Spacer Arms

The need for spacer arms was realized early on, since placing the ligand directly adjacent to the matrix might sterically hinder the interaction with the protein (Figure 3). But it has been shown that, as most spacer arms are hydrophobic, they can interact with other parts of the protein in a relatively nonspecific way. This has both advantages and disadvantages: the advantage, as described below, is that these nonspecific hydrophobic interactions can add to the binding strength of otherwise weak, though highly specific, interactions between a protein and its natural ligand. When inert, i.e. hydrophilic, spacer arms were introduced, many previously successful affinity methods did not work because the binding of the desired component was now too weak. The disadvantage of the use of spacer arms is that hydrophobic interactions with unwanted proteins may be so strong that these proteins may be bound as well.

Figure 3 Demonstration of the role of a spacer arm in keeping the bound protein away from the matrix backbone. Without the arm, it may be impossible for the protein to interact with the ligand, W.

Spacer arms can either be introduced as part of the activation process (see Table 1), or added afterwards in a multi-step process of creating the adsorbent. Sometimes it is beneficial to synthesize chemically a suitable spacer arm (ending with an amine) attached to the ligand itself, when the ligand does not have a suitable active nucleophile for direct coupling (Figure 2B).

Thus, spacer arms have two possible functions in a successful affinity adsorbent. First, they place the ligand away from the matrix to avoid physical interference from the matrix backbone. Second, they can provide some weak nonspecific interactions that increase the overall binding energy so that adsorption is more complete.

Solar Panel Basics

Solar Panel Basics

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