Synthesis of Neoglycopolymers by a Combination of “Click Chemistry” and Living Radical Polymerization

Preparation of carbohydrate-oligonucleotide conjugates using the squarate spacer

Attachment of carbohydrates to oligonucleotides has proven to induce receptor-mediated endocytosis. A facile method for the formation of covalent linkages between glycans and oligonucleotides is herein described. Thus, use of 3,4-diethoxy-3-cyclobutene-1,2-dione as a linking reagent provides easy conjugation between carbohydrates bearing an amino group at the reducing end and oligonucleotides bearing an aminoalkyl modification.

Preparation of biotinylated glyconanoparticles via a photochemical process and study of their bioconjugation to streptavidin

We report here the preparation of novel biotinylated glyconanoparticles from well-defined biotinylated glycopolymers and poly(N-isopropylacrylamide) (PNIPAAm) synthesized via the reversible addition fragmentation chain transfer (RAFT) polymerization process. The in situ reduction of the biotinylated glycopolymers, PNIPAAm, poly(ethylene glycol), and HAuCl4 via a photochemical process resulted in the formation of biotinylated gold nanoparticles. The multifunctional biotinylated glyconanoparticles were then evaluated for their bioconjugation toward streptavidin using UV-vis spectroscopy and surface plasmon resonance (SPR). The biotinylated nanoparticles underwent aggregation in the presence of streptavidin as revealed by spectrophotometry, which indicates the accessibility of the biotin for conjugation. These results were further confirmed by surface plasmon resonance even in the case of surface-immobilized streptavidin.

Synthesis of branched Man5 oligosaccharides and an unusual stereochemical observation

Branched mannopentaoses were synthesized through two routes. While assembly from the nonreducing end to the reducing end was more convergent with fewer intermediate steps, two diastereomers were obtained. On the other hand, synthesis from the reducing end to the nonreducing end yielded the mannopentaose with the desired stereochemistry as a single isomer. Our results suggest that it is still challenging to reliably predict stereochemical outcome of a glycosylation reaction. It is necessary to thoroughly characterize anomeric configurations of newly formed glycosidic linkages in complex oligosaccharide synthesis.

Well-defined polymers with activated ester and protected aldehyde side chains for bio-functionalization

Polymers with reactive side chains and narrow molecular weight distributions were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, and the potential to utilize these polymers to prepare drug carriers was demonstrated. p-Nitrophenyl methacrylate (NPMA) and diethoxypropyl methacrylate (DEPMA) were polymerized utilizing cumyl dithiobenzoate (CDB) as the chain transfer agent and azobisisobutyronitrile (AIBN) as the initiator to high conversions (> or = 86%). The resulting pNPMA and pDEPMA had narrow molecular weight distributions (polydispersity indices < 1.3). The ability to functionalize these polymers was confirmed. For pNPMA, up to 86% of the side chains were substituted with the amino acid, glycine methyl ester. The side chains of pDEPMA were hydrolyzed to aldehydes and reaction with O-benzylhydroxylamine and O-methylhydroxylamine to form stable oxime bond conjugates was demonstrated. The percent substitution depended on the feed ratios. Conjugation of an aminooxy-functionalized RGD peptide was also demonstrated.

Amino Acid and Peptide Synthesis and Functionalization by the Reaction of Thioacids with 2,4-Dinitrobenzenesulfonamides

Readily prepared amino thioacids react at room temperature in DMF in the presence of cesium carbonate with 2,4-dinitrobenzenesulfonamides to give amides. When the sulfonamide is derived from an amino acid the method results in peptide bond formation, whereas the use of carbohydrate derived sulfonamides gives neoglycoconjugates.

Efficient synthesis of linear multifunctional poly(ethylene glycol) by copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition

Poly(ethylene glycol) (PEG) is a versatile biocompatible polymer. Improvement of its limited functionality (two chain termini) may significantly expand its current applications. In this communication, a simple and yet highly efficient strategy for the synthesis of linear multifunctional PEGs with "click" chemistry is reported. A short acetylene-terminated PEG was linked by 2,2-bis(azidomethyl)propane-1,3-diol using Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition in water at room temperature. High-molecular-weight PEGs with pendant hydroxyl groups were obtained and characterized by 1H NMR and size-exclusion chromatography. A prototype bone-targeting polymeric drug delivery system was also successfully synthesized based on this new method. It demonstrates strong biomineral-binding ability and the ease of incorporating therapeutic agents into the delivery system. This simple "click" reaction approach provides a useful tool for the development of novel functional polymers and their conjugates for biomedical applications.

Triazole: the Keystone in Glycosylated Molecular Architectures Constructed by a Click Reaction

The copper(I)-catalyzed modern version of the Huisgen-type azide-alkyne cycloaddition to give a 1,4-disubstituted 1,2,3-triazole unit is introduced as a powerful ligation method for glycoconjugation. Owing to its high chemoselectivity and tolerance of a variety of reaction conditions, this highly atom-economic and efficient coupling reaction is especially useful for the effective construction of complex glycosylated structures such as clusters, dendrimers, polymers, peptides, and macrocycles. In all cases the triazole ring plays a key role by locking into position the various parts of these molecular architectures. The examples reported and briefly discussed in this short review highlight the use of this reaction in carbohydrate chemistry and pave the way to further developments and applications.

Synthesis of molecular brushes by "grafting onto" method: combination of ATRP and click reactions

Molecular brushes (densely grafted polymers or bottle-brush macromolecules) were synthesized by the "grafting onto" method via combination of atom transfer radical polymerization (ATRP) and "click" reactions. Linear poly(2-hydroxyethyl methacrylate) (PHEMA) polymers were synthesized first by ATRP. After esterification reactions between pentynoic acid and the hydroxyl side groups, polymeric backbones with alkynyl side groups on essentially every monomer unit (PHEMA-alkyne) were obtained. Five kinds of azido-terminated polymeric side chains (SCs) with different chemical compositions and molecular weights were used, including poly(ethylene glycol)-N3 (PEO-N3), polystyrene-N3, poly(n-butyl acrylate)-N3, and poly(n-butyl acrylate)-b-polystyrene-N3. All click coupling reactions between alkyne-containing polymeric backbones (PHEMA-alkyne) and azido-terminated polymeric SCs were completed within 3 h. The grafting density of the obtained molecular brushes was affected by several factors, including the molecular weights and the chemical structures of the linear SCs, as well as the initial molar ratio of linear chains to alkynyl groups. When linear polymers with "thinner" structure and lower molecular weight, e.g., PEO-N3 with Mn = 775 g/mol, were reacted with PHEMA-alkyne (degree of polymerization = 210) at a high molar ratio of linear chains to alkynyl groups in the backbone, the brush copolymers with the highest grafting density were obtained (Y(grafting) = 88%). This result indicates that the average number of SCs was ca. 186 per brush molecule and the average molecular weight of the brush molecules was ca. 190 kg/mol.

Facile preparation of glyconanoparticles and their bioconjugation to streptavidin

Well-defined glycopolymers containing linear and cyclic carbohydrate moieties as pendent groups were prepared by reversible addition fragmentation chain transfer polymerization (RAFT). The RAFT synthesized glycopolymers were used for the aqueous synthesis of stabilized glyconanoparticles. The in situ reduction of the glycopolymers and HAuCl4 resulted in the formation of highly stable modified gold nanoparticles with diameters ranging from 40 to 80 nm in aqueous media. Multifunctional glyconanoparticles were also generated in the presence of varying amounts of biotinylated-polyethyleneglycol (bio-PEG-SH) having terminal thiol groups. The gold nanoparticles underwent aggregation in the presence of streptavidin as revealed by UV-vis spectroscopy. The availability of the biotin for conjugation to streptavidin was also confirmed using surface plasmon resonance (SPR).

1,3-dipolar cycloadditions of azides and alkynes: a universal ligation tool in polymer and materials science

In 2001, Sharpless and co-workers introduced "click" chemistry, a new approach in organic synthesis that involves a handful of almost perfect chemical reactions. Among these carefully selected reactions, Huisgen 1,3-dipolar cycloadditions were shown to be the most effective and versatile and thus became the prime example of click chemistry. Hence, these long-neglected reactions were suddenly re-established in organic synthesis and, in particular, have gained popularity in materials science. The number of publications dealing with click chemistry has grown exponentially over the last two years. The Minireview discusses whether click chemistry is a miracle tool or an ephemeral trend.

Instantaneous inclusion of a polynucleotide and hydrophobic guest molecules into a helical core of cationic beta-1,3-glucan polysaccharide

We succeeded in the quantitative and selective introduction of an ammonium cationic group into the C6 position of Curdlan (CUR) by "Click Chemistry", and the obtained cationic Curdlan (CUR-N+) showed good solubility in water. ORD studies suggested that CUR-N+ adopts a single-stranded structure, different from a right-handed, triple-stranded helical structure of beta-1,3-glucan polysaccharides in water. It has been revealed that the polymeric complexes of CUR-N+ with polymeric guest molecules, such as polycytidylic acid (poly(C)), permethyldecasilane (PMDS), and single-walled carbon nanotubes (SWNTs), can be easily obtained by just mixing them in water with sonication. The characterization of the resultant CUR-N+-poly(C) complexes by UV-vis, CD spectroscopic measurements, and AFM and TEM observations revealed that they have stoichiometric, nanosized fibrous structures. From these experimental results as well as our precedent studies (e.g., refs 6 and 23), we propose that the complexation would be driven by the cooperative action of (1) the hydrogen-bonding interaction between the OH group at the C2 position and hydrogen-bonding sites of the cytosine ring (ref 6d), (2) the electrostatic interaction between the ammonium cation and the phosphate anion (ref 23), as well as (3) the background hydrophobic interaction. In addition, the complexed polynucleotide chain showed a strong resistance against enzymatic hydrolysis. Likewise, the dispersion of PMDS and SWNTs in water by CUR-N+ and the fibrous structures of the complexes were confirmed by spectroscopic measurements as well as microscopic observations. These binding properties of CUR-N+, which can proceed spontaneously in water, clearly differ from those of schizophyllan (SPG), which inevitably require a denature-renature process corresponding to a conversion of a triple strand to single strands induced by DMSO or base for inclusion of polymeric guest molecules.