Biomedical Engineering Reference
In-Depth Information
large charge transfer processes on the former surface whereas almost no
charge transfer is detected on the latter. A plausible explanation for all these
observations is that electrostatic repulsion between a cathodically polarized
surface and fi brinogen, in addition to the outward fl ow of ionic species, pre-
vents a high level of fi brinogen deposition.
6.12.3 Surface-dependent structure of fi brinogen under
aqueous conditions
AFM has been used to image fi brinogen on various surfaces like SAMs,
mica and graphite,
178,179,183
metals
184
as well as on polymeric materials with
a rough surface (e.g. polydimethylsiloxane) (PDMS).
185
In a recent study,
AFM is used to image adsorbed fi brinogen on three model surfaces, hydro-
phobic and positively charged SAM, and negatively charged mica, provid-
ing information on its three-dimensional structure.
179
Results show that hydrated fi brinogen structurally resembles the trinod-
ular model as it displays a three-globular domain-like conformation on
all three tested surfaces. However, due to the extent of the hydration, the
coiled-coil region is only marginally visible. Moreover, other proposed fea-
tures such as the αC domain are not directly visualized. Therefore, hydrated
fi brinogen is better modeled as three overlapping ellipsoids, which is sub-
stantially different from the anhydrous structure. Additionally, fi brinogen
appears to be quite fl exible. For instance, although many fi brinogen mole-
cules on mica display linear conformation, a small number of them are bent.
Overall, it appears that the degree of spreading of fi brinogen on different
surfaces decreases in the following order: hydrophobic > positively charged
> negatively charged. This observation is confi rmed by dimensional analy-
sis in which the molecular length and domain width increase, whereas the
domain height decreases. Consequently, the extent of unfolding and struc-
tural rearrangement is most pronounced on a hydrophobic surface. Also,
there would be less surface coverage on a hydrophobic surface than on a
hydrophilic one. In a separate study, time-dependent observation reveals
the formation of fi brinogen fi lm on hydrophobic graphite but not on hydro-
philic mica, implying the importance of protein-protein interaction during
the adsorption process.
183
These results are qualitatively consistent with
results obtained on phospholipids
99,106
and SAM surfaces
95,96
using differ-
ent surface analytical methods. In contrast, while the presence of fi brinogen
on PDMS cannot be distinguished from conventional topographical images
because the roughness of the substrate is at the same or higher order of
fi brinogen, the difference in viscoelastic properties allows the detection of
fi brinogen from the PDMS background.
185
