Polysaccharides reductive amination

In some instances, reducing sugars are present that can be reductively aminated without prior periodate treatment. A reducing end of a monosaccharide, a disaccharide, or a polysaccharide chain may be coupled to a diamine by reductive amination to yield an aminoalkyl derivative bound by a secondary amine linkage (Figure 1.96). Also see Section 4.6, this chapter, for an extensive discussion on carbohydrate modification techniques. 

A second method of immunotoxin preparation by reductive amination involves the use a polysaccharide spacer. Soluble dextran may be oxidized with periodate to form a multifunctional crosslinking polymer. Reaction with antibodies and cytotoxic molecules in the presence of a reducing agent forms multivalent immunotoxin conjugates. The following sections discuss these options. 

Figure 23.5 Oxidation of the polysaccharide components of HRP produces reactive aldehyde groups. Conjugation to avidin then may be done by reductive amination.

Polysaccharides can be derivatized by reductive amination at the reducing end group. With highly fluorescing labels like 2-aminoanthracene (2-AA), dex-trans up to a molecular mass of 20 kDa can be visualized and hence analyzed by CE [35]. The separation of the AA derivative (only one chromophore introduced) of a synthetic dextran (4-6 kDa) is shown in Fig. 15. This opens new possibilities in the characterization of polysaccharides. 

Mishnick and co-workers have developed several techniques for determining the positions of substitutions on monosaccharide units and distribution of substituents along deriva-tized polysaccharide chains, primarily cellulose derivatives. Examples are found in references [61,62,63,64,65,66,67,68,69,70]. An important method involves perdeuteromethylation followed by partial acid-catalyzed hydrolysis, then reductive amination with propylamine, and finally permethylation to yield completely O- and A-alkylated charged oligosaccharides that are analyzed by MALDI-TOF-MS [68,70]. Statistical methods are then applied. 

Amination can be achieved by reductive amination of carboxyl groups using sodium cyano-borohydride [124]. By this method, 2-amino-2-deoxy functionalized amylose and cellulose have been produced. Reductive amination can also be applied to the synthesis of other amino derivatives of various oxidized polysaccharides such us xanthan [125] or alginic acid and galactomannans [126]. 

Incorporating aldehydic fiinctions in a polysaccharide can be accomplished by reacting the polysaccharide with chloroacetaldehyde dimethyl acetal. After unmasking of the acetal, the modified polysaccharide is reacted with proteins by reductive amination [33]. Since the initial reaction of chloroacetaldehyde dimethyl acetal with polysaccharide lacks site-specificity, this method suffers from randomness as well as multiplicity of the attachment site on the polysaccharide. 

---