The first step in attempting to understand the mechanism by which proteins foul membranes which is different than that for other types of particles is to understand the structure of the protein molecule. A protein molecule is a linear chain of amino acids which is formed within a cell.
Proteins are generated within the cell by the action of
ribonucleic acid (RNA). The RNA
molecule dictates the particular sequence of the amino acids for
the protein which it
replicates. However, the structure of the protein molecule is
further defined due to
intermolecular forces between the amino acid molecules of the
protein. Proteins within a cell
are present in aqueous solutions. For this reason, hydrophobic
regions of the protein chain
tend to attract each other and subsequently orient themselves
within the center of the protein,
as depicted in the figure below:
This folding occurs due to a thermodynamic driving force to
reduce the hydrophobic surface
area of these regions These interactions are what give each
protein its specific three
dimensional shape, which in turn allows the protein to carry on
its specific functions within
the cell. The figure below depicts the structure of a typical
protein, myoglobulin.
Protein Aggregation
Proteins in solution have the ability to interact with one another. When proteins interact, they tend to mass together to form large particles called protein aggregates. Protein aggregation is similar to coagulation of colloidal particles in solution. It is this aggregation of proteins, the formation of particles much larger than individual protein molecules which is one of the reasons protein solutions tend to foul filtration membranes to an extent greater than non- protein containing solutions (Case C fouling). There appear to be several different mechanisms by which proteins aggregate. These include hydrophobic interactions and thiol oxidation / thiol disulfide interchange reactions .