Affinity separations are popular methods for the purification of biological molecules and other biological entities. They can readily be implemented on the laboratory scale but a number of additional factors have to be considered when these techniques are to be used for production purposes. Under these circumstances it is necessary to apply biochemical engineering principles to the design, scale-up and optimization of affinity separations. These topics are the subject of this article.

Selective interactions are exploited in affinity separations in order to achieve greater adsorbent selectivity for the desired molecule. Subtle differences in physical properties such as charge, size and hydro-phobicity are often found to be insufficient for the required degree of purification in many separations of biological compounds. Many separations require the isolation of a minority component from a highly complex feedstock which may contain large amounts of similar compounds. As a consequence, it has been necessary to devise recovery flow sheets that consist of an extensive sequence of different steps - a sequence that may result in low overall yields and excessive costs. Hence affinity separations have been developed as alternatives to the more widely used separations based on ion exchange, hydrophobic interaction and size exclusion methods. Provided

Tjerneld F, Johansson G and Joelsson M (1987) Affinity liquid-liquid extraction of lactate dehydrogenase on a large scale. Biotechnology and Bioengineering 30: 809-816.

Walter H and Johansson G (eds) (1974) Methods in En-zymology, Vol. 228, Aqueous Two-phase Systems. San Diego, CA: Academic Press.

a ligand can be obtained which is truly selective for the desired component, it is possible to recover that component from a complex feedstock to a high degree of purity and in high yield. Typically the ligand is used in heterogeneous phase separations in which it is immobilized on to the surfaces of a porous solidphase matrix material and employed in chromato-graphic and other adsorption techniques. Other approaches including the use of affinity ligands in selective precipitation and in modifying the phase selectivities in aqueous two-phase separations (ATPS) have been reported, but are not considered further here.

A variety of ligands with a wide range of molecular complexities have been developed for use in affinity separations and these are reviewed extensively elsewhere in this work. In many examples, duplication of the selective interactions that occur during the normal function of biomolecules have been exploited during such affinity separations; the affinity ligand is frequently one of the components of a recognition interaction. Examples include the recognition between an enzyme and its inhibitor or co-factor, or the highly specific interaction between an antigen and an antibody raised against it. Biomimetic molecules have been developed to mimic the recognition sites of more complex molecules, either by exploiting fortuitous interactions shown by readily available compounds (e.g. textile dyes) or as a result of the identification of new compounds either by studying the detailed three-dimensional structure of the target, or by the techniques of combinatorial synthesis. Selective molecular recognition can also be achieved without mimicking any naturally occurring

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Solar Panel Basics

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