Improved procedures for the conjugation of oligosaccharides to protein by reductive amination

The rate of coupling of oligosaccharides having aldose end groups to protein by reductive amination was significantly increased by changing the temperature and pH of the reaction, and even more significantly by the addition of borate ions. Under optimized conditions half of the lysine residues of bovine serum albumin could be derivatized by lactose in 7 h and their complete derivatization was achieved in approximately 24 h. All attempts to carry out similar reductive amination procedures using oligosaccharides having ketose (D-fructose, 3-deoxy-D-manno-octulosonic acid (KDO), and sialic acid) failed owing to the slowness of the reaction. Model studies on the coupling of D-fructose and KDO to glycine indicate that any coupling procedure based on reductive amination of ketose residues would of necessity require the prior introduction of a small functionalized spacer molecule.