction and free radical polymerization yields molecularly imprinted nanoparticles (MINPs) with guest-complementary binding sites. An important property of MINP-based receptors is the surface-cross-linking between the propargyl groups of the surfactants and a diazide cross-linker. Decreasing the number of carbons in between the two azides enhanced the binding affinity of the MINPs, possibly by keeping the imprinted binding site more open prior to the guest binding. The depth of the binding pocket can be controlled by the distribution of the hydrophilic/hydrophobic groups of the template and was found to influence the binding in addition to electrostatic interactions between oppositely charged MINPs and guests. Cross-linkers with an alkoxyamine group enabled two-stage double surface-cross-linking that strengthened the binding constants by an order of magnitude, possibly by expanding the binding pocket of the MINP into the polar region. The binding selectivity among very similar isomeric structures also improved.

Sequence-Selective Recognition of Peptides in Aqueous Solution: A Supramolecular Approach through Micellar Imprinting

Sequence-selective recognition of peptides in water has been one of the most important and yet unsolved problems in bioorganic and supramolecular chemistry. The motivation comes from not only the importance of these molecules in biology but also the fundamental challenges involved in the research. Molecular imprinting in doubly cross-linked surfactant micelles offers a unique solution to this problem by creating a "supramolecular code" on the micelle surface that matches the structural features of the peptide chain. Hydrophobic "dimples" are constructed on imprinted micelles that match the hydrophobic side chains of the peptide precisely in size and shape. Polar binding functionalities are installed at correct positions to interact with specific acidic and basic groups on the peptide. Secondary hydrogen-bonding and electrostatic interactions are introduced through imprinting to enhance the binding affinity and specificity further. Binding affinities of tens of nanomolar are readily achieved in water for biological peptides with over a dozen residues. Excellent binding selectivity is observed even for subtly different peptides. The synthesis of these protein-sized nanoparticles involves a one-pot reaction complete within 2 days; purification requires nothing but precipitation and solvent washing. These features make the molecularly imprinted nanoparticles (MINPs) highly promising peptide-binding "artificial antibodies" for chemical and biological applications.

Enhancing adsorption capacity while maintaining specific recognition performance of mesoporous silica: a novel imprinting strategy with amphiphilic ionic liquid as surfactant

In order to facilitate the broad applications of molecular recognition materials in biomedical areas, it is critical to enhance their adsorption capacity while maintaining their excellent recognition performance. In this work, we designed and synthesized well-defined peptide-imprinted mesoporous silica (PIMS) for specific recognition of an immunostimulating hexapeptide from human casein (IHHC) by using amphiphilic ionic liquid as the surfactant to anchor IHHC via a combination of one-step sol-gel method and docking oriented imprinting approach. Thereinto, theoretical calculation was employed to reveal the multiple binding interactions and dual-template configuration between amphiphilic ionic liquid and IHHC. The fabricated PIMS was characterized and an in-depth analysis of specific recognition mechanism was conducted. Results revealed that both adsorption and recognition capabilities of PIMS far exceeded that of the NIMS's. More significantly, the PIMS exhibited a superior binding capacity (60.5 mg g^-1), which could increase 18.9% than the previous work. The corresponding imprinting factor and selectivity coefficient could reach up to 4.51 and 3.30, respectively. The PIMS also possessed lickety-split kinetic binding for IHHC, where the equilibrium time was only 10 min. All of these merits were due to the high surface area and the synergistic effect of multiple interactions (including hydrogen bonding, π-π stacking, ion-ion electrostatic interactions and van der Waals interactions, etc) between PIMS and IHHC in imprinted sites. The present work suggests the potential application of PIMS for large-scale and high-effective separation of IHHC, which may lead to their broad applications in drug/gene deliver, biosensors, catalyst and so on.

Molecularly imprinted artificial esterases with highly specific active sites and precisely installed catalytic groups

A difficult challenge in synthetic enzymes is the creation of substrate-selective active sites with accurately positioned catalytic groups. Covalent molecular imprinting in cross-linked micelles afforded such active sites in protein-sized, water-soluble nanoparticle catalysts. Our method allowed a systematic tuning of the distance of the catalytic group to the bound substrate. The catalysts displayed enzyme-like kinetics and easily distinguished substrates with subtle structural differences.

Surface Ligands in the Imprinting and Binding of Molecularly Imprinted Cross-Linked Micelles

Molecular recognition in water is challenging but water-soluble molecularly imprinted nanoparticle (MINP) receptors were produced readily by double cross-linking of surfactant micelles in the presence of suitable template molecules. When the micellar surface was decorated with different polyhydroxylated ligands, significant interactions could be introduced between the surface ligands and the template. Flexible surface ligands worked better than rigid ones to interact with the polar moiety of the template, especially for those template molecules whose water-exposed surface is not properly solvated by water. The importance of these hydrophilic interactions was examined in the context of different substrates, density of the surface ligands, and surface-cross-linking density of the MINP. Together with the hydrophobic interactions in the core, the surface hydrophilic interactions can be used to enhance the binding of guest molecules in water.

Selective Binding of Folic Acid and Derivatives by Imprinted Nanoparticle Receptors in Water

Folate receptors are overexpressed on cancer cells and frequently used for targeted delivery. Creation of synthetic receptors to bind folic acid and its analogues in water, however, is challenging because of its complex hydrogen-bonding patterns and competition for hydrogen bonds from the solvent. Micellar imprinting within cross-linkable surfactants circumvented these problems because the nonpolar micellar environment strengthened the hydrogen bonds between the amide group in the surfactant and the template molecule. Incorporation of polymerizable thiouronium functional monomers further enhanced the binding through hydrogen-bond-reinforced ion pairs with the glutamate moiety of the template. The resulting imprinted micelles were able to bind folate and their analogues with submicromolar affinity and distinguish small changes in the hydrogen-bonding patterns as well as the number/position of carboxylic acids. The binding constant obtained was 2-3 orders of magnitude higher than those reported for small-molecule synthetic receptors. Our binding study also revealed interesting details in the binding. For example, the relative contributions of different segments of the molecule to the binding followed the order of carboxylates > pyrimidine ring > pyrazine ring.