Polymeric Nanoparticle Receptors as Synthetic Antibodies for Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)

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.

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.

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.

Selenium-chelating corn oligopeptide as a potential antioxidant supplement: investigation of the protein conformational changes and identification of the antioxidant fragment composition

A selenium-chelating corn oligopeptide (Se-COP) with high protein and low molecular weight was prepared as a selenium supplement. We utilized infrared (IR), ultraviolet (UV), and circular dichroism (CD) spectroscopy, 1-anilinonaphthalene-8-sulfonate (ANS)-binding fluorescence spectra, and isothermal titration calorimetry (ITC) to analyze and describe Se-COP and its reactions. It was concluded that the chelation reaction was a spontaneous process driven by enthalpy and entropy, with ΔH=3.79 × 104 ± 4075 cal/mol, ΔS = 146 cal/mol, ΔG = –23356.30 ± 126.94 cal/mol, binding constant Ka = 1.18 × 104 ± 855 M–1, and binding site number n = 0.13 ± 0.0126, and described as coordination bonds forming and hydrophobic interaction, as well as protein conformational changes including secondary and tertiary hydrophobic structure. Se-COP had strong antioxidant capacity, and mass spectrometry (MS) was used to identify the antioxidant peptide fragment, which was characterized as LLPPY and quantified at 428.95 ng/mg. This study indicated that Se-COP prepared by chelation may be a Se supplement with antioxidant capacity that can be applied in functional foods or ingredients.

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).

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.

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.

Biomimetic and bioinspired nanoparticles for targeted drug delivery

In drug targeting, the urgent need for more effective and less iatrogenic therapies is pushing toward a complete revision of carrier setup. After the era of 'articles used as homing systems', novel prototypes are now emerging. Newly conceived carriers are endowed with better biocompatibility, biodistribution and targeting properties. The biomimetic approach bestows such improved functional properties. Exploiting biological molecules, organisms and cells, or taking inspiration from them, drug vector performances are now rapidly progressing toward the perfect carrier. Following this direction, researchers have refined carrier properties, achieving significant results. The present review summarizes recent advances in biomimetic and bioinspired drug vectors, derived from biologicals or obtained by processing synthetic materials with a biomimetic approach.

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.

Water-Soluble Nanoparticle Receptors Supramolecularly Coded for Acidic Peptides

Sequence-specific recognition of peptides is of enormous importance to many chemical and biological applications, but has been difficult to achieve due to the minute differences in the side chains of amino acids. Acidic peptides are known to play important roles in cell growth and gene expression. In this work, we report molecularly imprinted micelles coded with molecular recognition information for the acidic and hydrophobic side chains of acidic peptides. The imprinted receptors could distinguish acidic amino acids from other polar and nonpolar amino acids, with dissociation constants of tens of nanomolar for biologically active peptides containing up to 18 amino acids.

Fluorescent nanoparticle sensors with tailor-made recognition units and proximate fluorescent reporter groups

Molecular imprinting in micelles followed by covalent modification of the binding pocket yielded fluorescent sensors with precisely constructed binding pockets.

Peptide-Binding Nanoparticle Materials with Tailored Recognition sites for Basic Peptides

Peptides rich in basic residues such as lysine and arginine play important roles in biology such as bacterial defense and cell penetration. Although peptide-binding materials with high sequence-specificity have broad potential applications, the diverse functionalities of peptide side chains make their molecular recognition extremely difficult. By covalently capturing micelles of a doubly cross-linkable surfactant with solubilized peptide templates, we prepared water-soluble molecularly imprinted nanoparticles with high sequence-specificity for basic peptides. The nanoparticles interact with the side chains of lysine and arginine through hydrogen bonds strengthened by the nonpolar environment of the micelle. They have hydrophobic pockets in their core complementary to the hydrophobic side chains in size and shape. These recognition sites allowed the micelles to bind basic biological peptides strongly in water, with tens to hundreds of nanomolar in binding affinity.

Imprinted micelles for chiral recognition in water: shape, depth, and number of recognition sites

Chiral molecular recognition is important to biology, separation, and asymmetric catalysis. Because there is no direct correlation between the chiralities of the host and the guest, it is difficult to design a molecular receptor for a chiral guest in a rational manner. By cross-linking surfactant micelles containing chiral template molecules, we obtained chiral nanoparticle receptors for a number of 4-hydroxyproline derivatives. Molecular imprinting allowed us to transfer the chiral information directly from the guest to host, making the molecular recognition between the two highly predictable. Hydrophobic interactions between the host and the guest contributed strongly to the enantio- and diastereoselective differentiation of these compounds in water, whereas ion-pair interactions, which happened near the surface of the micelle, were less discriminating. The chiral recognition could be modulated by tuning the size and shape of the binding pockets.

Water-Soluble Molecularly Imprinted Nanoparticle Receptors with Hydrogen-Bond-Assisted Hydrophobic Binding

Molecularly imprinted nanoparticles (MINPs) were prepared when surfactants with a tripropargylammonium headgroup and a methacrylate-functionalized hydrophobic tail were cross-linked in the micelle form on the surface and in the core in the presence of hydrophobic template molecules. With the surfactants containing an amide bond near the headgroup, the MINPs had a layer of hydrogen-bonding groups in the interior that strongly influenced their molecular recognition. Templates/guests with strong hydrogen-bonding groups in the midsection of the molecule benefited most, especially if the hydrophobe of the template could penetrate the amide layer to reach the hydrophobic core of the cross-linked micelles. The location and the orientation of the hydrophilic groups were also important, as they determined how the template interacted with the surfactant micelles and, ultimately, with the MINP receptors.

Selective Recognition of d-Aldohexoses in Water by Boronic Acid-Functionalized, Molecularly Imprinted Cross-Linked Micelles

Molecular imprinting within cross-linked micelles using 4-vinylphenylboronate derivatives of carbohydrates provided water-soluble nanoparticle receptors selective for the carbohydrate templates. Complete differentiation of d-aldohexoses could be achieved by these receptors if a single inversion of hydroxyl occurred at C2 or C4 of the sugar or if two or more inversions took place. Glycosides with a hydrophobic aglycan displayed stronger binding due to increased hydrophobic interactions.

Surface-Cross-Linked Micelles as Multifunctionalized Organic Nanoparticles for Controlled Release, Light Harvesting, and Catalysis

Surfactant micelles are dynamic entities with a rapid exchange of monomers. By "clicking" tripropargylammonium-containing surfactants with diazide cross-linkers, we obtained surface-cross-linked micelles (SCMs) that could be multifunctionalized for different applications. They triggered membrane fusion through tunable electrostatic interactions with lipid bilayers. Antenna chromophores could be installed on them to create artificial light-harvesting complexes with efficient energy migration among tens to hundreds of chromophores. When cleavable cross-linkers were used, the SCMs could break apart in response to redox or pH signals, ejecting entrapped contents quickly as a result of built-in electrostatic stress. They served as caged surfactants whose surface activity was turned on by environmental stimuli. They crossed cell membranes readily. Encapsulated fluorophores showed enhanced photophysical properties including improved quantum yields and greatly expanded Stokes shifts. Catalytic groups could be installed on the surface or in the interior, covalently attached or physically entrapped. As enzyme mimics, the SCMs enabled rational engineering of the microenvironment around the catalysts to afford activity and selectivity not possible with conventional catalysts.

Molecularly Responsive Binding through Co-occupation of Binding Space: A Lock-Key Story

When two guest molecules co-occupy a binding pocket of a water-soluble host, the first guest could be used as a signal molecule to turn on the binding of the second. This type of molecularly responsive binding strongly depends on the size of the two guests and the location of the signal molecule.

Enhancing binding affinity and selectivity through preorganization and cooperative enhancement of the receptor

When direct host–guest binding interactions are weakened by unfavorable solvent competition, guest-triggered intrareceptor interactions could be used to augment the binding.