Biomedical Engineering Reference
In-Depth Information
2.1 Interaction between Surfaces and Biomolecules
If you were a biomolecule, you would ind that surfaces are very
sticky
, and that once stuck to
them, it would be very hard for you to release yourself. Also, the forces you would feel likely
would be so strong as to latten you against the surface, not unlike a hypergravity world. In
surface science language, the phenomenon in which a molecule adheres to a surface is called
adsorption
. If the adsorption is mediated purely by physical forces, it is termed
physisorption
.
If a chemical bond forms between the molecule and the surface, then we speak of
chemisorp-
tion
. hese interactions exert forces on the biomolecule, distorting its shape. Extreme shape
changes can result in loss of (some or all) bioactivity, a phenomenon known as
denaturation
.
(Denaturation can occur through all sorts of causes, not just by the proximity of a surface—for
example, the exposure to a solvent causes a protein to be denatured.)
Physisorption is mediated by four diferent forces or interactions, and in all of them, water
plays a crucial role:
Biomolecules are invariably polar, which means they have a dipole moment that is
attracted to the charges and dipoles present in the solid (whose surface we are consid-
ering). Dipoles always induce dipoles in other nearby molecules or atoms. he added
action of all the luctuating, dipole-induced dipole interactions is known as
van der
Waals force
, which is always attractive. he range of this force for two simple mole-
cules in a vacuum is only a few nanometers. he situation becomes extremely complex
in the presence of a solvent such as water, which must be considered a third player in
its own right.
Biomolecules are also invariably charged, and so are most surfaces in physiological
luids. he
electrostatic force
between a biomolecule and a surface depends on the ionic
concentrations present in the solution and on the surface functional groups. Unlike the
well-known Coulomb force between two charges in a vacuum, the electrostatic forces
in a physiological luid are hard to predict because they are extremely sensitive to very
small concentrations of ionic species (oten undetermined) in the solution, which par-
tially can shield the electrostatic ield felt by more distant ions. he forces associated
with the formation of complexes of ions and water are termed
hydration forces
.
A hydrogen atom can dissociate partially from a donor (electronegative) atom and be
shared with another electronegative atom, forming a
hydrogen bond
. When a highly
polar group such as -OH (abundant on glass surfaces and many cell culture polymer
substrates) is immersed in water, the hydrogen atom is so loosely bound to the polar
group that it oten prefers to altogether lose its electron (becoming a
proton
,
H
+
) and leave
the group -O
-
on the surface. he proton can then associate with either water (form-
ing the
hydronium ion
H
3
O
+
) or another polar group on the surface of a biomolecule
(through a hydrogen bond). his competition highlights the important role of water in
the interaction between biomolecules and surfaces. Needless to say, this process is highly
dynamic: protons actually hop from molecule to molecule millions of times a second.
As a result of the strong associations that water forms with polar groups, nonpolar
groups and water tend to repel each other (a net, loosely deined interaction referred
to as
hydrophobic force
); thus, nonpolar groups are let with no other choice but to
stick to each other. It is not so much that nonpolar groups “like” each other but that
they are “bullied” into “hugging” each other by the surrounding water network of
protons and hydronium ions.
As a corollary of the above, it is hard to design (or ind) a surface to which proteins and
nucleic acids (both large biopolymers with many charged groups and dipoles) will
not
physisorb.
However, there are also molecular-scale random events that can counteract this propensity to
adhere to surfaces:
