Immobilisation on polystyrene of diazirine derivatives of mono- and disaccharides: biological activities of modified surfaces

Enzymatic reactions on immobilised substrates

This review gives an overview of enzymatic reactions that have been conducted on substrates attached to solid surfaces. Such biochemical reactions have become more important with the drive to miniaturisation and automation in chemistry, biology and medicine. Technical aspects such as choice of solid surface and analytical methods are discussed and examples of enzyme reactions that have been successful on these surfaces are provided.

Specific immobilization of biotinylated fusion proteins NGF and Sema3A utilizing a photo-cross-linkable diazirine compound for controlling neurite extension

In this study we report the successful synthesis of N-(2-mercaptoethyl)-3-(3-methyl-3H-diazirine-3-yl) propanamide (N-MCEP-diazirine), with sulfhydryl and amine photoreactive ends to allow recombinant protein tethering to chitosan films. This regimen allows mimicry of the physiological endeavor of axon pathfinding in the nervous system where neurons rely on cues for guidance during development and regeneration. Our strategy incorporates strong covalent and noncovalent interactions, utilizing N-MCEP-diazirine, maleimide-streptavidin complex, and two custom biotinylated-fusion proteins, nerve growth factor (bNGF), and semaphorin3A (bSema3A). Synthetic yield of N-MCEP-diazirine was 87.3 ± 1.9%. Characteristic absorbance decrease at 348 nm after N-MCEP-diazirine exposure to UV validated the photochemical properties of the diazirine moiety, and the attachment of cross-linker to chitosan films was verified with Fourier transform infrared spectroscopy (FTIR). Fluorescence techniques showed no significant difference in the detection of immobilized proteins compared to absorbing the proteins to films (p < 0.05); however, in vitro outgrowth of dorsal root ganglia (DRG) was more responsive to immobilized bNGF and bSema3A compared to adsorbed bNGF and bSema3A over a 5 day period. Immobilized bNGF significantly increased DRG length over time (p < 0.0001), but adsorbed bNGF did not increase in axon extension from day 1 to day 5 (p = 0.4476). Immobilized bSema3A showed a significant decrease in neurite length (524.42 ± 57.31 μm) at day 5 compared to adsorbed bSema3A (969.13 ± 57.31 μm). These results demonstrate the superiority of our immobilization approach to protein adsorption because biotinylated-fusion proteins maintain their active confirmation and their tethering can be spatially controlled via a UV activated N-MCEP-diazirine cross-linker.

Glycoarray by DNA-directed immobilization

Glycoarrays have become a powerful platform to investigate the interactions of many biological events involving carbohydrates. The carbohydrates immobilization on the surface of the substrates is a key step of glycoarray fabrication. Plenty of strategies have been applied to the immobilization process. Herein a protocol for the synthesis of oligonucleotide glycomimetic conjugates is proposed. The resulting molecules are immobilized by hybridization on a DNA microarray (DNA-directed immobilization; DDI). DDI has been proved to be a very efficient and site-selective. This protocol provides detailed procedures for the preparation of fluorescent oligonucleotide trigalactosylmimetic conjugates and for the preparation of carbohydrate microarrays by DDI on glass slides.

Specific recognition of lectins by oligonucleotide glycoconjugates and sorting on a DNA microarray

Two glycoconjugates bearing different DNA tags are mixed in solution with lectins; both interact with their specific lectin and the resulting complexes are sorted, according to their DNA sequences, at the surface of micro-reactors bearing the immobilised complementary DNA sequences.

DNA-directed immobilisation of glycomimetics for glycoarrays application: comparison with covalent immobilisation, and development of an on-chip IC50 measurement assay

Glycoarrays are powerful tools for the understanding of protein/carbohydrate interactions and should find applications in the diagnosis of diseases involving these interactions. Immobilisation of the carbohydrate probe is a key issue in the elaboration of high performance devices. In the present study, we have compared the fluorescent signal intensity and determined the lower detection limit of glycoconjugates immobilised at two concentrations (0.5 and 25 microM) by DNA-directed immobilisation (DDI), to glycoconjugates covalently immobilised on the solid support (borosilicate glass slide). At 0.5 microM, DDI led to a stronger fluorescence signal (by a factor of 4.5) and to a lower detection limit (20 nM) than covalent immobilisation (higher than 200 nM). We also report the development of an IC(50) measurement assay of DDI immobilised glycoconjugates. We found that the relative affinity per galactose residue of RCA 120 for glycoconjugates bearing one or three galactose residues was different by a factor of 23 when measured under IC(50) conditions or by direct fluorescence reading.

Photo-Generation of Carbohydrate Microarrays

The unparalleled structural diversity of carbohydrates among biological molecules has been recognized for decades. Recent studies have highlighted carbohydrate signaling roles in many important biological processes, such as fertilization, embryonic development, cell differentiation and cellȁ4cell communication, blood coagulation, inflammation, chemotaxis, as well as host recognition and immune responses to microbial pathogens. In this chapter, we summarize recent progress in the establishment of carbohydrate-based microarrays and the application of these technologies in exploring the biological information content in carbohydrates. A newly established photochemical platform of carbohydrate microarrays serves as a model for a focused discussion.

Glycoarrays--tools for determining protein-carbohydrate interactions and glycoenzyme specificity

Carbohydrate arrays (glycoarrays) have recently emerged as a high-throughput tool for studying carbohydrate-binding proteins and carbohydrate-processing enzymes. A number of sophisticated array platforms that allow for qualitative and quantitative analysis of carbohydrate binding and modification on the array surface have been developed, including analysis by fluorescence spectroscopy, mass spectrometry and surface plasmon resonance spectroscopy. These platforms, together with examples of biologically-relevant applications are reviewed in this Feature Article.

Surface plasmon resonance study of protein-carbohydrate interactions using biotinylated sialosides

Lectins are carbohydrate binding proteins found in plants, animals, and microorganisms. They serve as important models for understanding protein-carbohydrate interactions at the molecular level. We report here the fabrication of a novel sensing interface of biotinylated sialosides to probe lectin-carbohydrate interactions using surface plasmon resonance spectroscopy (SPR). The attachment of carbohydrates to the surface using biotin-NeutrAvidin interactions and the implementation of an inert hydrophilic hexaethylene glycol spacer (HEG) between the biotin and the carbohydrate result in a well-defined interface, enabling desired orientational flexibility and enhanced access of binding partners. The specificity and sensitivity of lectin binding were characterized using Sambucus nigra agglutinin (SNA) and other lectins including Maackia amurensis lectin (MAL), concanavalin A (Con A), and wheat germ agglutinin (WGA). The results indicate that alpha2,6-linked sialosides exhibit high binding affinity to SNA, while alteration in sialyl linkage and terminal sialic acid structure compromises the affinity by a varied degree. Quantitative analysis yields an equilibrium dissociation constant (KD) of 777 +/- 93 nM for SNA binding to Neu5Ac alpha2,6-LHEB. Transient SPR kinetics confirms the K D value from the equilibrium binding studies. A linear relationship was obtained in the 10-100 microg/mL range with limit of detection of approximately 50 nM. Weak interactions with MAL, Con A, and WGA were also quantified. The control experiment with bovine serum albumin indicates that nonspecific interaction on this surface is insignificant over the concentration range studied. Multiple experiments can be performed on the same substrate using a glycine stripping buffer, which selectively regenerates the surface without damaging the sialoside or the biotin-NeutrAvidin interface. This surface design retains a high degree of native affinity for the carbohydrate motifs, allowing distinction of sialyl linkages and investigation pertaining to the effect of functional group on binding efficiency. It could be easily modified to identify and quantify binding patterns of any low-affinity biologically relevant systems, opening new avenues for probing carbohydrate-protein interactions in real time.

Characterization of secondary and tertiary conformational changes of beta-lactoglobulin adsorbed on silica nanoparticle surfaces

Nanoparticles possess unique properties as a result of their large surface area per unit volume and therefore can be functionalized by the immobilization of enzymes for a variety of biosensing applications. Changes in the tertiary conformation of beta-lactoglobulin adsorbed on 90 nm silica nanoparticles with time were inferred using tryptophan fluorescence and Fourier transform infrared spectroscopy (FTIR) for different surface concentrations, temperature, pH, ionic strength, and 2,2,2-trifluoroethanol (TFE) and dithiothreitol (DTT) concentrations. Rapid initial unfolding followed by a much slower rate at longer times was observed, with the extent of unfolding being higher at lower surface concentrations, higher ionic strengths, higher temperature, higher TFE and DTT concentrations, and pI. The effect of temperature on the unfolding of adsorbed protein on the nanoparticle surface was similar to that in the bulk even though the extent of unfolding was higher for adsorbed protein molecules. The results of the extent of change in tertiary conformation using FTIR as indicated by the change in the ratio of amide II'/amide I were consistent with those obtained by tryptophan fluorescence whereas the rates of conformational changes given by FTIR were found to be much faster. Circular dichroism (CD) spectra showed that altering the surface concentration by itself did not change the secondary structure of beta-lactoglobulin on the surface. TFE was found to increase the alpha helix content at the expense of the fraction of the beta sheet, whereas the beta sheet was converted to an unordered conformation in the presence of DTT.

Sugar Chips immobilized with synthetic sulfated disaccharides of heparin/heparan sulfate partial structure

Carbohydrate chip technology has a great potential for the high-throughput evaluation of carbohydrate-protein interactions. Herein, we report syntheses of novel sulfated oligosaccharides possessing heparin and heparan sulfate partial disaccharide structures, their immobilization on gold-coated chips to prepare array-type Sugar Chips, and evaluation of binding potencies of proteins by surface plasmon resonance (SPR) imaging technology. Sulfated oligosaccharides were efficiently synthesized from glucosamine and uronic acid moieties. Synthesized sulfated oligosaccharides were then easily immobilized on gold-coated chips using previously reported methods. The effectiveness of this analytical method was confirmed in binding experiments between the chips and heparin binding proteins, fibronectin and recombinant human von Willebrand factor A1 domain (rh-vWf-A1), where specific partial structures of heparin or heparan sulfate responsible for binding were identified.

Glyco-array technology for efficient monitoring of plant cell wall glycosyltransferase activities

The plant cell wall is a complex network of polysaccharides. The diversity in the linkage types connecting all monosaccharides within these polysaccharides would need a large set of glycosyltransferases to catalyze their formation. Development of a methodology that would allow monitoring of glycosyltransferase activities in an easy and high-throughput manner would help assign biochemical functions, and understand their roles in building this complex network. A microarray-based method was optimized for testing glycosyltransferases involved in plant wall biosynthesis using an alpha(1,2)fucosyltransferase involved in xyloglucan biosynthesis. The method is simple, sensitive, and easy to implement in any lab. Tamarind xyloglucan polymer and trimer, and a series of cello-oligosaccharides were immobilized on a thin-coated photo-activable glass slide. The slide with the attached sugars was then used to estimate the incorporation of [(14)C]Fuc onto xyloglucan polymer and trimer. [(14)C]-radiolabel incorporation is revealed with a standard phosphoimager scanner, after exposure of the glycochip to a phosphor screen and detection. The method proved to be sensitive enough to detect as low as 45 cpm/spot. Oriented anchoring of small oligosaccharides (trimer) was required for optimal transferase activities. The glycochip was also used to monitor and estimate xyloglucan fucosyltransferase activity in detergent-solubilized crude extracts from pea microsomes that are known to contain this enzyme activity. Our data indicate that the methodology can be used for efficient and rapid monitoring of glycosyltransferase activities involved in plant wall polysaccharides biosynthesis.

Photons to illuminate the universe of sugar diversity through bioarrays

In this mini-review, we summarize the photochemical approaches for developing high-throughput carbohydrate microarray technologies. Newly established methods for photo-immobilizing unmodified monosaccharides, oligosaccharides and polysaccharides onto photoactive surfaces and coupling of photoactive carbohydrates onto polymer surfaces are reviewed.

Development and application of diazirines in biological and synthetic macromolecular systems

Many different reagents and methodologies have been utilised for the modification of synthetic and biological macromolecular systems. In addition, an area of intense research at present is the construction of hybrid biosynthetic polymers, comprised of biologically active species immobilised or complexed with synthetic polymers. One of the most useful and widely applicable techniques available for functionalisation of macromolecular systems involves indiscriminate carbene insertion processes. The highly reactive and non-specific nature of carbenes has enabled a multitude of macromolecular structures to be functionalised without the need for specialised reagents or additives. The use of diazirines as stable carbene precursors has increased dramatically over the past twenty years and these reagents are fast becoming the most popular photophors for photoaffinity labelling and biological applications in which covalent modification of macromolecular structures is the basis to understanding structure-activity relationships. This review reports the synthesis and application of a diverse range of diazirines in macromolecular systems.

RGD modified polymers: biomaterials for stimulated cell adhesion and beyond

Since RGD peptides (R: arginine; G: glycine; D: aspartic acid) have been found to promote cell adhesion in 1984 (Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule, Nature 309 (1984) 30), numerous materials have been RGD functionalized for academic studies or medical applications. This review gives an overview of RGD modified polymers, that have been used for cell adhesion, and provides information about technical aspects of RGD immobilization on polymers. The impacts of RGD peptide surface density, spatial arrangement as well as integrin affinity and selectivity on cell responses like adhesion and migration are discussed.