Totally synthetic polymer with lectin-like function: Induction of killer cells by the copolymer of 3-acrylamidophenylboronic acid withN,N-dimethylacrylamide

A Gadolinium DO3A Amide m-Phenyl Boronic Acid MRI Probe for Targeted Imaging of Sialated Solid Tumors

We developed a new probe, Gd-DO3A-Am-PBA, for imaging tumors. Our results showed active targeting of Gd-DO3A-Am-PBA to sialic acid (SA) moieties, with increased cellular labeling in vitro and enhanced tumor accumulation and retention in vivo, compared to the commercial Gadovist. The effectiveness of our newly synthesized probe lies in its adequate retention phase, which is expected to provide a suitable time window for tumor diagnosis and a faster renal clearance, which will reduce toxicity risks when translated to clinics. Hence, this study can be extended to other tumor types that express SA on their surface. Targeting and MR imaging of any type of tumors can also be achieved by conjugating the newly synthesized contrast agent with specific antibodies. This study thus opens new avenues for drug delivery and tumor diagnosis via imaging.

Boronic Acid Ligands Can Target Multiple Subpopulations of Pancreatic Cancer Stem Cells via pH-Dependent Glycan-Terminal Sialic Acid Recognition

Eradication of cancer stem cells (CSCs) is an ultimate goal in cancer chemotherapy. Although a ligand-assisted targeting approach seems rational, the existence of subpopulations of CSCs and their discrimination from those present on healthy sites makes it a severe challenge. Some boronic acid (BA) derivatives are known for the ability to bind with glycan-terminal sialic acid (SA), in a manner dependent on the acidification found in hypoxic tumoral microenvironment. Taking advantage of this feature, here we show that the BA-ligand fluorescence conjugate can effectively target multiple CSC subpopulations in parallel, which otherwise must be independently aimed when using antibody--ligands.

Potentiometric Determination of Circulating Glycoproteins by Boronic Acid End-Functionalized Poly(ethylene glycol)-Modified Electrode

Despite tremendous complexity in glycan structure, sialic acid (SA) provides an analytically accessible index for glycosylation, owing to its uniquely anionic nature and glycan-chain terminal occupation. Taking advantage of boronic acid (BA) based SA-recognition chemistry, we here demonstrate a label-free, no enzymatic, potentiometric determination of fetuin, a blood-circulating glycoprotein implicated in physiological and various pathological states. A phenylboronic acid (PBA) ω-end-functionalized poly(ethylene glycol) (PEG) with an α-tethering unit bearing pendent alkyne groups was "grafted-to" a gold electrode modified with 11-azide-undecathiol by a copper-catalyzed azide-alkyne cycloaddition reaction. Using the electrode, fetuin was potentiometrically detectable with a μM-order-sensitivity that is comparable to what is found in blood-collected specimen. Our finding may have implications for developing a remarkably economic hemodiagnostic technology with ease of downsizing and mass production.

Chemo-physical Strategies to Advance the in Vivo Functionality of Targeted Nanomedicine: The Next Generation

The past few decades have witnessed an evolution of nanomedicine from biologically inert entities to more smart systems, aimed at advancing in vivo functionality. However, we should recognize that most systems still rely on reasonable explanation-including some over-explanation-rather than definitive evidence, which is a watershed radically determining the speed and extent of advancing nanomedicine. Probing nano-bio interactions and desirable functionality at the tissue, cellular, and molecular levels is most frequently overlooked. Progress toward answering these questions will provide instructive insight guiding more effective chemo-physical strategies. Thus, in the next generation, we argue that much effort should be made to provide definitive evidence for proof-of-mechanism, in lieu of creating many new and complicated systems for similar proof-of-concept.

Direct Observation of Cell Surface Sialylation by Atomic Force Microscopy Employing Boronic Acid-Sialic Acid Reversible Interaction

Tracing cell surface sialylation dynamics at a scale of the glycolipoprotein microdomain (lipid rafts) formations remains an intriguing challenge of cellular biology. Here, we demonstrate that this goal is accessible, taking advantage of a boronic acid (BA)-based reversible molecular recognition chemistry. A BA-end-functionalized poly(ethylene glycol) was decorated onto an atomic force microscopy (AFM) cantilever, which provided a dynamic and sialic acid (SA)-specific imaging mode. Using this technique, we were able to heat map the SA expression levels not only on protein-decorated substrates but also directly on the cell surfaces, with a submicrometer scale resolution that may be relevant to that of the lipid rafts formation. The SA specificity and the binding reversibility of the probe were confirmed from its pH-dependent characteristics and an inhibition assay using free state SA. This finding may provide a noninvasive means for assessing a variety of SA-involved glycosylation dynamics spanning from physiology to pathology.

Heterocyclic boronic acids display sialic acid selective binding in a hypoxic tumor relevant acidic environment

A group of heterocyclic boronic acids demonstrating unusually high affinity and selectivity for sialic acids are described, with strong interactions under the weakly acidic pH conditions associated with a hypoxic tumoral microenvironment.

Boronic acids are well known for their ability to reversibly interact with the diol groups found in sugars and glycoproteins. However, they are generally indiscriminate in their binding. Herein we describe the discovery of a group of heterocyclic boronic acids demonstrating unusually high affinity and selectivity for sialic acids (SAs or N-acetylneuraminic acid), which are sugar residues that are intimately linked with tumor growth and cancer progression. Remarkably, these interactions strengthen under the weakly acidic pH conditions associated with a hypoxic tumoral microenvironment. In vitro competitive binding assays uncovered a significantly higher ability of 5-boronopicolinic acid, one of the derivatives identified in this work as a strong SA-binder, to interact with cell surface SA in comparison to a gold-standard structure, 3-propionamidophenylboronic acid, which has proven to be an efficient SA-binder in numerous reports. This structure also proved to be suitable for further chemical conjugation with a well-preserved SA-binding capability. These findings suggest an attractive alternative to other ongoing boronic acid based chemistry techniques aiming to achieve tumor-specific chemotherapies and diagnoses.

Nanoscaled boron-containing delivery systems and therapeutic agents for cancer treatment

Significant efforts have recently been made to develop nanoscaled boron-containing delivery systems for improving drug delivery in cancer therapy. On one hand, borate ester chemistry has shown importance in ligand-mediated tumor targeting owing to the recognition ability of boronic acid to polyol residues in cell membranes. In particular, the phenylboronic acid-functionalized nanocarriers for specific targeting to sialic acid groups which are overexpressed on tumor cells have made great achievements. On the other hand, nanoscaled boron neutron capture therapy agents show growing potential in efficiently transporting boron to tumor. The current review outlines the recent developments in the application of borate ester chemistry in tumor targeting by nanoparticles, then summarizes recent work on the development of boron-based nanomaterials as boron neutron capture therapy agents.

Thermo- and glucose-responsive micelles self-assembled from phenylborate ester-containing brush block copolymer for controlled release of insulin at physiological pH

The micelles present temperature- and glucose-responses, and can achieve the controlled release of insulin by altering temperature and glucose concentration.

Mucoadhesive and enzymatic inhibitory nanoparticles for transnasal insulin delivery

AIM

To develop a novel nanocarrier with mucoadhesion and enzymatic inhibition for transnasal insulin delivery. METHODS & METHODS: The physicochemical characterization of the nanoparticles included size and morphology, as well as mucoadhesion and enzymatic inhibition. The in vitro release of insulin from the nanoparticles was evaluated in 3 mg/ml glucose medium. The cytocompatibility of the nanoparticles was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The interactions of the nanoparticles with Caco-2 cells and nasal epithelia, and the effect of the nanoparticles on transnasal insulin delivery were estimated.

RESULTS

The nanoparticles were spherical in shape, with an average size of 100 nm, and presented strong enzymatic inhibitory activity and high mucin adsorption ability. The insulinloaded nanoparticles showed the rapid insulin release in 3 mg/ml glucose medium. The nanoparticles were noncytotoxic to Caco-2 cells. Furthermore, the insulin-loaded nanoparticles overcame mucosal barriers and significantly decreased plasma glucose levels.

Current and emerging challenges of field effect transistor based bio-sensing

Field-effect-transistor (FET) based electrical signal transduction is an increasingly prevalent strategy for bio-sensing. This technique, often termed "Bio-FETs", provides an essentially label-free and real-time based bio-sensing platform effective for a variety of targets. This review highlights recent progress and challenges in the field. A special focus is on the comprehension of emerging nanotechnology-based approaches to facilitate signal-transduction and amplification. Some new targets of Bio-FETs and the future perspectives are also discussed.

Facile phenylboronate modification of silica by a silaneboronate

Macroscopic and colloidal silica surfaces were readily modified with alkoxysilaneboronate, IV, yielding silica surfaces with covalently bonded phenylboronic acid groups. XPS and neutron activation confirmed the presence of boron. The ability of these surfaces to specifically interact with polyols was demonstrated with polyol-coated latex and ARS, a dye that specifically couples to boronic acid groups immobilized on colloidal or macroscopic silica. This is a new, direct approach for introduction of phenylboronic acid groups onto silica surfaces.

Applications of reversible covalent chemistry in analytical sample preparation

Reversible covalent chemistry (RCC) adds another dimension to commonly used sample preparation techniques like solid-phase extraction (SPE), solid-phase microextraction (SPME), molecular imprinted polymers (MIPs) or immuno-affinity cleanup (IAC): chemical selectivity. By selecting analytes according to their covalent reactivity, sample complexity can be reduced significantly, resulting in enhanced analytical performance for low-abundance target analytes. This review gives a comprehensive overview of the applications of RCC in analytical sample preparation. The major reactions covered include reversible boronic ester formation, thiol-disulfide exchange and reversible hydrazone formation, targeting analyte groups like diols (sugars, glycoproteins and glycopeptides, catechols), thiols (cysteinyl-proteins and cysteinyl-peptides) and carbonyls (carbonylated proteins, mycotoxins). Their applications range from low abundance proteomics to reversible protein/peptide labelling to antibody chromatography to quantitative and qualitative food analysis. In discussing the potential of RCC, a special focus is on the conditions and restrictions of the utilized reaction chemistry.

Boronate-containing polymers form affinity complexes with mucin and enable tight and reversible occlusion of mucosal lumen by poly(vinyl alcohol) gel

Copolymers of N-acryloyl-m-aminophenylboronic acid (NAAPBA) with N,N-dimethylacrylamide (DMAA) formed insoluble interpolymer complexes with mucin from porcine stomach at pH 9.0. The complex formation based on boronate-sugar interactions took place between the similarly charged macromolecules and resulted in coacervate particles formation, which depended both on pH and ionic strength of the solution. The coacervation rate displayed a maximum at the intermediate DMAA-NAAPBA copolymer: mucin weight ratio, that is a pattern typical of interpolymer complex formation. The effective hydrodynamic particle diameter of the coacervates monotonously grew from 155+/-20 nm up to 730+/-120 nm in 2 days in 0.1M sodium bicarbonate buffer solution, pH 9.0. Electrophoretic mobility of the resultant nanoparticles was intermediate between those of individual polymers, whereas the particles zeta-potential was -7.5+/-0.4 mV in the above buffer solution. Pre-treatment of the inner mucosal epithelium of excised male pig urethras with 5% (w/v) solutions of acrylamide-NAAPBA or DMAA-NAAPBA copolymers at pH 8.8 allowed for tight occlusion of the lumen by poly(vinyl alcohol)-borax gel injected via a two-way catheter. Leakage of 0.15M NaCl solution through the thus occluded organs could be prevented, while the leakage through the organs occluded by the gel without the pre-treatment was unavoidable. The gel plug could be quickly dissolved on demand after injection of 5% (w/v) aqueous fructose solution into the lumen. The described technique may be useful for temporal occlusion of mucosal lumens in living organisms. In contrast to the conventional mucoadhesive polymers like polyacrylic acid or chitosan, the boronate-containing copolymers display their mucoadhesivity at weakly alkaline pH of 8-9 and physiological ionic strength.

A Fluorescent Polymeric Heparin Sensor

Linear copolymers have been developed which carry binding sites tailored for sulfated sugars. All binding monomers are based on the methacrylamide skeleton and ensure statistical radical copolymerization. They are decorated with o-aminomethylphenylboronates for covalent ester formation and/or alkylammonium ions for noncovalent Coulomb attraction. Alcohol sidechains maintain a high water solubility; a dansyl monomer was constructed as a fluorescence label. Statistical copolymerization of comonomer mixtures with optimized ratios was started by AIBN (AIBN=2,2'-azoisobutyronitrile) and furnished water-soluble comonomers with an exceptionally high affinity for glucosaminoglucans. Heparin can be quantitatively detected with an unprecedented 30 nM sensitivity, and a neutral polymer without any ammonium cation is still able to bind the target with almost micromolar affinity. From this unexpected result, we propose a new binding scheme between the boronate and a sulfated ethylene glycol or aminoethanol unit. Although the mechanism of heparin binding involves covalent boronate ester formation, it can be completely reversed by protamine addition, similar to heparin's complex formation with antithrombin III.

Tapping into Boron/?-Hydroxycarboxylic Acid Interactions in Sensing and Catalysis

Whereas interaction of boron acids (boric and boronic) with diols and neutral sugar ligands has received much global research attention in recent years, the binding of simple α-hydroxycarboxylic and sugar acids by boron has received less attention. Applications of boron-based fluorescent sensors and chemoselective catalysts targeting this functional motif have appeared only in the past 5 years. The present synopsis will focus on rapid developments that have occurred in both areas during this half decade.

Boron-Containing Functional Copolymers for Bioengineering Applications

This review summarizes the some advances and results of our recent investigations, outlining the synthesis, structural phenomena, unique properties and application areas of boron-containing bioengineering copolymers. Particular attention is paid to the effects of hydrogen bonding in copolymerization reactions of boron-containing monomers, and in formation of supramacromolecular structures, bioconjugates, physically and chemically crosslinkable copolymers, and other important bioengineering materials.

Temperature-Controlled Release of Diols fromN-Isopropylacrylamide-co-Acrylamidophenylboronic Acid Microgels

N-Isopropylacrylamide-co-acrylamidophenylboronic acid (NIPAM-co-PBA) microgels were prepared by free radical polymerization in water. The release of glucose and Alizarin Red S (ARS) from the microgels as a function of temperature has been investigated by using laser light scattering (LLS) and ultrasensitive differential scanning calorimetry (US-DSC). Such microgels can bind glucose and ARS via boronic acids at a lower temperature. As the temperature increases, the microgels shrink, and the diols are released. The release could be controlled by temperature. The effect of the structure of the microgels on the release is also discussed.

Affinity Adhesion of Carbohydrate Particles and Yeast Cells to Boronate-Containing Polymer Brushes Grafted onto Siliceous Supports

Cross-linked agarose particles (Sepharose CL-6B) and baker's yeast cells were found to adhere to siliceous supports end-grafted with boronate-containing copolymers (BCCs) of N,N-dimethylacrylamide at pH> or =7.5, due to boronate interactions with surface carbohydrates of the particles and the cells. These interactions were registered both on macroscopic and on molecular levels: the BCCs spontaneously adsorbed on the agarose gel at pH> or =7.5, with adsorption increasing with pH. Agarose particles and yeast cells stained with Procion Red HE-3B formed stable, monolayer-like structures at pH 8.0, whereas at pH 7.0-7.8 the structures on the copolymer-grafted supports were less stable and more random. At pH 9.0, 50 % saturation of the surface with adhering cells was attained in 2 min. Stained cells formed denser and more stable layers on the copolymer-grafted supports than they did on supports modified with self-assembled organosilane layers derivatized with low-molecular-weight boronate, presumably due to a higher reactivity of the grafted BCCs. Quantitative detachment of adhered particles and cells could be achieved by addition of 20 mM fructose--a strong competitor for binding to boronates--at pH 7.0-9.0. Regeneration of the grafted supports allowed several sequential adhesion and detachment cycles with stained yeast cells. Affinity adhesion of micron-sized carbohydrate particles to boronate-containing polymer brushes fixed on solid supports is discussed as a possible model system suggesting a new approach to isolation and separation of living cells.

Effects of polyols, saccharides, and glycoproteins on thermoprecipitation of phenylboronate-containing copolymers

The copolymer of 3-(acrylamido)phenylboronic acid and N-isopropylacrylamide (82:18, Mn = 47000 g/mol) was prepared by free radical polymerization. The copolymer showed typical thermoprecipitation behavior in aqueous solutions; its phase transition temperature (TP) was 26.5 +/- 0.2 degrees C in 0.1 M glycine-NaOH buffer containing 0.1 M NaCl, pH 9.2. Due to specific complex formation of the pendant boronates with sugars, TP was strongly affected by the type of sugar and its concentration at pH 9.2. Fructose, lactulose, and glucose caused the largest increase in TP (up to 4 degrees C) at 0.56 mM concentration, attributed to the high binding affinity of the sugars to borate and phenylboronate. Among the sugars typical of nonreducing ends of oligosaccharides, N-acetylneuraminic acid had the strongest effect on TP (ca. 2 degrees C at 0.56 mM concentration and pH 9.2), while the effects of other sugars are well expressed at the higher concentrations (16 and 80 mM) and decreased in the order xylose approximately galactose >or= N-acetyllactosamine >or= mannose approximately fucose >> N-acetylglucosamine. The effect exerted on the phase transition by glycoproteins was the strongest with mucin from porcine stomach and decreased in the series mucin > horseradish peroxidase > human gamma-globulin at pH 9.2. As a first approximation, the weight percentage and/or the number of oligosaccharides in glycoproteins determined the character of their interaction with the pendant phenylboronates and, therefore, the effect on the copolymer phase transition.

Boronate-Containing Copolymers: Polyelectrolyte Properties and Sugar-Specific Interaction with Agarose Gel

Copolymers of N-acryloyl-m-aminophenylboronic acid (NAAPBA) with acryamide (AA), N,N-dimethylacrylamide (DMAA), and N-isopropylacrylamide (NIPAM) were found to adsorb on cross-linked agarose gel (Sepharose CL-6B) in the pH range from 7.5-9.2, due to specific boronate-sugar interactions. The molar percentages of phenylboronic acid (PBA) groups in the boronate-containing copolymers (BCCs), as estimated by 1H NMR spectroscopy, were 13, 10, and 16%, respectively, whereas the apparent ionization constants, the pKa values, of the copolymers were similar and equal to 9.0 +/- 0.2 at 20 degrees C. The copolymers adsorption capacities were in the range of 15-30 mg x ml(-1) gel (14-36 micromol pendant PBA ml(-1) gel) at pH 9.2 and decreased with decreasing pH value. The interaction of monomeric NAAPBA with Sepharose CL-6B was characterized by an equilibrium association constant of 53 +/- 17 M(-1), the chromatographic capacity factor k' = 1.8, and a total content of binding sites of 27 +/- 10 micromol x ml(-1) gel at pH 9.2. The weak reversible binding of monomeric NAAPBA and almost irreversible binding of NAAPBA copolymers to the gel at pH 9.2 suggested a multivalent character of the copolymer adsorption. At pH 7.5, the maximal adsorption capacity was displayed by the AA-NAAPBA copolymer (15 mg x ml(-1) gel). All the BCCs could be completely desorbed from the gel by 0.1 M fructose in aqueous buffered media with pH values from 7.5-9.2. The strong adsorption of AA-NAAPBA on agarose gel probably relates to the conformation of the copolymer in aqueous solution and provides opportunities for biomedical applications of the copolymer under physiological conditions. Multivalent, weak-affinity adsorption of BCCs to the agarose gel seems to be a tentative model for the copolymers' binding to oligo- and polysaccharides of cell membranes and mucosal surfaces.

Interaction of sugars, polysaccharides and cells with boronate-containing copolymers: from solution to polymer brushes

Interaction of mono- and disaccharides, polysaccharide particles and yeast cells with boronate-containing copolymers (BCC) of N-acryloyl-m-aminophenylboronic acid (NAAPBA) with N,N-dimethylacrylamide (DMAA) or N-isopropylacrylamide (NIPAM) was studied. The binding of saccharides to BCC of NIPAM resulted in a shift of its phase transition temperature (DeltaTP), which provided a quantitative measure for the complex formation. Among the sugars typical of non-reducing ends of glycoproteins the DeltaTP decreased in the order: N-acetylneuraminic acid > xylose approximately galactose > mannose approximately fucose >> N-acetylglucosamine. Strong specific adsorption of the BCC on the cross-linked agarose gel Sepharose CL-6B (15-30 mg/ml gel at pH 9.2) was registered. The copolymers adsorption was due to boronate-sugar interactions and decreased with pH. Multivalent interaction of the BCC with the agarose gel has been proven by liquid column chromatography exhibiting a weak reversible adsorption of NAAPBA and almost irreversible adsorption of DMAA-NAAPBA copolymer from 0.1 M sodium phosphate buffer, pH 7.9. The two studied BCCs could be completely desorbed from the gel by 0.1 M fructose in aqueous buffered media with pH from 7.5 to 9.2. In turn, the agarose particles and yeast cells were found to adhere to siliceous supports end-grafted with boronate-BCC of N,N-dimethylacrylamide at pH > or = 7.5, due to the actions. Quantitative detachment of adhered particles or cells could be attained by addition of 20 mM or 100 mM fructose, respectively, in the pH range from 7.5 to 9.2. Affinity adhesion of micron-size carbohydrate particles to boronate-containing polymer brushes fixed on solid supports was considered as a model system suggesting a new approach to isolation and separation of living cells.

Binding of Adenosine to Pendant Phenylboronate Groups of Thermoresponsive Copolymer: a Quantitative Study

Binding of adenosine to the thermosensitive copolymer of N-isopropylacrylamide and 3-(acrylamido)aminophenylboronic acid (82:18, Mn = 47,000 g . mol(-1)) was studied by equilibrium dialysis at 22 degrees C and 37 degrees C, in a 0.1 M glycine buffer containing 0.1 M NaCl at pH 9.2. The copolymer exhibited a the phase transition temperature (T(p)) of 26.5 degrees C under the above conditions. At 22 degrees C the binding of adenosine to the water-soluble copolymer was well described by a Langmuir model, accounting for preferential ionisation of the boronate-nucleoside complexes and, therefore, restricted reactivity of the rest of boronates. At saturation, the copolymer contained 38% of its phenylboronic acid groups in the form of complexes, whereas the association constant was 1,400 M(-1). At 37 degrees C no binding of adenosine to thermally precipitated copolymer was found, presumably owing to interaction of the phenylboronates with hydrophobic segments of polyNIPAM. At high loading of the copolymer by the reversibly bound adenosine the T(p) steeply increases with increasing fraction of the phenylboronate-adenosine complexes in the chains. The increase of the T(p) observed above the saturating adenosine concentration (>1 x 10(-3) M, 22 degrees C) very probably testifies to competition of the nucleoside with hydrophobic polyNIPAM segments for binding to the pendant phenylboronates.

Molecular Recognition of Sialic Acid End Groups by Phenylboronates

A multinuclear NMR study of the interaction between phenylboronic acid (PBA) and sialic acid (Neu5 Ac) has been performed. The latter compound is known to be overexpressed on the cell surface of tumor cells. The results of this investigation suggest that the binding of PBA to sialic acid is pH dependent. 17O NMR experiments with glycolic acid as the model compound prove that an interaction at the alpha-hydroxycarboxylate occurs at pH < 9, while a study with threonic and erythronic acids shows that the PBA group interacts selectively with the vicinal diol functions at higher pH. Similarly, Neu5 Ac binds PBA through its alpha-hydroxycarboxylate at low pH (< 9) and through its glycerol side chain at higher pH values. The conditional stability constant of the phenylboronate ester at pH 7.4 is 11.4. On cell surfaces, sialic acid is connected to the neighboring sugar unit through the 2-hydroxy group. To mimic this the 2-alpha-O-methyl derivative of Neu5 Ac was included in this study. The erythro configuration of the hydroxy substituents prevents stable-complex formation at positions C7 and C8 and, consequently, the strongest interaction is observed at positions C8 and C9, leading to a five-membered 2-boron-1,3-dioxalate. In addition, a relatively small amount of the C7-C9 six-membered complex was observed. Molecular modeling studies confirm that the C8-C9 boronate complex has the lowest energy.

Identification of novel small-Molecule Ulex europaeus I mimetics for targeted drug delivery

Lectin mimetics have been identified that may have potential application towards targeted drug delivery. Synthetic multivalent polygalloyl constructs effectively competed with Ulex europaeus agglutinin I (UEA1) for binding to intestinal Caco-2 cell membranes.

Anomalous binding profile of phenylboronic acid with N-acetylneuraminic acid (Neu5Ac) in aqueous solution with varying pH

Borates are known to interact with carbohydrate moieties expressed on the surface of biological membranes of a variety of cells, viruses, bacteria, and fungi. This study revealed the anomalous binding profile of borate in aqueous solution with N-acetylneuraminic acid (Neu5Ac, sialic acid) as a potential receptor site on the surfaces of biological membranes using (11)B, (1)H, (13)C, and (15)N nuclear magnetic resonance spectroscopies. 3-(Propionamido)phenylboronic acid (PAPBA) was chosen as the model borate compound. The equilibrium constant (K) for Neu5Ac binding to PAPBA was compared with those for glucose, mannose, and galactose, which are the major carbohydrate constituents of glycoproteins and glycolipids expressed on biological membranes. In the Neu5Ac/PAPBA system, the unusual pH dependency of the K values, a decrease in K with increasing pH, was observed, suggesting the formation of a trigonal-formed complex stabilized by the coordination of an amide group of Neu5Ac at the C-5 position to the boron atom, forming intramolecular B-N or B-O bonding. Furthermore, the anomalously high complexing ability at physiological pH 7.4 was confirmed for this system, with the K value 37.6 which is approximately 7 times higher than that for glucose. This exceptionally high value of K at physiological pH, compared to those of other sugars, strongly suggests that the boronic acid selectively recognizes the Neu5Ac residues of the glycosylated components including glycoproteins and gangliosides existing on the surface of the biological membranes.

Immunomodulating activities of soluble synthetic polymer-bound drugs

The introduction of a synthetic material into the body always affects different body systems, including the defense system. Synthetic polymers are usually thymus-independent antigens with only a limited ability to elicit antibody formation or to induce a cellular immune response against them. However, there are many other ways that they influence or can be used to influence the immune system of the host. Low-immunogenic water-soluble synthetic polymers sometimes exhibit significant immunomodulating activity, mainly concerning the activation/suppression of NK cells, LAK cells and macrophages. Some of them, such as poly(ethylene glycol) and poly[N-(2-hydroxypropyl)methacrylamide], can be used as effective protein carriers, as they are able to reduce the immunogenicity of conjugated proteins and/or to reduce non-specific uptake of liposome/nanoparticle-entrapped drugs and other therapeutic agents. Recently, the development of vaccine delivery systems prepared from biodegradable and biocompatible water-soluble synthetic polymers, microspheres, liposomes and/or nanoparticles has received considerable attention, as they can be tailored to meet the specific physical, chemical, and immunogenic requirements of a particular antigen and some of them can also act as adjuvants.

Molecular imprinting within hydrogels

Hydrogels have been used primarily in the pharmaceutical field as carriers for delivery of various drugs, peptides and proteins. These systems have included stimuli-responsive gels that exhibit reversible swelling behavior and hence can show modulated release in response to external stimuli such as pH, temperature, ionic strength, electric field, or specific analyte concentration gradients. The focus of this article is to review molecular imprinting within hydrogels and discuss recent efforts on analyte-responsive intelligent gels, specifically suggesting the possibility of utilizing molecular imprinting strategies to impart analyte specificity and responsiveness within these systems. Molecular imprinting is an emerging field that produces precise chemical architecture that can bind analytes and differentiate between similar molecules with enantiomeric resolution. On the forefront of imprinting gel systems are intelligent, stimuli-sensitive imprinted gels that modify their swelling behavior and in turn modulate their analyte binding abilities. We discuss the challenges creating an imprinting effect in hydrogels and the possibilities of using molecularly imprinted mechanisms within controlled release gels.