The Spacer Arm

The various groups that are attached to the sold matrix constituting the chain (which is finally linked to the ligand) is termed as the spacer arm. Certain features of the spacer arm are highly critical these include:

1. The length of the chain
2. The nature of the atoms that constitute the spacer arm.
3. The hydrophilic nature of the chain.

Even a minor change in any one of the above features will eventually prevent the binding of the component being purified to the ligand. The following example illustrates the point.

1. The ligand p-aminophenyl-beta-thiogalactopyranoside has affinity to the active site of the enzyme bacterial beta-galactosidase.
2. This ligand is bound to the solid support in structure A, no separation of the enzyme is possible because there is no binding between the ligand and the enzyme galactosidase. The ligand is too close to the solid support sterically it is very difficult for the ligand to reach the active site and bind.
3. Some separation was observed by placing the ligand at a moderate distance of 10 A, as in structure B.
4. A distance of 20 A as in structure C was found to be most effective.

It is also necessary that the spacer arm is made of polar groups as well (with electronegative atoms). The chain should not be completely hydrophobic.