Biomedical Engineering Reference
In-Depth Information
from the bulk, electrostatic interaction is the main intermolecular interac-
tion. Additional factors that can shift the electrostatic force balance are pH
and ionic strength, which can also affect the electrostatic properties of the
protein. For instance, the degree of protonation of the dissociation groups is
dependent on the pH, which can induce conformational changes to proteins
both in solution and on the surface to a variable extent. Experimental evi-
dence also suggests that the charged surface affects the pKa of amino acid
side-chains and hence the electrostatic properties of the adsorbed proteins.
Co-adsorption of low molecular weight ions occurs during protein adsorp-
tion on charged surfaces. The surface charges of the adsorbent are likely
to be incompletely neutralized by the net charge of the adsorbed protein,
even if they carry different sign of overall charges. Co- and counter-ions are
almost always found in the protein/adsorbent region. Even when both the
protein and adsorbent carry the same sign of overall charges, adsorption
is still observed as co-adsorption of low molecular weight ions effectively
neutralizes the opposing repulsion between like charges. Within the protein/
adsorbent regions, the dielectric constant is relatively low and a small net
charge can give rise to a large electrostatic potential.
The creation of a net electrostatic potential at or near a charged surface
and the adsorption of co- and counter-ions from the bulk solution form the
basis of the double electrical layer. In a one-dimensional case, the surface
electrostatic potential decreases non-linearly with respect to distance mea-
sured from the surface. In comparison, the electrostatic potential of the
surface with a layer of adsorbed proteins can show a sharp decrease with
respect to distance from the surface. Its actual value and sign are mainly
dependent on the charge distribution of the protein/adsorbent system and
dielectric constant of the electrical double layers.
6.3 Factors affecting protein adsorption
Although no single factor is dominant for protein adsorption at solid/liquid
interface, several factors involved in this process can be outlined, and these
are discussed in the following sections.
5,6,20-24
6.3.1 Proteins and their properties in aqueous media
The number of non-charged groups and hence the surface hydrophobic-
ity, in addition to the hydrophobic core, is a major factor in determining
the extent of protein adsorption, especially for a hydrophobic surface.
Structural rearrangement of adsorbed protein accompanies hydrophobic
dehydration to some extent as apolar domains unfold and adsorb to the
surface. Since this results in protein spreading and a reduction of the space
