Biomedical Engineering Reference
In-Depth Information
available for adsorption of another protein molecule, the adsorbed pro-
tein mass is likely to be inversely proportional to the hydrophobicity of an
adsorbed protein, which is indeed observed experimentally. In contrast, the
presence of charged groups and the charge distribution of the protein are
closely related to the overall hydrophobicity of the protein. Knowing the
presence and location of charged groups and their distribution on the pro-
tein defi nitely offers some clues to its adsorption behavior. The infl uence of
electrostatic interaction can even be outweighed by the hydrophobic dehy-
dration effect.
The folded state of a protein is only marginally stable as numerous exper-
imental results suggest that proteins undergo some kind of structural rear-
rangement upon adsorption at the solid/liquid interface, regardless of the
net charge sign of the surface. The less stable the protein structure is in its
native state, the more likely the adsorbed protein is to proceed to highly
denatured states. This leads to the classifi cation of so-called 'soft' proteins,
which generally favor adsorption on a surface, as opposed to 'rigid' proteins.
The time-dependent changes in the content of secondary structure such as
α-helix and β-sheet can be used as indicators of the extent a particular pro-
tein would adsorb on a surface.
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Structural rearrangement is at a mini-
mum when the adsorption occurs at the pI of the system.
Other global properties of proteins that infl uence the adsorption behav-
ior include the effective surface area of a protein and protein concentration
in the bulk solution. The former information is useful for quantifying pro-
tein adsorption. Unfortunately, the exact value may never be known. For
the latter, it has been found that the less concentrated the bulk protein solu-
tion, the higher the degree of protein unfolding. This suggests that available
adsorbent surface area and the nature of protein-protein interactions are
important in various aspects of protein adsorption.
6.3.2 Properties of the adsorbent
The availability of surface area, the homogeneity of the adsorption sites
and the contact area between the protein and the adsorbent are important
information for quantifying protein adsorption. Moreover, in contrast to the
effect of protein hydrophobicity, protein adsorption is shown to be maximal
when the adsorbent possesses intermediate polarity.
6.3.3 Effects of adsorbent and protein charge
If both protein and adsorbent surfaces are hydrophilic, the extent of hydra-
tion on both protein and adsorbent surfaces is believed to undergo small
changes following protein adsorption. This is because water molecules
