Nucleophilic Catalysis of Carbohydrate Oxime Formation by Anilines

Neo-Glycolipid Oximes as Intestinal Permeation Enhancers for Peptide Hormone PYY3-36

Herein, we describe the design and synthesis of 16 neo-glycolipids that are potential permeation enhancers for oral drug delivery of peptide therapeutics. These amphiphilic neo-glycolipids are composed of fatty acids and various carbohydrates (d-glucose, lactose, cellobiose, maltose) via an oxime linker. The ability of the synthesized neo-glycolipids to enhance permeation of fluorescein-labelled dextran (4 kDa) or 3H-mannitol across intestinal epithelium was investigated in vitro using monolayers of human epithelial Caco-2 cells. Their effects were compared with (pre-)clinically known enhancers as reference compounds; sodium salts of octanoic, decanoic, and dodecanoic acid, and sodium salcaprozate (SNAC). Most neo-glycolipids increased the permeation of the model compounds, proving that neo-glycolipids, which possess vastly different properties from the reference compounds, e. g., in terms of clogD and polar surface area, are effective permeation enhancers. The neo-glycolipid based on decanoic acid and glucose was more potent than related compounds based on disaccharides. Significant differences in solubility and cellular compatibility were found for neo-glyolipids based on different carbohydrates. Finally, neo-glycolipids were evaluated as permeation enhancers for the peptide hormone PYY3-36. Glucose- and maltose-derived neo-glycolipids based on decanoic and dodecanoic acid showed promising enhancements in PYY3-36 permeation in vitro while maintaining good cellular compatibility, relevant for oral delivery of obesity treatments.

Kinetics of Strain-Promoted Alkyne-Nitrone Cycloadditions (SPANC) with Unprotected Carbohydrate Scaffolded Nitrones

The use of unprotected carbohydrate-derived nitrones as partners in strain-promoted alkyne-nitrone cycloadditions was investigated as a new tool for bioconjugation. The observed second-order reactions displayed rate constants of 3.4 × 10-4-5.8 × 10-2 M-1 s-1, which is the common order of magnitude of reaction kinetics with other simple aliphatic or aromatic nitrones. Applicability of this method to aqueous media was demonstrated by performing a one-pot protocol, which combines sequential formation of the nitrone and cycloaddition with cyclooctyne in water.

Ca2+-Responsive Glyco-insulin

Chemical modification of peptides and proteins, such as PEGylation and lipidation, creates conjugates with new properties. However, they are typically not dynamic or stimuli-responsive. Self-assembly controlled by a stimulus will allow adjusting properties directly. Here, we report that conjugates of oligogalacturonic acids (OGAs), isolated from plant-derived pectin, are Ca²⁺-responsive. We report the conjugation of OGA to human insulin (HI) to create new glyco-insulins. In addition, we coupled OGA to model peptides. We studied their self-assembly by dynamic light scattering, small-angle X-ray scattering, and circular dichroism, which showed that the self-assembly to form nanostructures depended on the length of the OGA sequence and Zn²⁺ and Ca²⁺ concentrations. Subcutaneous administration of OGA12-HI with Zn²⁺ showed a stable decrease in blood glucose over a longer period of time compared to HI, despite the lower receptor binding affinity.

Multivalent glycocyclopeptides: conjugation methods and biological applications

Click chemistry was extensively used to decorate synthetic multivalent scaffolds with glycans to mimic the cell surface glycocalyx and to develop applications in glycosciences. Conjugation methods such as oxime ligation, copper(I)-catalyzed alkyne-azide cycloaddition, thiol-ene coupling, squaramide coupling or Lansbury aspartylation proved particularly suitable to achieve this purpose. This review summarizes the synthetic strategies that can be used either in a stepwise manner or in an orthogonal one-pot approach, to conjugate multiple copies of identical or different glycans to cyclopeptide scaffolds (namely multivalent glycocyclopeptides) having different size, valency, geometry and molecular composition. The second part of this review will describe the potential of these structures to interact with various carbohydrate binding proteins or to stimulate immunity against tumor cells.

Bioorthogonal, Bifunctional Linker for Engineering Synthetic Glycoproteins

Post-translational glycosylation of proteins results in complex mixtures of heterogeneous protein glycoforms. Glycoproteins have many potential applications from fundamental studies of glycobiology to potential therapeutics, but generating homogeneous recombinant glycoproteins using chemical or chemoenzymatic reactions to mimic natural glycoproteins or creating homogeneous synthetic neoglycoproteins is a challenging synthetic task. In this work, we use a site-specific bioorthogonal approach to produce synthetic homogeneous glycoproteins. We develop a bifunctional, bioorthogonal linker that combines oxime ligation and strain-promoted azide-alkyne cycloaddition chemistry to functionalize reducing sugars and glycan derivatives for attachment to proteins. We demonstrate the utility of this minimal length linker by producing neoglycoprotein inhibitors of cholera toxin in which derivatives of the disaccharide lactose and GM1os pentasaccharide are attached to a nonbinding variant of the cholera toxin B-subunit that acts as a size- and valency-matched multivalent scaffold. The resulting neoglycoproteins decorated with GM1 ligands inhibit cholera toxin B-subunit adhesion with a picomolar IC₅₀.

Rapid glycoconjugation with glycosyl amines

Conjugation of unprotected carbohydrates to surfaces or probes by chemoselective ligation reactions is indispensable for the elucidation of their numerous biological functions. In particular, the reaction with oxyamines leading to the formation of carbohydrate oximes which are in equilibrium with cyclic N-glycosides (oxyamine ligation) has an enormous impact in the field. Although highly chemoselective, the reaction is rather slow. Here, we report that the oxyamine ligation is significantly accelerated without the need for a catalyst when starting with glycosyl amines. Reaction rates are increased up to 500-fold compared to the reaction of the reducing carbohydrate. For comparison, aniline-catalyzed oxyamine ligation is only increased 3.8-fold under the same conditions. Glycosyl amines from mono- and oligosaccharides are easily accessible from reducing carbohydrates via the corresponding azides by using Shoda's reagent (2-chloro-1,3-dimethylimidazolinium chloride, DMC) and subsequent reduction. Furthermore, glycosyl amines are readily obtained by enzymatic release from N-glycoproteins making the method suited for glycomic analysis of these glycoconjugates which we demonstrate employing RNase B. Oxyamine ligation of glycosyl amines can be carried out at close to neutral conditions which makes the procedure especially valuable for acid-sensitive oligosaccharides.

A new method for carbohydrate-oxyamine ligation starting from glycosyl amines 1 instead of the commonly used reducing sugars 2 results in tremendously increased ligation rates without the need for a catalyst, such as aniline.

Development of programmable gemcitabine-GnRH pro-drugs bearing linker controllable "click" oxime bond tethers and preclinical evaluation against prostate cancer

Peptide-drug conjugates (PDCs) are gaining considerable attention as anti-neoplastic agents. However, their development is often laborious and time-consuming. Herein, we have developed and preclinically evaluated three PDCs with gemcitabine as the anticancer cytotoxic unit and D-Lys⁶-GnRH (gonadotropin-releasing hormone; GnRH) as the cancer-targeting unit. These units were tethered via acid-labile programmable linkers to guide a differential drug release rate from the PDC through a combination of ester or amide and "click" type oxime ligations. The pro-drugs were designed to enable the selective targeting of malignant tumor cells with linker guided differential drug release rates. We exploited the oxime bond responsiveness against the acidic pH of the tumor microenvironment and the GnRH endocytosis via the GnRH-R GPCR which is overexpressed on cancer cells. The challenging metabolic properties of gemcitabine were addressed during design of the PDCs. We developed a rapid (1 hour) and cost-effective "click" oxime bond ligation platform to assemble in one-pot the 3 desired PDCs that does not require purification, surpassing traditional time-ineffective and low yield methods. The internalization of the tumor-homing peptide unit in cancer cells, overexpressing the GnRH-R, was first validated through confocal laser microscopy and flow cytometry analysis. Subsequently, the three PDCs were evaluated for their in vitro antiproliferative effect in prostate cancer cells. Their stability and the release of gemcitabine over time were monitored in vitro in cell culture and in human plasma using LC-MS/MS. We then assessed the ability of the developed PDCs to internalize in prostate cancer cells and to release gemcitabine. The most potent analog, designated GOXG₁, was used for pharmacokinetic studies in mice. The metabolism of GOXG₁ was examined in liver microsomes, as well as in buffers mimicking the pH of intracellular organelles, resulting in the identification of two metabolites. The major metabolite at low pH emanated from the cleavage of the pH-labile oxime bond, validating our design approach. NMR spectroscopy and in vitro radioligand binding assays were exploited for GOXG₁ to validate that upon conjugating the drug to the peptide, the peptide microenvironment responsible for its GnRH-R binding is not perturbed and to confirm its high binding potency to the GnRH-R. Finally, the binding of GOXG₁ to the GnRH-R and the associated elicitation of testosterone release in mice were also determined. The facile platform established herein for the rapid assembly of PDCs with linker controllable characteristics from aldehyde and aminooxy units through rapid "click" oxime ligation, that does not require purification steps, could pave the way for a new generation of potent cancer therapeutics, diagnostics and theranostics.

Protecting Group Free Synthesis of Glyconanoparticles Using Amino-Oxy-Terminated Polymer Ligands

Glycomaterials display enhanced binding affinity to carbohydrate-binding proteins due to the nonlinear enhancement associated with the cluster glycoside effect. Gold nanoparticles bearing glycans have attracted significant interest in particular. This is due to their versatility, their highly tunable gold cores (size and shape), and their application in biosensors and diagnostic tools. However, conjugating glycans onto these materials can be challenging, necessitating either multiple protecting group manipulations or the use of only simple glycans. This results in limited structural diversity compared to glycoarrays which can include hundreds of glycans. Here we report a method to generate glyconanoparticles from unprotected glycans by conjugation to polymer tethers bearing terminal amino-oxy groups, which are then immobilized onto gold nanoparticles. Using an isotope-labeled glycan, the efficiency of this reaction was probed in detail to confirm conjugation, with 25% of end-groups being functionalized, predominantly in the ring-closed form. Facile post-glycosylation purification is achieved by simple centrifugation/washing cycles to remove excess glycan and polymer. This streamlined synthetic approach may be particularly useful for the preparation of glyconanoparticle libraries using automation, to identify hits to be taken forward using more conventional synthetic methods. Exemplar lectin-binding studies were undertaken to confirm the availability of the glycans for binding and show this is a powerful tool for rapid assessment of multivalent glycan binding.

Development of a Microwave-assisted Chemoselective Synthesis of Oxime-linked Sugar Linkers and Trivalent Glycoclusters

A rapid, high-yielding microwave-mediated synthetic procedure was developed and optimized using a model system of monovalent sugar linkers, with the ultimate goal of using this method for the synthesis of multivalent glycoclusters. The reaction occurs between the aldehyde/ketone on the sugars and an aminooxy moiety on the linker/trivalent core molecules used in this study, yielding acid-stable oxime linkages in the products and was carried out using equimolar quantities of reactants under mild aqueous conditions. Because the reaction is chemoselective, sugars can be incorporated without the use of protecting groups and the reactions can be completed in as little as 30 min in the microwave. As an added advantage, in the synthesis of the trivalent glycoclusters, the fully substituted trivalent molecules were the major products produced in excellent yields. These results illustrate the potential of this rapid oxime-forming microwave-mediated reaction in the synthesis of larger, more complex glycoconjugates and glycoclusters for use in a wide variety of biomedical applications.

Achieving Controlled Biomolecule-Biomaterial Conjugation

The conjugation of biomolecules can impart materials with the bioactivity necessary to modulate specific cell behaviors. While the biological roles of particular polypeptide, oligonucleotide, and glycan structures have been extensively reviewed, along with the influence of attachment on material structure and function, the key role played by the conjugation strategy in determining activity is often overlooked. In this review, we focus on the chemistry of biomolecule conjugation and provide a comprehensive overview of the key strategies for achieving controlled biomaterial functionalization. No universal method exists to provide optimal attachment, and here we will discuss both the relative advantages and disadvantages of each technique. In doing so, we highlight the importance of carefully considering the impact and suitability of a particular technique during biomaterial design.

Oxime Ligation via in situ Oxidation of N-Phenylglycinyl Peptides

Mild conditions for oxime ligations via in situ generation of α-imino amide intermediates are reported. The evaluation of a variety of N-terminal N-phenylglycine residues revealed that a metal-free, chemoselective oxidation was possible using oxygen as the only oxidant in buffer at pH 7.0. Moreover, selective unmasking of an inert residue by addition of potassium ferricyanide is demonstrated. These simple and mild conditions, which can be fine-tuned by the electronic properties of the N-phenylglycine residue, offer unique advantages over conventional approaches for oxime ligations.

Lectin-Binding Specificity of the Fertilization-Relevant Protein PDC-109 by Means of Surface Plasmon Resonance and Carbohydrate REcognition Domain EXcision-Mass Spectrometry

Seminal plasma proteins are relevant for sperm functionality and some appear responsible for establishing sperm interactions with the various environments along the female genital tract towards the oocyte. In recent years, research has focused on characterizing the role of these proteins in the context of reproductive biology, fertility diagnostics and treatment of related problems. Herein, we focus on the main protein of bovine seminal plasma, PDC-109 (BSP-A1/-A2), which by virtue of its lectin properties is involved in fertilization. By means of surface plasmon resonance, the interaction of PDC-109 with a panel of the most relevant glycosidic epitopes of mammals has been qualitatively and quantitatively characterized, and a higher affinity for carbohydrates containing fucose has been observed, in line with previous studies. Additionally, using the orthogonal technique of Carbohydrate REcognition Domain EXcision-Mass Spectrometry (CREDEX-MS), the recognition domain of the interaction complexes between PDC-109 and all fucosylated disaccharides [(Fuc-α1,(3,4,6)-GlcNAc)] has been defined, revealing the specific glycotope and the peptide domain likely to act as the PDC-109 carbohydrate binding site.

Glycoconjugate Oxime Formation Catalyzed at Neutral pH: Mechanistic Insights and Applications of 1,4-Diaminobenzene as a Superior Catalyst for Complex Carbohydrates

The reaction of unprotected carbohydrates with aminooxy reagents to provide oximes is a key method for the construction of glycoconjugates. Aniline and derivatives serve as organocatalysts for the formation of oximes from simple aldehydes, and we have previously reported that aniline also catalyzes the formation of oximes from the more complex aldehydes, carbohydrates. Here, we present a comprehensive study of the effect of aniline analogues on the formation of carbohydrate oximes and related glycoconjugates depending on organocatalyst structure, pH, nucleophile, and carbohydrate, covering more than 150 different reaction conditions. The observed superiority of the 1,4-diaminobenzene (PDA) catalyst at neutral pH is rationalized by NMR analyses and DFT studies of reaction intermediates. Carbohydrate oxime formation at pH 7 is demonstrated by the formation of a bioactive glycoconjugate from a labile, decorated octasaccharide originating from exopolysaccharides of the soil bacterium Mesorhizobium loti. This study of glycoconjugate formation includes the first direct comparison of aniline-catalyzed reaction rates and equilibrium constants for different classes of nucleophiles, including primary oxyamines, secondary N-alkyl oxyamines, as well as aryl and arylsulfonyl hydrazides. We identified 1,4-diaminobenzene as a superior catalyst for the construction of oxime-linked glycoconjugates under mild conditions.

Quantitative profiling of carbonyl metabolites directly in crude biological extracts using chemoselective tagging and nanoESI-FTMS

The extensive range of chemical structures, wide range of abundances, and chemical instability of metabolites present in the metabolome pose major analytical challenges that are difficult to address with existing technologies. To address these issues, one approach is to target a subset of metabolites that share a functional group, such as ketones and aldehydes, using chemoselective tagging. Here we report a greatly improved chemoselective method for the quantitative analysis of hydrophilic and hydrophobic carbonyl-containing metabolites directly in biological samples. This method is based on direct tissue or cells extraction with simultaneous derivatization of stable and labile carbonylated metabolites using N-[2-(aminooxy)ethyl]-N,N-dimethyl-1-dodecylammonium (QDA) and 13CD3 labeled QDA. We combined innovations of direct quenching of biological sample with frozen derivatization conditions under the catalyst N,N-dimethyl-p-phenylenediamine, which facilitated the formation of oxime stable-isotope ion pairs differing by m/z 4.02188 while minimizing metabolite degradation. The resulting oximes were extracted by HyperSep C8 tips to remove interfering compounds, and the products were detected using nano-electrospray ionization interfaced with a Thermo Fusion mass spectrometer. The quaternary ammonium tagging greatly increased electrospray MS detection sensitivity and the signature ions pairs enabled simple identification of carbonyl compounds. The improved method showed the lower limits of quantification for carbonyl standards to be in the range of 0.20-2 nM, with linearity of R2 > 0.99 over 4 orders of magnitude. We have applied the method to assign 66 carbonyls in mouse tumor tissues, many of which could not be assigned solely by accurate mass and tandem MS. Fourteen of the metabolites were quantified using authentic standards. We also demonstrated the suitability of this method for determining 13C labeled isotopologues of carbonyl metabolites in 13C6-glucose-based stable isotope-resolved metabolomic (SIRM) studies.

Reductive oxyamination: a method for the qualitative and quantitative analysis of monosaccharides with a new aminooxy reagent using high-performance liquid chromatography with fluorescence detection

Derivatization of carbohydrates with aminooxy agents to form oximes can be used for qualitative and quantitative analysis of carbohydrates; however, the formation of isomeric products limits its application. A new reductive oxyamination procedure developed for the analysis of monosaccharides with a novel fluorescent O-substituted aminooxy reagent, 4-((aminooxy)methyl)-6-chloro-7-hydroxycoumarin (AOCC), is reported. In this procedure, monosaccharides undergo an oxime formation reaction with AOCC and are then readily reduced with 2-picoline-borane, followed by analysis with high-performance liquid chromatography with fluorescence detection. Good separation of five monosaccharide derivatives was achieved within 40 min with acetonitrile-water-tetrahydrofuran as the mobile phase. The detection limits were on the order of femtomoles. The linear range was 0.2-4000 nM, with a good correlation coefficient (R ≥ 0.9985). Furthermore, the method was applied for analysis of real samples, such as bovine milk powder, without complicated and tedious sample treatment. This reductive oxyamination method circumvents the problem caused by oxime isomers and can be used for the highly sensitive and selective analysis of monosaccharides with high accuracy, providing an effective and promising method for the analysis of carbonyls with aminooxy agents.

Preparation of glycoconjugates from unprotected carbohydrates for protein-binding studies

Glycobiology, in particular the study of carbohydrate-protein interactions and the events that follow, has become an important research focus in recent decades. To study these interactions, many assays require homogeneous glycoconjugates in suitable amounts. Their synthesis is one of the methodological challenges of glycobiology. Here, we describe a versatile, three-stage protocol for the formation of glycoconjugates from unprotected carbohydrates, including those purified from natural sources, as exemplified here by rhizobial Nod factors and exopolysaccharide fragments. The first stage is to add an oligo(ethylene glycol) linker (OEG-linker) that has a terminal triphenylmethanethiol group to the reducing end of the oligosaccharide by oxime formation catalyzed by aniline. The triphenylmethyl (trityl) tag is then removed from the linker to expose a thiol (stage 2) to allow a conjugation reaction at the thiol group (stage 3). There are many possible conjugation reactions, depending on the desired application. Examples shown in this protocol are as follows: (i) coupling of the oligosaccharide to a support for surface plasmon resonance (SPR) studies, (ii) fluorescence labeling for microscale thermophoresis (MST) or bioimaging, and (iii) biotinylation for biolayer interferometry (BLI) studies. This protocol starts from unprotected carbohydrates and provides glycoconjugates in milligram amounts in just 2 d.

Versatile Methodology for Glycosurfaces: Direct Ligation of Nonderivatized Reducing Saccharides to Poly(pentafluorophenyl acrylate) Grafted Surfaces via Hydrazide Conjugation

In this work, we report a convenient and versatile strategy for surface-grafted glycopolymer constructs with the goal of surface modification that controls the chemical presentation and grafting density of carbohydrate side chains. This approach employs a difunctional hydrazine linker, chemically modified to an active ester containing poly(pentafluorophenyl acrylate) grafted scaffold, to conjugate a variety of saccharides through the reducing end. The successive conjugation steps are carried out under mild conditions and yield high surface densities of sugars, as high as 4.8 nmol·cm^-2, capable of multivalency, with an intact structure and retained bioactivity. We also demonstrate that this glycosylated surface can bind specific lectins according to the structure of its pendant carbohydrate. To demonstrate bioactivity, this surface platform is used to study the binding events of a human respiratory tract pathogen, Mycoplasma pneumoniae, on surfaces conjugated with sialylated sugars.

Convergent dendrimer synthesis by olefin metathesis and studies toward glycoconjugation

The synthesis of novel hyperbranched monodisperse linear dendrimers, based on 2,2-bis-(hydroxymethyl)-propionic acid (bis-MPA), has been achieved by convergent metathesis-mediated coupling between the alkene-terminated focal point of bis-MPA dendrons. On their surface, dendrimers present 4, 8 and 16 functional groups. Glycodendrimers exposing multiple saccharide moieties have also been obtained. To the best of our knowledge, this is the first example of the use of metathesis for focal point coupling.

Oximes and Hydrazones in Bioconjugation: Mechanism and Catalysis

The formation of oximes and hydrazones is employed in numerous scientific fields as a simple and versatile conjugation strategy. This imine-forming reaction is applied in fields as diverse as polymer chemistry, biomaterials and hydrogels, dynamic combinatorial chemistry, organic synthesis, and chemical biology. Here we outline chemical developments in this field, with special focus on the past ∼10 years of developments. Recent strategies for installing reactive carbonyl groups and α-nucleophiles into biomolecules are described. The basic chemical properties of reactants and products in this reaction are then reviewed, with an eye to understanding the reaction's mechanism and how reactant structure controls rates and equilibria in the process. Recent work that has uncovered structural features and new mechanisms for speeding the reaction, sometimes by orders of magnitude, is discussed. We describe recent studies that have identified especially fast reacting aldehyde/ketone substrates and structural effects that lead to rapid-reacting α-nucleophiles as well. Among the most effective new strategies has been the development of substituents near the reactive aldehyde group that either transfer protons at the transition state or trap the initially formed tetrahedral intermediates. In addition, the recent development of efficient nucleophilic catalysts for the reaction is outlined, improving greatly upon aniline, the classical catalyst for imine formation. A number of uses of such second- and third-generation catalysts in bioconjugation and in cellular applications are highlighted. While formation of hydrazone and oxime has been traditionally regarded as being limited by slow rates, developments in the past 5 years have resulted in completely overturning this limitation; indeed, the reaction is now one of the fastest and most versatile reactions available for conjugations of biomolecules and biomaterials.

Aminooxylated Carbohydrates: Synthesis and Applications

Among other classes of biomolecules, carbohydrates and glycoconjugates are widely involved in numerous biological functions. In addition to addressing the related synthetic challenges, glycochemists have invested intense efforts in providing access to structures that can be used to study, activate, or inhibit these biological processes. Over the past few decades, aminooxylated carbohydrates have been found to be key building blocks for achieving these goals. This review provides the first in-depth overview covering several aspects related to the syntheses and applications of aminooxylated carbohydrates. After a brief introduction to oxime bonds and their relative stabilities compared to related C═N functions, synthetic aspects of oxime ligation and methodologies for introducing the aminooxy functionality onto both glycofuranosyls and glycopyranosyls are described. The subsequent section focuses on biological applications involving aminooxylated carbohydrates as components for the construcion of diverse architectures. Mimetics of natural structures represent useful tools for better understanding the features that drive carbohydrate-receptor interaction, their biological output and they also represent interesting structures with improved stability and tunable properties. In the next section, multivalent structures such as glycoclusters and glycodendrimers obtained through oxime ligation are described in terms of synthetic design and their biological applications such as immunomodulators. The second-to-last section discusses miscellaneous applications of oxime-based glycoconjugates, such as enantioselective catalysis and glycosylated oligonucleotides, and conclusions and perspectives are provided in the last section.

Polyvalent Catalysts Operating on Polyvalent Substrates: A Model for Surface-Controlled Reactivity

Unusually fast rates of nucleophilic catalysis of hydrazone ligation were observed when polyvalent anthranilic acid catalysts operating on polyvalent aldehyde substrates were used with PAMAM dendrimers as the common platform. When presented in this way, the catalyst has a strong accelerating effect at concentrations 40-400 times lower than those required for similar monovalent catalysts and displays unique kinetic parameters. We attribute these properties to polyvalent engagement between the dendrimer surface groups, and a potential "rolling" effect leading to fast interparticle kinetic turnover. The phenomenon is sensitive to the density of functional groups on each dendrimer, and insensitive to factors that promote or inhibit nonspecific particle aggregation. These findings constitute a rare experimental example of an underappreciated phenomenon in biological and chemical systems that are organized on interacting surfaces.

Oxime conjugation in protein chemistry: from carbonyl incorporation to nucleophilic catalysis

Use of oxime forming reactions has become a widely applied strategy for peptide and protein bioconjugation. The efficiency of the reaction and robust stability of the oxime product has led to the development of a growing list of methods to introduce the required ketone or aldehyde functionality site specifically into proteins. Early methods focused on site-specific oxidation of an N-terminal serine or threonine and more recently transamination methods have been developed to convert a broader set of N-terminal amino acids into a ketone or aldehyde. More recently, site-specific modification of protein has been attained through engineering enzymes involved in posttranslational modifications in order to accommodate aldehyde-containing substrates. Similarly, a growing list of unnatural amino acids can be introduced through development of selective amino-acyl tRNA synthetase/tRNA pairs combined with codon reassignment. In the case of glycoproteins, glycans can be selectively modified chemically or enzymatically to introduce aldehyde functional groups. Finally, the total chemical synthesis of proteins complements these biological and chemoenzymatic approaches. Once introduced, the oxime ligation of these aldehyde and ketone groups can be catalyzed by aniline or a variety of aniline derivatives to tune the activity, pH preference, stability and solubility of the catalyst. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Fabrication of Alkoxyamine-Functionalized Magnetic Core-Shell Microspheres via Reflux Precipitation Polymerization for Glycopeptide Enrichment

As a facile method to prepare hydrophilic polymeric microspheres, reflux precipitation polymerization has been widely used for preparation of polymer nanogels. In this article, we synthesized a phthalamide-protected N-aminooxy methyl acrylamide (NAMAm-p) for preparation of alkoxyamine-functionalized polymer composite microspheres via reflux precipitation polymerization. The particle size and functional group density of the composite microspheres could be adjusted by copolymerization with the second monomers, N-isopropyl acrylamide, acrylic acid or 2-hydroxyethyl methacrylate. The resultant microspheres have been characterized by TEM, FT-IR, TGA and DLS. The experimental results showed that the alkoxyamine group density of the microspheres could reach as high as 1.49 mmol/g, and these groups showed a great reactivity with ketone/aldehyde compounds. With the aid of magnetic core, the hybrid microspheres could capture and magnetically isolate glycopeptides from the digested mixture of glycopeptides and non-glycopeptides at a 1:100 molar ratio. After that, we applied the composite microspheres to profile the glycol-proteome of a normal human serum sample, 95 unique glycopeptides and 64 glycoproteins were identified with these enrichment substrates in a 5 μL of serum sample.

One-pot preparation of labelled mannan-peptide conjugate, model for immune cell processing

An efficient method for preparation of fluorescently labelled mannan-peptide glycoconjugates has been developed. After selective Dess-Martin periodinane oxidation of mannan, it was conjugated to the fluorescent label alone and a peptide with the label via reductive amination. Prepared glycoconjugates were characterised by HPSEC, FTIR-ATR and UV-VIS spectroscopy. Finally, the fluorescently labelled mannan and mannan-peptide conjugate were used for microscopic visualization of their accumulation in intracellular organelles of RAW 264.7 cells.

New organocatalyst scaffolds with high activity in promoting hydrazone and oxime formation at neutral pH

The discovery of two new classes of catalysts for hydrazone and oxime formation in water at neutral pH, namely 2-aminophenols and 2-(aminomethyl)benzimidazoles, is reported. Kinetics studies in aqueous solutions at pH 7.4 revealed rate enhancements up to 7-fold greater than with classic aniline catalysis. 2-(Aminomethyl)benzimidazoles were found to be effective catalysts with otherwise challenging aryl ketone substrates.

Rapid End-Group Modification of Polysaccharides for Biomaterial Applications in Regenerative Medicine

Polysaccharides have emerged as important functional materials because of their unique properties such as biocompatibility, biodegradability, and availability of reactive sites for chemical modifications to optimize their properties. The overwhelming majority of the methods to modify polysaccharides employ random chemical modifications, which often improve certain properties while compromising others. On the other hand, the employed methods for selective modifications often require excess of coupling partners, long reaction times and are limited in their scope and wide applicability. To circumvent these drawbacks, aniline-catalyzed oxime formation is developed for selective modification of a variety of polysaccharides through their reducing end. Notably, it is found that for efficient oxime formation, different conditions are required depending on the composition of the specific polysaccharide. It is also shown how our strategy can be applied to improve the physical and functional properties of alginate hydrogels, which are widely used in tissue engineering and regenerative medicine applications. While the randomly and selectively modified alginate exhibits similar viscoelastic properties, the latter forms significantly more stable hydrogel and superior cell adhesive and functional properties. Our results show that the developed conjugation reaction is robust and should open new opportunities for preparing polysaccharide-based functional materials with unique properties.

Photoactivated bioconjugation between ortho-azidophenols and anilines: a facile approach to biomolecular photopatterning

Methods for the surface patterning of small molecules and biomolecules can yield useful platforms for drug screening, synthetic biology applications, diagnostics, and the immobilization of live cells. However, new techniques are needed to achieve the ease, feature sizes, reliability, and patterning speed necessary for widespread adoption. Herein, we report an easily accessible and operationally simple photoinitiated reaction that can achieve patterned bioconjugation in a highly chemoselective manner. The reaction involves the photolysis of 2-azidophenols to generate iminoquinone intermediates that couple rapidly to aniline groups. We demonstrate the broad functional group compatibility of this reaction for the modification of proteins, polymers, oligonucleotides, peptides, and small molecules. As a specific application, the reaction was adapted for the photolithographic patterning of azidophenol DNA on aniline glass substrates. The presence of the DNA was confirmed by the ability of the surface to capture living cells bearing the sequence complement on their cell walls or cytoplasmic membranes. Compared to other light-based DNA patterning methods, this reaction offers higher speed and does not require the use of a photoresist or other blocking material.

Synthesis of Hydrophilic Aminooxy Linkers and Multivalent Cores for Chemoselective Aldehyde/Ketone Conjugation

A series of three linear and two trivalent aminooxy-containing hydrophilic linkers and cores were synthesized. The five molecules contain from one to three aminooxy groups, and all but one contain an ether for enhanced aqueous solubility. These unique and versatile molecules can be utilized in the chemoselective conjugation of aldehyde/ketone-containing molecules, including reducing sugars, under mild aqueous conditions, and give rise to oxime-containing conjugates useful in a wide variety of applications and studies. The value of these aminooxy-based molecules and the ease and speed of preparation of both monovalent and multivalent oxime-linked molecules is demonstrated in two examples using the disaccharide cellobiose; one with a linear linker, and the second with a trivalent core.

Regenerative glycosylation under nucleophilic catalysis

This article describes 3,3-difluoroxindole (HOFox)-mediated glycosylation. The uniqueness of this approach is that both the in situ synthesis of 3,3-difluoro-3H-indol-2-yl (OFox) glycosyl donors and activation thereof can be conducted in a regenerative fashion as is a typical reaction performed under nucleophilic catalysis. Only a catalytic amount of the OFox imidate donor and a Lewis acid activator are present in the reaction medium. The OFox imidate donor is constantly regenerated upon its consumption until glycosyl acceptor has reacted.

A quartz crystal microbalance method to study the terminal functionalization of glycosaminoglycans

QCM-D is demonstrated as a novel method to quantify the reaction yields and stability of the reducing-end conjugation of glycosaminoglycans.

Enhanced catalysis of oxime-based bioconjugations by substituted anilines

The conjugation of biomolecules by chemoselective oxime ligation is of great interest for the site-specific modification of proteins, peptides, nucleic acids, and carbohydrates. These conjugations proceed optimally at a reaction pH of 4-5, but some biomolecules are not soluble or stable under these conditions. Aniline can be used as a nucleophilic catalyst to enhance the rate of oxime formation, but even in its presence, the reaction rate at neutral pH can be slower than desired, particularly at low reagent concentrations and/or temperature. Recently, alternative catalysts with improved properties were reported, including anthranilic acid derivatives for small molecule ligations, as well as m-phenylenediamine at high concentrations for protein conjugations. Here, we report that p-substituted anilines containing an electron-donating ring substituent are superior catalysts of oxime-based conjugations at pH 7. One such catalyst, p-phenylenediamine, was studied in greater detail. This catalyst was highly effective at neutral pH, even at the low concentration of 2 mM. In a model oxime ligation using aminooxy-functionalized PEG, catalysis at pH 7 resulted in a 120-fold faster rate of protein PEGylation as compared to an uncatalyzed reaction, and 19-fold faster than the equivalent aniline-catalyzed reaction. p-Phenylenediamine (10 mM) was also an effective catalyst under acidic conditions and was more efficient than aniline throughout the pH range 4-7. This catalyst allows efficient oxime bioconjugations to proceed under mild conditions and low micromolar concentrations, as demonstrated by the PEGylation of a small protein.

18F-Fluorodeoxyglycosylamines: Maillard reaction of 18F-fluorodeoxyglucose with biological amines

The Maillard reaction of sugars and amines resulting in the formation of glycosylamines and Amadori products is of biological significance, for drug delivery, role in central nervous system, and other potential applications. We have examined the interaction of (18) F-fluorodeoxyglucose ((18) F-FDG) with biological amines to study the formation of (18) F-fluorodeoxyglycosylamines ((18) F-FDGly). Respective amines N-allyl-2-aminomethylpyrrolidine (NAP) and 2-(4'-aminophenyl)-6-hydroxybenzothiazole (PIB precursor) were mixed with FDG to provide glycosylamines, FDGNAP and FDGBTA. Radiosynthesis using (18) F-FDG (2-5 mCi) was carried out to provide (18) F-FDGNAP and (18) F-FDGBTA. Binding of FDGBTA and (18) F-FDGBTA was evaluated in human brain sections of Alzheimer's disease (AD) patients and control subjects using autoradiography. Both FDGNAP and FDGBTA were isolated as stable products. Kinetics of (18) F-FDGNAP reaction indicated a significant product at 4 h (63% radiochemical yield). (18) F-FDGBTA was prepared in 57% yield. Preliminary studies of FDGBTA showed displacement of (3) H-PIB (reduced by 80%), and (18) F-FDGBTA indicated selective binding to Aβ-amyloid plaques present in postmortem AD human brain, with a gray matter ratio of 3 between the AD patients and control subjects. We have demonstrated that (18) F-FDG couples with amines under mild conditions to form (18) F-FDGly in a manner similar to click chemistry. Although these amine derivatives are stable in vitro, stability in vivo and selective binding is under investigation.

Nucleophilic catalysis of MeON-neoglycoside formation by aniline derivatives

Neoglycosylations are increasingly being employed in the synthesis of natural products, drug candidates, glycopeptide mimics, oligosaccharide analogues, and other applications, but the efficiency of these reactions is usually limited by slow reaction times. Here, we show that aniline derivatives such as 2-amino-5-methoxybenzoic acid enhance the rate of acid-catalyzed neoglycosylation for a range of sugar substrates up to a factor of 32 relative to the uncatalyzed reaction.

Kinetic analysis of inhibition of glucoamylase and active site mutants via chemoselective oxime immobilization of acarbose on SPR chip surfaces

We here report a quantitative study on the binding kinetics of inhibition of the enzyme glucoamylase and how individual active site amino acid mutations influence kinetics. To address this challenge, we have developed a fast and efficient method for anchoring native acarbose to gold chip surfaces for surface plasmon resonance studies employing wild type glucoamylase and active site mutants, Y175F, E180Q, and R54L, as analytes. The key method was the chemoselective and protecting group-free oxime functionalization of the pseudo-tetrasaccharide-based inhibitor acarbose. By using this technique we have shown that at pH 7.0 the association and dissociation rate constants for the acarbose-glucoamylase interaction are 10(4)M(-1)s(-1) and 10(3)s(-1), respectively, and that the conformational change to a tight enzyme-inhibitor complex affects the dissociation rate constant by a factor of 10(2)s(-1). Additionally, the acarbose-presenting SPR surfaces could be used as a glucoamylase sensor that allowed rapid, label-free affinity screening of small carbohydrate-based inhibitors in solution, which is otherwise difficult with immobilized enzymes or other proteins.