Dextran hydroxyl functionalities

The hydroxylic content of the dextran sugar backbone makes the polymer very hydrophilic and easily modified for coupling to other molecules. Unlike PEG, discussed previously, which has modifiable groups only at the ends of each linear polymer, the hydroxyl functional groups of dextran are present on each monomer in the chain. The monomers contain at least 3 hydroxyls (4 on the terminal units) that may undergo derivatization reactions. This multivalent nature of dextran allows molecules to be attached at numerous sites along the polymer chain. 

Modification of dextran polymers with 1,4-butanediol diglycidyl ether results in ether derivatives of the dextran hydroxyl groups, which then contain hydrophilic spacers with terminal epoxy functions (Figure 25.16). 

These reagents have a number of drawbacks. First of all, they are toxic especially via contact with skin. The LD50 (dermal, rat) of DCC is 71 mg kg. This should always be considered if the reaction is used for the preparation of materials for biological applications. Moreover, the N.N -dialkylurea formed during the reaction is hard to remove from the polymer except for preparation in DMF and DMSO, where it can be filtered off. In case of esterification of polysaccharides in DMSO in the presence of these reagents, oxidation of hydroxyl functions may occur due to a Moffatt type reaction (Fig. 25, [188]). The oxidation products formed can be detected with the aid of 2,4-dinitrophenylhydrazine, e.g. in case of the conversion of dextran with DCC in DMSO [189],... 

An example of the use of 1,4-butanediol diglycidyl ether for the activation of soluble dex-tran polymers is given in Chapter 25, Section 2.3. One end of the fezs-epoxide reacts with the hydroxylic sugar residues of dextran to form ether linkages, which terminate in epoxy functionalities. The epoxides of the activated derivative then can be used to couple additional mol-ecules-containing nucleophilic groups to the dextran backbone. 

Figure 25.14 An amine derivative of dextran may be prepared through a two-step process involving the reac-tion of chloroacetic acid with the hydroxyl groups of the polymer to create carboxylates. Next, ethylene diamine is coupled in excess using a carbodiimide-mediated reaction to give the primary amine functional groups.

Figure 25.16 An epoxy-functional dextran derivative may be prepared by the reaction of 1,4-butanediol diglycidyl ether with the hydroxyl groups of the polymer.

Figure 2. Degree of substitution (DS) of hydroxyl groups as a function of reaction time for the reaction of dextran with 2-chloroethylamine (80°C).

The freshly cleaned crystal is immersed in an unstirred 1 mM ethanolic solution of 11-mercaptoundecanol at room temperature, in the dark, for 48 h. The solution of 11-mercaptoundecanol is freshly prepared before use (2 mg of the thiol in lOmL of ethanol). The crystal is then washed with ethanol and milliQ water and sonicated for 10 min in ethanol to remove the excess of thiol. The hydroxylic surface is treated with a 600 mM solution of epichlorohydrin in a 1 1 mixture of 400 mM NaOH and bis-2-methoxyethyl ether (diglyme) for 4h. After washing with water and ethanol, the crystal is immersed for 20h in a basic dextran solution (3g of dextran in lOmL of NaOH lOOmM). The surface is further functionalized with a carboxymethyl group using bromoacetic acid (1M solution in 2M NaOH for 16h). All the reactions are performed at room temperature. The coated crystals can be stored at 4°C immersed in milliQ water for 15 days. For their use, the crystals are washed with water and placed in the cell. 

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