Biomedical Engineering Reference
In-Depth Information
direction, becoming to point to the external charge, which makes the interaction
between the water and the charge even stronger. This will result in more difficulties
when water molecules move along the nanotube, thus the net flux across the
nanochannel decreases.
It seems that the water permeation across the bio-mimicked nanochannel with
single-filed water molecules inside the nanochannel is also effectively resistant to
charge noises and sensitive to available charge signals.
Now we have come to the structure of aquaporins again. The structure of
aquaporins is appropriate that the water molecules inside the pore are single filed.
Moreover, compared to the CNT, aquaporins are much more complex while the
functions of most of the structures are unknown. There are two impressive parts in
those complex structures: the two highly conserved fingerprint Asp-Pro-Ala (NPA)
motifs in the pore center are proposed to determine the selectivity of permeation for
the channel, and the ar/R region near extracellular side, formed by the aromatic side
chains of Phe and His, and Arg, is proposed to function as a proton filter [
32
]. We
note that there are some important charged parts on these regions: the Arg residue on
ar/R region is positively charged (
C
1
e
); the two Asn residues on NPA motifs can be
considered to be carrying an effective charge of
C
0.5
e
on each residue, since they
are at the ends of two '-helixes (it is known that an '-helix has a net dipole moment
whose magnitude corresponds to a charge of 0.5-0.7
e
at each end of the helix [
41
]).
With further steps to understand the functions of those parts in the aquaporins and
trying to design nanochannels inspired by aquaporins, we assign the charges on
those parts near a CNT, as shown in Fig.
1.30
.
Explicitly, an SWNT 23 A in length and 8.1 A in diameter is embedded along
the vertical direction between two graphite sheets, and three positive charges with
charge magnitudes of 1.0
e
,0.5
e
, and 0.5
e
are positioned at
z
D
d
,
0.7 A, and
0.7 A, respectively, where
z
D
0 corresponds to the center of the channel. All of
the positive charges are at the same radial distance, ı, from the carbon atoms.
Numerically, they have found that when ı
0.5 A, there is a considerable net flux
from top to bottom.
1.1.2
Manipulating Biomolecules with Aqueous Liquids
Confined Within Nanoscale Channels
One thing we are inspired from this molecular water pump is that there is a
strong interaction between the charge outside the nanotube and the water molecules
inside the nanotube. This is because of the ordered dipole orientations of the water
molecules inside the nanotube when there is a charge outside the nanotube. It is
the ordering of the water dipole orientations that makes the interaction between the
charge and the water molecules strong. In Fig.
1.31
, we have shown the averaged
dipole orientation < > of water molecules when there is a charge outside the
nanotube (see inset in Fig.
1.32
), where
r
is the distance of a water molecule inside
