Biomedical Engineering Reference
In-Depth Information
Fig. 1 Hydrophilic or hydrophobic in vitro surfaces in contact with a biological environment.
a Protein adsorption, b cell adhesion (
water molecules,
polar amino acid side chains)
state and their orientation will be different [ 2 ]. Surface wettability is regarded
as one of the most important surface parameters governing protein adsorption [ 3 ].
A common observation is that hydrophilic and hydrophobic surfaces bind proteins
differently, i.e. proteins may adsorb intact or may undergo unfolding to minimize
the free energy of the system (Fig. 1 a). Water-soluble proteins in a physiological
environment commonly show a globular shape with a hydrophobic core and
hydrophilic and charged amino acid side chains exposed to the solution. Thus, on
hydrophilic surfaces protein adsorption occurs through polar and ionic interactions
(Fig. 1 a, left panel). No conformational changes are induced and the proteins bind
in their native conformation with intact water shells [ 4 ]. This leads to a rather
weak, mostly reversible protein adsorption.
On hydrophobic surfaces the proteins are often irreversibly bound due to
dehydration of the interface and the associated absence of intervening water shells.
Dehydration of both the substrate and the protein surface provides an entropic
driving force for the adsorption on hydrophobic surfaces. This leads inevitably to a
significant rearrangement of the protein conformation with partial or total
unfolding. The hydrophobic amino acids of the protein core are exposed to the
substrate surface to allow for hydrophobic interactions with the surface [ 5 ]. Most
of the polar and charged amino acid residues are oriented towards the aqueous
solution (Fig. 1 a, right panel). The degree of the surface-induced conformational
change mirrors the balance between the strength of protein-surface interactions
and the internal conformational stability of the protein [ 6 ]. The adsorbed protein
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