Dialysis is a crucial step in protein purification, particularly following the salting out process, which often leaves a high concentration of salt in the protein solution. Excessive salt can lead to the loss of protein activity or even denaturation, making it essential to remove these salts to protect the target protein.
The dialysis process utilizes a semipermeable membrane, which allows certain molecules to pass while blocking others. In this case, a dialysis bag containing the protein solution is submerged in a low salt solution. The larger protein molecules, which cannot fit through the membrane's pores, remain inside the bag, while smaller salt molecules diffuse out into the surrounding solution. This selective diffusion effectively reduces the salt concentration within the bag, thereby minimizing the risk of protein denaturation.
To illustrate, consider the dialysis bag filled with a protein solution that initially has a high salt concentration. As dialysis progresses, the smaller salt molecules exit the bag through the membrane's pores, while the larger protein molecules are retained. This results in a significant decrease in salt concentration inside the bag, allowing the proteins to remain stable and active.
The size of the pores in the dialysis membrane is approximately 24 angstroms, while the proteins can be around 160 angstroms in size. This size difference is critical, as it ensures that proteins cannot pass through the membrane, thus maintaining their integrity during the purification process.
Ultimately, dialysis serves to purify proteins from salts and small molecules, but it does not separate different proteins from one another. Therefore, further purification techniques will be necessary to isolate the target protein of interest in subsequent steps.
