Biomedical Engineering Reference
In-Depth Information
are likely to be found between the adsorbent and the adsorbed protein.
However, if either one or both surfaces are hydrophobic, hydrophobic dehy-
dration between these two surfaces becomes a strong driving force for the
adsorption process.
Experimental results indicate that global electrostatic interaction plays
an important role in protein adsorption when the adsorbed protein and
the adsorbent carry opposite charges. However, even when they carry the
same charge, adsorption is still observed, suggesting that global electrostatic
interaction is not the most dominant force behind adsorption. Moreover,
in many protein adsorption systems, regardless of the surface charge sign,
the adsorbed protein mass displays a bell-shaped curve with respect to the
change in pH. This further supports the argument that global electrostatic
interaction is only one of the many possible factors that infl uence protein
adsorption. pH also has an effect on the structural stability of a native pro-
tein. pH values that differ signifi cantly from the protein's pI are likely to
induce certain structural rearrangements rendering the protein more prone
to unfold or denature upon adsorption.
Experimental measurements imply that protein adsorption is at max-
imum when the pH is at the pI of the protein/adsorbent system. This is
when the charges on both protein and adsorbent compensate each other
exactly. This means that electrostatic interaction is involved in the process.
During adsorption, the electrical double layers of both the protein and the
adsorbent overlap. The adsorption process involves protein-surface, pro-
tein-media and surface-media interactions. The electrostatic interaction is
accompanied by the redistribution of charged groups at the protein/adsor-
bent interface. Theoretical calculations imply that proteins may have a pre-
ferred conformation for adsorption due to the dipole moment formed by
the charged groups.
6.3.4 Aqueous media
Dielectric constant of water is an important parameter affecting protein
adsorption due to its infl uence on intermolecular forces. For instance, the
strength of the electrical double layer is affected by the value of dielec-
tric constant. The frequency response of dielectric constant proves impor-
tant in determining the relative strength of van der Waals and electrostatic
forces. Moreover, the role of water molecules in hydrophobic interactions
and hydrogen bond formation are among other factors that need to be con-
sidered in protein adsorption, which is supported by a study showing that
the structure of the water molecules constituting the surrounding hydra-
tion layer of a protein is a decisive factor with regard to the conformational
states (native vs. unfolded).
27
