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The interactions of a number of commercially available dextran preparations with the lectin Concanavalin A (ConA) have been investigated. Dextrans over the molecular mass range 6 × 10³-2 × 10⁶ g mol⁻¹ were initially characterised in terms of their branching and hence terminal ligand density, using NMR. This showed a range of branching ratios between 3% and 5%, but no clear correlation with molecular mass. The bio-specific interaction of these materials with ConA was investigated using microcalorimetry. The data obtained were interpreted using a number of possible binding models reflecting the known structure of both dextran and the lectin. The results of this analysis suggest that the interaction is most appropriately described in terms of a two-site model. This offers the best compromise for the observed relationship between data and model predictions and the number of parameters used based on the chi-squared values obtained from a nonlinear least-squares fitting procedure. A two-site model is also supported by analysis of the respective sizes of the dextrans and the ConA tetramer. Using this model, the relationship between association constants, binding energy and molecular mass was determined., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Dextran solutions intended for use as plasma extenders have been observed to form insoluble precipitates. Earlier studies of precipitation have shown that in solutions of 50% and 60% w/w of dextran molecular mass 6000 g mol(-1) beaded precipitates are formed over a two-week period. This study considers dextran precipitation over a wider molecular mass range and the kinetics, of formation, morphology and potential utility of these precipitates is investigated. Results show precipitation occurs over the dextran molecular mass range 6000-17,000 g mol(-1), with lower molecular mass material showing more rapid precipitation. As bead formation is accompanied by an increase in turbidity, formation kinetics were quantified spectrophotometrically confirming that precipitation rates were inversely proportional to molecular mass. The utility of these precipitates for drug delivery applications was assessed using bovine serum albumin as a protein drug analogue. The results showed that the inclusion of protein did not prevent bead formation and that entrapped protein was subsequently released from dextran beads in a time dependant manner. This suggests that dextran beads of this type may find application in the drug delivery area, as they combine the advantages of mild entrapment conditions with the use of an unmodified clinically approved polymer.

A facile method for the characterization of hydrogel swelling is described which is based on the determination of changes in the liquid phase concentration of an excluded tracer as gel swells in a constant volume system. The utility of this approach is demonstrated with two responsive hydrogel preparations, one where swelling is influenced by system pH, the other by changes in specific solute concentration.

A fast and simple method for the preparation of pH-sensitive hydrogel membranes for drug delivery and tissue engineering applications has been developed using carbodiimide chemistry. The hydrogels were formed by the intermolecular cross-linking of carboxymethyl dextran (CM-dextran) using 1-ethyl-(3-3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). Infrared spectra of the hydrogels suggest the formation of ester bonds between the hydroxyl and carboxyl groups in the CM-dextran. The porosity of the hydrogels produced, as shown by protein diffusion, increases in response to changes in the pH and the ionic strength of the external medium. The results show pH-dependent swelling behaviour arising from the acidic pedant groups in the polymer network. The diffusion of the protein lysozyme through the hydrogel membranes increased with increases in both pH (5.0-9.0) and ionic strength. The effect of changes of pH and ionic strength on the hydrogel's permeability was shown to be reversible. Scanning electron microscopy of these hydrogels showed that pH-dependent changes in permeability are mirrored by morphological changes in gel structure.

Glucose-sensitive hydrogel membranes have been synthesized and characterized for their rate-of-delivery of macromolecules. The mechanism for changing this rate is based on variable displacement of the affinity interaction between dextran and concanavalin A (con A). Our main objective was to characterize the diffusion of model proteins (insulin, lysozyme, and BSA) through the membrane, in response to changes in environmental glucose concentrations. Membranes were constructed from crosslinked dextrans to which con A was coupled via a spacer arm. Changes in the porosity of the resulting hydrogel in the presence of glucose led to changes in the diffusion rate observed for a range of proteins. Gels of specified thickness were cast around to nylon gauze support (pore size, 0.1 mm) to improve mechanical strength. Diffusion of proteins through the gel membrane was determined using a twin-chamber diffusion cell with the concentrations being continuously monitored using a UV-spectrophotometer. Changes in the transport properties of the membranes in response to glucose were explored and it was found that, while 0.1M D-glucose caused a substantial, but saturateable, increase in the rates of diffusion of both insulin and lysozyme, controls using glycerol or L-glucose (0.1M) had no significant effect. Sequential addition and removal of external glucose in a stepwise manner showed that permeability changes were reversible. As expected, diffusion rates were inversely proportional to membrane thickness. A maximum increase in permeability was observed at pH 7.4 and at 37 degrees C. The results demonstrate that this hydrogel membrane functions as a smart material allowing control of solute delivery in response to specific changes in its external environment., (Copyright 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 82: 47-53, 2003.)

Soluble dextran-ATP complexes have been synthesized using a bifunctional oxirane as the coupling agent. The degree of coupling is time-dependent, allowing materials of varying coenzyme loadings to be produced very simply. Characterization studies have shown that at the maximum coenzyme loading obtained (34 molecules per complex) all coenzyme moieties were coenzymically active with hexokinase. The extent of coenzyme loading was shown to have a considerable influence on the values of Km and Vmax of the complex as a substrate for hexokinase. Enzyme activity was also found with acetate kinase and myokinase, and coenzyme recycling (ATP, ADP) was demonstrated in an ultrafiltration reactor.