Strategies For The Isolation And Purification Of Biochemicals Using Ion Exchange And Adsorbent Resins

A wide variety of commercial products are being manufactured today through a fermentation process. These range from common commodity products such as animal feed-grade amino acids, food and chemical grades of organic acids, to higher valued products such as vitamins, pharmaceuticals, fine chemicals, and flavor components for foods. One of the challenges of the manufacturing of products made by a fermentation process is the recovery, isolation and purification of the target product from a fermentation broth. In fact, the potential commercial viability of many products as produced from a fermentation or biochemical process can depend upon the processing costs associated with recovery and purification. Synthetic resins can offer a wide range of possibilities for dealing with separation problem's (1).

Ion exchange and adsorbent resins are quite versatile tools; although they will not solve all purification needs they do offer an array of alternatives for solving separation problems. There are a wide number of tools to choose from which vary by functional group, ionic group, base polymer, degrees of cross-linking, particle size and bead porosity. In looking at the commercial offerings for styrene-divinylbenzene and acrylic based resin products there are more than 1000 resin tools available in the market place today. Examples of such resin beads can be seen Figure 1, which compares a uniformed particle size strong acid cation, chromatographic separation resin with a Fine Mesh strong base anion exchange resin. Table 1 summarizes some of the resin tools available for screening against a complex matrix application; a more complete listing can be found in a number of specialized text books including, "Separation and Purification Techniques in Biotechnology", F.J. Dechow, Noyes, 1989 and "Ion Exchange" K. Dorfner, Walter de Gruyter, 1991.

With so many different tools, and since no two products produced through a fermentation process yield the same exact chemistries, each purification problem has unique separation challenges. However, there are some common themes and goals that can be considered when attacking a biotech application. One of the first steps is to define the overall purification need and then divide the purification work into a series of steps that can be handled as more controllable unit operations. These generally fall into three categories: matrix reduction, process purification and final purification.

A matrix reduction is the earliest of the purification steps with the goal of reducing the bulk of the unwanted chemical or background materials from a process stream while pooling and concentrating the desired target solute. The overall goal of a matrix reduction step is to concentrate the "goodie" while eliminating a portion of the background matrix material(s). The scheme in Figure 2, outlines ways of attacking a matrix reduction problem. Basically, if there is enough understood about the chemistry of target solute and matrix materials, such as the functional group chemistries or chemical conditions under which ion exchange or adsorption conditions, where one fraction would be favored over another then one resin type could be screened over another. This could be a known anionic or cationic functional group that is ionized and retained by resin or a key functional group that could be manipulated by pH such to favor an ion exchange or adsorption. Or if the matrix is ill defined or very diverse, possible purification tool(s) can be screened via an isotherm test screening with a number of different resins. To apply such a screen generally a selection of five different type of resins ; weak base anion, strong base anion, weak acid cation, strong acid cation and an adsorbent resin is a good place to start. By focusing upon what is eluted and what is retained, by a given class of resin and then selecting which yields the best results. The key to a successful matrix reduction step is to think in-terms of process pools, a successful step at this point should yield good recovery of the target solute in one pool, while concentrating it 5-20 fold and reducing 20-50% of unwanted matrix components. It may be critical to target components that have limited chemical stability at this point.