Protein dextran polymers

The following sections describe the major activation and coupling methods used with dextran polymers. The reactive derivatives may be used to couple with proteins and other molecules containing the appropriate functional groups. 

Proteins may be modified with oxidized dextran polymers under mild conditions using sodium cyanoborohydride as the reducing agent. The reaction proceeds primarily through e-amino groups of lysine located at the surface of the protein molecules. The optimal pH for the reductive amination reaction is an alkaline environment between pH 7 and 10. The rate of reaction is greatest at pH 8-9 (Kobayashi and Ichishima, 1991), reflecting the efficiency of Schiff base formation at this pH. 

Add 10 mg of the protein to be coupled to the dextran solution. Other ratios of dextran-to-protein may be used as appropriate. For instance, if more than one protein or a protein plus a smaller molecule are both to be conjugated to the dextran backbone, the amount of protein added initially may have to be scaled back to allow the second molecule to be coupled latter. Many times, a small molecule such as a drug will be coupled to the dextran polymer first, and then a targeting protein such as an antibody conjugated secondarily. The optimal ratio of components forming the dextran conjugate should be determined experimentally to obtain the best combination possible. 

Fig. 7. Separation of SDS protein complexes with an exchangeable dextran polymer solution. Separation conditions L=30-37 cm, =300 V cnr1, T= 20 °C (Beckman SDS protein kit) detection 214 nm analytes Orange G (M),carboanhydrase (1),ovalbumin (2),bovine serum albumin (3), phosphorylase B (4), (3-galactosidase (5), myosin (6)...

Shibusawa, Y. Ito, Y. Countercurrent chromatography of proteins with polyethylene glycol-dextran polymer phase systems using type-XLLL cross-axis coil planet centrifuge. J. Liq. Chromatogr. 1992, 15, 2787. 

Although PEG-phosphate systems yield a high-efficiency separation, some proteins show a low solubility due to a high salt concentration in the solvent system. In this case, the PEG-dextran polymer-phase system with a low salt concentration can be alternatively used for the separation of such proteins. Because the dex-tran-PEG system has a high viscosity and an extremely low interfacial tension, it tends to cause emulsification and loss of the stationary phase in the XLL or XL cross-axis CPCs. This problem is minimized using the XLLL cross-axis CPC, which provides a strong lateral centrifugal force to provide a more stable retention of the stationary phase. 

Figure 4.6 Hydrodynamic radius for globular proteins, dextran and DNA polymers. Diffusion coefficients were obtained as described in Table 4.2. The hydrodynamic radius was calculated for proteins (squares), DNA (circles), and dextran (triangles).

Polysaccharides are among the most versatile polymers because of their vast structural diversity and nontoxicity. Among polysaccharides, chitosan, alginate, pectin, hylauronic acid, and dextran have received much attention. Protein-based polymers such as albumin, casein, and gelatin have also been investigated for oral peptide delivery. 

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