Another mechanism by which proteins may form aggregates involves chemical bonding between individual protein molecules. The oxidation of thiol groups (-SH) located on two protein molecules can be described by the following equation:

2R - SH -------- R-S-S-R + 2H⁺ + 2e^-

Since protein molecules often have several thiol groups which may react, aggregates may be formed in this manner.

Proteins may also interact by thiol-disulfide interaction, which is described by the following equation:

R₁S^- + R₂S-SR₃-------- R₁S-SR₃ + R₂S^-

Again, since protein molecules often have several thiol groups and several disulfide bonds which may react, aggregates may also be formed in this manner.

The tendency of proteins to form aggregates becomes important when one tries to optimize the filtration of protein-containing solutions. Most of the commonly used methods to counteract membrane fouling during the filtration of solutions which do not contain proteins do not work on protein-containing solutions.